Contents
Introduction
Assembly language is a low-level programming language for niche platforms such as IoTs, device drivers, and embedded systems. Usually, it’s the sort of language that Computer Science students should cover in their coursework and rarely use in their future jobs. From TIOBE Programming Community Index, assembly language has enjoyed a steady rise in the rankings of the most popular programming languages recently.
In the early days, when an application was written in assembly language, it had to fit in a small amount of memory and run as efficiently as possible on slow processors. When memory becomes plentiful and processor speed is dramatically increased, we mainly rely on high level languages with ready made structures and libraries in development. If necessary, assembly language can be used to optimize critical sections for speed or to directly access non-portable hardware. Today assembly language still plays an important role in embedded system design, where performance efficiency is still considered as an important requirement.
In this article, we’ll talk about some basic criteria and code skills specific to assembly language programming. Also, considerations would be emphasized on execution speed and memory consumption. I'll analyze some examples, related to the concepts of register, memory, and stack, operators and constants, loops and procedures, system calls, etc.. For simplicity, all samples are in 32-bit, but most ideas will be easily applied to 64-bit.
All the materials presented here came from my teaching [1] for years. Thus, to read this article, a general understanding of Intel x86-64 assembly language is necessary, and being familiar with Visual Studio 2010 or above is assumed. Preferred, having read Kip Irvine’s textbook [2] and the MASM Programmer's Guide [3] are recommended. If you are taking an Assembly Language Programming class, this could be a supplemental reading for studies.
About instruction
The first two rules are general. If you can use less, don’t use more.
1. Using less instructions
Suppose that we have a 32-bit DWORD variable:
.data
var1 DWORD 123
The example is to add var1 to EAX. This is correct with MOV and ADD:
mov ebx, var1
add eax, ebx
But as ADD can accept one memory operand, you can just
add eax, var1
2. Using an instruction with less bytes
Suppose that we have an array:
.data
array DWORD 1,2,3
If want to rearrange the values to be 3,1,2, you could
mov eax,array
xchg eax,[array+4]
xchg eax,[array+8]
xchg array,eax
But notice that the last instruction should be MOV instead of XCHG. Although both can assign 3 in EAX to the first array element, the other way around in exchange XCHG is logically unnecessary.
Be aware of code size, MOV takes 5-byte machine code but XCHG takes 6, as another reason to choose MOV here:
00000011 87 05 00000000 R xchg array,eax
00000017 A3 00000000 R mov array,eax
To check machine code, you can generate a Listing file in assembling or open the Disassembly window at runtime in Visual Studio. Also, you can look up from the Intel instruction manual.
About register and memory
In this section, we’ll use a popular example, the nth Fibonacci number, to illustrate multiple solutions in assembly language. The C function would be like:
unsigned int Fibonacci(unsigned int n)
{
unsigned int previous = 1, current = 1, next = 0;
for (unsigned int i = 3; i <= n; ++i)
{
next = current + previous;
previous = current;
current = next;
}
return next;
}
3. Implementing with memory variables
At first, let’s copy the same idea from above with two variables previous and current created here
.data
previous DWORD ?
current DWORD ?
We can use EAX store the result without the next variable. Since MOV cannot move from memory to memory, a register like EDX must be involved for assignment previous = current. The following is the procedure FibonacciByMemory. It receives n from ECX and returns EAX as the nth Fibonacci number calculated:
FibonacciByMemory PROC
mov eax,1
mov previous,0
mov current,0
L1:
add eax,previous
mov edx, current
mov previous, edx
mov current, eax
loop L1
ret
FibonacciByMemory ENDP
4. If you can use registers, don’t use memory
A basic rule in assembly language programming is that if you can use a register, don’t use a variable. The register operation is much faster than that of memory. The general purpose registers available in 32-bit are EAX, EBX, ECX, EDX, ESI, and EDI. Don’t touch ESP and EBP that are for system use.
Now let EBX replace the previous variable and EDX replace current. The following is FibonacciByRegMOV, simply with three instructions needed in the loop:
FibonacciByRegMOV PROC
mov eax,1
xor ebx,ebx
xor edx,edx
L1:
add eax,ebx
mov ebx,edx
mov edx,eax
loop L1
ret
FibonacciByRegMOV ENDP
A further simplified version is to make use of XCHG which steps up the sequence without need of EDX. The following shows FibonacciByRegXCHG machine code in its Listing, where only two instructions of three machine-code bytes in the loop body:
000000DF FibonacciByRegXCHG PROC
000000DF 33 C0 xor eax,eax
000000E1 BB 00000001 mov ebx,1
000000E6 L1:
000000E6 93 xchg eax,ebx
000000E7 03 C3 add eax,ebx
000000E9 E2 FB loop L1
000000EB C3 ret
000000EC FibonacciByRegXCHG ENDP
In concurrent programming
The x86-64 instruction set provides many atomic instructions with the ability to temporarily inhibit interrupts, ensuring that the currently running process cannot be context switched, and suffices on a uniprocessor. In someway, it also would avoid the race condition in multi-tasking. These instructions can be directly used by compiler and operating system writers.
5. Using atomic instructions
As seen above used XCHG, so called as atomic swap, is more powerful than some high level language with just one statement:
xchg eax, var1
A classical way to swap a register with a memory var1 could be
mov ebx, eax
mov eax, var1
mov var1, ebx
Moreover, if you use the Intel486 instruction set with the .486 directive or above, simply using the atomic XADD is more concise in the Fibonacci procedure. XADD exchanges the first operand (destination) with the second operand (source), then loads the sum of the two values into the destination operand. Thus we have
000000EC FibonacciByRegXADD PROC
000000EC 33 C0 xor eax,eax
000000EE BB 00000001 mov ebx,1
000000F3 L1:
000000F3 0F C1 D8 xadd eax,ebx
000000F6 E2 FB loop L1
000000F8 C3 ret
000000F9 FibonacciByRegXADD ENDP
Two atomic move extensions are MOVZX and MOVSX. Another worth mentioning is bit test instructions, BT, BTC, BTR, and BTS. For the following example
.data
Semaphore WORD 10001000b
.code
btc Semaphore, 6
Imagine the instruction set without BTC, one non-atomic implementation for the same logic would be
mov ax, Semaphore
shr ax, 7
xor Semaphore,01000000b
Little-endian
An x86 processor stores and retrieves data from memory using little-endian order (low to high). The least significant byte is stored at the first memory address allocated for the data. The remaining bytes are stored in the next consecutive memory positions.
6. Memory representations
Consider the following data definitions:
.data
dw1 DWORD 12345678h
dw2 DWORD 'AB', '123', 123h
by3 BYTE 'ABCDE', 0FFh, 'A', 0Dh, 0Ah, 0
w1 WORD 123h, 'AB', 'A'
For simplicity, the hexadecimal constants are used as initializer. The memory representation is as follows:
As for multiple-byte DWORD and WORD date, they are represented by the little-endian order. Based on this, the second DWORD initialized with 'AB' should be 00004142h and next '123' is 00313233h in their original order. You can't initialize dw3 as 'ABCDE' that contains five bytes 4142434445h, while you really can initialize by3 in a byte memory since no little-endian for byte data. Similarly, see w1 for a WORD memory.
7. A code error hidden by little-endian
From the last section of using XADD, we try to fill in a byte array with first 7 Fibonacci numbers, as 01, 01, 02, 03, 05, 08, 0D. The following is such a simple implementation but with a bug. The bug does not show up an error immediately because it has been hidden by little-endian.
FibCount = 7
.data
FibArray BYTE FibCount DUP(0ffh)
BYTE 'ABCDEF'
.code
mov edi, OFFSET FibArray
mov eax,1
xor ebx,ebx
mov ecx, FibCount
L1:
mov [edi], eax
xadd eax, ebx
inc edi
loop L1
To debug, I purposely make a memory 'ABCDEF' at the end of the byte array FibArray with seven 0ffh initialized. The initial memory looks like this:
Let's set a breakpoint in the loop. When the first number 01 filled, it is followed by three zeros as this:
But OK, the second number 01 comes to fill the second byte to overwrite three zeros left by the first. So on and so forth, until the seventh 0D, it just fits the last byte here:
All fine with an expected result in FibArray because of little-endian. Only when you define some memory immediately after this FibArray, your first three byte will be overwritten by zeros, as here 'ABCDEF' becomes 'DEF'. How to make an easy fix?
About runtime stack
The runtime stack is a memory array directly managed by the CPU, with the stack pointer register ESP holding a 32-bit offset on the stack. ESP is modified by instructions CALL, RET, PUSH, POP, etc.. When use PUSH and POP or alike, you explicitly change the stack contents. You should be very cautious without affecting other implicit use, like CALL and RET, because you programmer and the system share the same runtime stack.
8. Assignment with PUSH and POP is not efficient
In assembly code, you definitely can make use of the stack to do assignment previous = current, as in FibonacciByMemory. The following is FibonacciByStack where only difference is using PUSH and POP instead of two MOV instructions with EDX.
FibonacciByStack
mov eax,1
mov previous,0
mov current,0
L1:
add eax,previous
push current
pop previous
mov current, eax
loop L1
ret
FibonacciByStack ENDP
As you can imagine, the runtime stack built on memory is much slower than registers. If you create a test benchmark to compare above procedures in a long loop, you’ll find that FibonacciByStack is the most inefficient. My suggestion is that if you can use a register or memory, don’t use PUSH and POP.
9. Using INC to avoid PUSHFD and POPFD
When you use the instruction ADC or SBB to add or subtract an integer with the previous carry, you reasonably want to reserve the previous carry flag (CF) with PUSHFD and POPFD, since an address update with ADD will overwrite the CF. The following Extended_Add example borrowed from the textbook [2] is to calculate the sum of two extended long integers BYTE by BYTE:
Extended_Add PROC
clc
L1:
mov al,[esi]
adc al,[edi]
pushfd
mov [ebx],al
add esi, 1
add edi, 1
add ebx, 1
popfd
loop L1
mov dword ptr [ebx],0
adc dword ptr [ebx],0
ret
Extended_Add ENDP
As we know, the INC instruction makes an increment by 1 without affecting the CF. Obviously we can replace above ADD with INC to avoid PUSHFD and POPFD. Thus the loop is simplified like this:
L1:
mov al,[esi]
adc al,[edi]
mov [ebx],al
inc esi
inc edi
inc ebx
loop L1
Now you might ask what if to calculate the sum of two long integers DWORD by DWORD where each iteration must update the addresses by 4 bytes, as TYPE DWORD. We still can make use of INC to have such an implementation:
clc
xor ebx, ebx
L1:
mov eax, [esi +ebx*TYPE DWORD]
adc eax, [edi +ebx*TYPE DWORD]
mov [edx +ebx*TYPE DWORD], eax
inc ebx
loop L1
Applying a scaling factor here would be more general and preferred. Similarly, wherever necessary, you also can use the DEC instruction that makes a decrement by 1 without affecting the carry flag.
10. Another good reason to avoid PUSH and POP
Since you and the system share the same stack, you should be very careful without disturbing the system use. If you forget to make PUSH and POP in pair, an error could happen, especially in a conditional jump when the procedure returns.
The following Search2DAry searches a 2-dimensional array for a value passed in EAX. If it is found, simply jump to the FOUND label returning one in EAX as true, else set EAX zero as false.
Search2DAry PROC
mov ecx,NUM_ROW
ROW:
push ecx
mov ecx,NUM_COL
COL:
cmp al, [esi+ecx-1]
je FOUND
loop COL
add esi, NUM_COL
pop ecx
loop ROW
mov eax, 0
jmp QUIT
FOUND:
mov eax, 1
QUIT:
ret
Search2DAry ENDP
Let’s call it in main by preparing the argument ESI pointing to the array address and the search value EAX to be 31h or 30h respectively for not-found or found test case:
.data
ary2D BYTE 10h, 20h, 30h, 40h, 50h
BYTE 60h, 70h, 80h, 90h, 0A0h
NUM_COL = 5
NUM_ROW = 2
.code
main PROC
mov esi, OFFSET ary2D
mov eax, 31h
call Search2DAry
exit
main ENDP
Unfortunately, it’s only working in not-found for 31h. A crash occurs for a successful searching like 30h, because of the stack leftover from an outer loop counter pushed. Sadly enough, that leftover being popped by RET becomes a return address to the caller.
Therefore, it’s better to use a register or variable to save the outer loop counter here. Although the logic error is still, a crash would not happen without interfering with the system. As a good exercise, you can try to fix.
Assembling time vs. runtime
I would like to talk more about this assembly language feature. Preferred, if you can do something at assembling time, don’t do it at runtime. Organizing logic in assembling indicates doing a job at static (compilation) time, not consuming runtime. Differently from high level languages, all operators in assembly language are processed in assembling such as +, -, *, and /, while only instructions work at runtime like ADD, SUB, MUL, and DIV.
11. Implementing with plus (+) instead of ADD
Let’s redo Fibonacci calculating to implement eax = ebx + edx in assembling with the plus operator by help of the LEA instruction. The following is FibonacciByRegLEA with only one line changed from FibonacciByRegMOV.
FibonacciByRegLEA
xor eax,eax
xor ebx,ebx
mov edx,1
L1:
lea eax, DWORD PTR [ebx+edx]
mov edx,ebx
mov ebx,eax
loop L1
ret
FibonacciByRegLEA ENDP
This statement is encoded as three bytes implemented in machine code without an addition operation explicitly at runtime:
000000CE 8D 04 1A lea eax, DWORD PTR [ebx+edx]
This example doesn’t make too much performance difference, compared to FibonacciByRegMOV. But is enough as an implementation demo.
12. If you can use an operator, don’t use an instruction
For an array defined as:
.data
Ary1 DWORD 20 DUP(?)
If you want to traverse it from the second element to the middle one, you might think of this like in other language:
mov esi, OFFSET Ary1
add esi, TYPE DWORD
mov ecx LENGTHOF Ary1
sub ecx, 1
div ecx, 2
L1:
Loop L1
Remember that ADD, SUB, and DIV are dynamic behavior at runtime. If you know values in advance, they are unnecessary to calculate at runtime, instead, apply operators in assembling:
mov esi, OFFSET Ary1 + TYPE DWORD
mov ecx (LENGTHOF Ary1 -1)/2
L1:
Loop L1
This saves three instructions in the code segment at runtime. Next, let’s save memory in the data segment.
13. If you can use a symbolic constant, don’t use a variable
Like operators, all directives are processed at assembling time. A variable consumes memory and has to be accessed at runtime. As for the last Ary1, you may want to remember its size in byte and the number of elements like this:
.data
Ary1 DWORD 20 DUP(?)
arySizeInByte DWORD ($ - Ary1)
aryLength DWORD LENGTHOF Ary1
It is correct but not preferred because of using two variables. Why not simply make them symbolic constants to save the memory of two DWORD?
.data
Ary1 DWORD 20 DUP(?)
arySizeInByte = ($ - Ary1)
aryLength EQU LENGTHOF Ary1
Using either equal sign or EQU directive is fine. The constant is just a replacement during code preprocessing.
14. Generating the memory block in macro
For an amount of data to initialize, if you already know the logic how to create, you can use macro to generate memory blocks in assembling, instead of at runtime. The following macro creates all 47 Fibonacci numbers in a DWORD array named FibArray:
.data
val1 = 1
val2 = 1
val3 = val1 + val2
FibArray LABEL DWORD
DWORD val1
DWORD val2
WHILE val3 LT 0FFFFFFFFh
DWORD val3
val1 = val2
val2 = val3
val3 = val1 + val2
ENDM
As macro goes to the assembler to be processed statically, this saves considerable initializations at runtime, as opposed to FibonacciByXXX mentioned before.
For more about macro in MASM, see my article Something You May Not Know About the Macro in MASM [4]. I also made a reverse engineering for the switch statement in VC++ compiler implementation. Interestingly, under some condition the switch statement chooses the binary search but without exposing the prerequisite of a sort implementation at runtime. It’s reasonable to think of the preprocessor that does the sorting with all known case values in compilation. The static sorting behavior (as opposed to dynamic behavior at runtime), could be implemented with a macro procedure, directives and operators. For details, please see Something You May Not Know About the Switch Statement in C/C++ [5].
About loop design
Almost every language provides an unconditional jump like GOTO, but most of us rarely use it based on software engineering principles. Instead, we use others like break and continue. While in assembly language, we rely more on jumps either conditional or unconditional to make control workflow more freely. In the following sections, I list some ill-coded patterns.
15. Encapsulating all loop logic in the loop body
To construct a loop, try to make all your loop contents in the loop body. Don’t jump out to do something and then jump back into the loop. The example here is to traverse a one-dimensional integer array. If find an odd number, increment it, else do nothing.
Two unclear solutions with the correct result would be possibly like:
mov ecx, LENGTHOF array
xor esi, esi
L1:
test array[esi], 1
jnz ODD
PASS:
add esi, TYPE DWORD
loop L1
jmp DONE
ODD:
inc array[esi]
jmp PASS
DONE:
| | mov ecx, LENGTHOF array
xor esi, esi
jmp L1
ODD:
inc array[esi]
jmp PASS
L1:
test array[esi], 1
jnz ODD
PASS:
add esi, TYPE DWORD
loop L1
|
However, they both do incrementing outside and then jump back. They make a check in the loop but the left does incrementing after the loop and the right does before the loop. For a simple logic, you may not think like this; while for a complicated problem, assembly language could lead astray to produce such a spaghetti pattern. The following is a good one, which encapsulates all logic in the loop body, concise, readable, maintainable, and efficient.
mov ecx, LENGTHOF array
xor esi, esi
L1:
test array[esi], 1
jz PASS
inc array[esi]
PASS:
add esi, TYPE DWORD
loop L1
16. Loop entrance and exit
Usually preferred is a loop with one entrance and one exit. But if necessary, two or more conditional exits are fine as shown in Search2DAry with found and not-found results.
The following is a bad pattern of two-entrance, where one gets into START via initialization and another directly goes to MIDDLE. Such a code is pretty hard to understand. Need to reorganize or refactor the loop logic.
je MIDDLE
START:
MIDDLE:
loop START
The following is a bad pattern of two-loop ends, where some logic gets out of the first loop end while the other exits at the second. Such a code is quite confusing. Try to reconsider with a label jumping to maintain one loop end.
START2:
je NEXT
loop START2
jmp DONE
NEXT:
loop START2
DONE:
17. Don’t change ECX in the loop body
The register ECX acts as a loop counter and its value is implicitly decremented when using the LOOP instruction. You can read ECX and make use of its value in iteration. As see in Search2DAry in the previous section, we compare the indirect operand [ESI+ECX-1] with AL. But never try to change the loop counter within the loop body that makes code hard to understand and hard to debug. A good practice is to think of the loop counter ECX as read-only.
mov ecx, 10
L1:
mov eax, ecx
mov ebx, [esi +ecx *TYPE DWORD]
mov ecx, edx
inc ecx
loop L1
18. When jump backward…
Besides the LOOP instruction, assembly language programming can heavily rely on conditional or unconditional jumps to create a loop when the count is not determined before the loop. Theoretically, for a backward jump, the workflow might be considered as a loop. Assume that jx and jy are desired jump or LOOP instructions. The following backward jy L2 nested in the jx L1 is probably thought of as an inner loop.
L1:
L2:
jy L2
jx L1
To have selection logic of if-then-else, it's reasonable to use a foreword jump like this as branching in the jx L1 iteration:
L1:
jy TrueLogic
jmp DONE
TrueLogic:
DONE:
jx L1
19. Implementing C/C++ FOR loop and WHILE loop
The high level language usually provides three types of loop constructs. A FOR loop is often used when a known number of iterations available in coding that allows to initiate a loop counter as a check condition, and to change the count variable each iteration. A WHILE loop may be used when a loop counter is unknown, e.g, it might be determined by the user input as an ending flag at runtime. A DO-WHILE loop executes the loop body first and then check the condition. However, the usage is not so strictly clear and limited, since one loop can be simply replaced (implemented) by the other programmatically.
Let's see how the assembly code implements three loop structures in high level language. The previously mentioned LOOP instruction should behave like the FOR loop, because you have to initialize a known loop counter in ECX. The "LOOP target" statement takes two actions:
- decre
ment ECX - if
ECX != 0, jump to target
To calculate the sum of n+(n-1)+...+2+1, we can have
mov ecx, n
xor eax, eax
L1:
add eax, ecx
loop L1
mov sum, eax
This is the same as the FOR loop:
int sum=0;
for (int i=n; i>0; i++)
sum += i;
How about the following logic - for a WHILE loop to add any non-zero input numbers until a zero entered:
int sum=0;
cin >> n;
while (n !=0)
{
sum += n;
cin >> n;
}
There is no meaning to use LOOP here, because you could not set or ignore any value in ECX. Instead, using a conditional jump to manually construct such a loop is required:
xor ebx, ebx
call ReadInt
L1:
or eax, eax
jz L2
add ebx, eax
call ReadInt
jmp L1
L2:
mov sum, ebx
Here the Irvine32 library procedure ReadInt is used to read an integer from the console into EAX. Using OR instead of CMP is just for efficiency, as OR doesn't affect EAX while affecting the zero flag for JZ. Next, considering the similar logic with DO-WHILE loop:
int sum=0;
cin >> n;
do
{
sum += n;
cin >> n;
}
while (n !=0)
Still with a conditional jump to have a loop here, the code looks more straight, as it does loop body first and then check:
xor ebx, ebx
call ReadInt
L1:
add ebx, eax
call ReadInt
or eax, eax
jnz L1
mov sum, ebx
20. Making your loop more efficient with a jump
Based on above understanding, we can now turn to the loop optimization in assembly code. For detailed instruction mechanisms, please see the Intel® 64 and IA-32 Architectures Optimization Reference Manual. Here, I only use an example of calculating the sum of n+(n-1)+...+2+1 to illustrate the performance comparison between iteration implementations of LOOP and conditional jumps. As code in the last section, I create our first procedure named as Using_LOOP:
Using_LOOP PROC
xor eax, eax
L1:
add eax, ecx
loop L1
ret
Using_LOOP ENDP
To manually simulate the LOOP instruction, I simply decrement ECX and if not zero, go back to the loop label. So I name the second one Using_DEC_JNZ:
Using_DEC_JNZ PROC
xor eax, eax
L1:
add eax, ecx
dec ecx
JNZ L1
ret
Using_DEC_JNZ ENDP
A similar alternative could be a third procedure by using JECXZ below, naming it as Using_DEC_JECXZ_JMP:
Using_DEC_JECXZ_JMP PROC
xor eax, eax
L1:
add eax, ecx
dec ecx
JECXZ L2
jmp L1
L2:
ret
Using_DEC_JECXZ_JMP ENDP
Now let's test three procedures by accepting a number n from the user input to save the loop counter, and then calling each procedure with a macro mCallSumProc (Here Clrscr, ReadDec, Crlf, and mWrite are from Irvine32 that will be mentioned shortly):
main PROC
call Clrscr
mWrite "To calculate 1+2+...+n, please enter n (1 - 4294967295): "
call ReadDec
mov ecx, eax
call Crlf
mCallSumProc Using_LOOP
mCallSumProc Using_DEC_JNE
mCallSumProc Using_DEC_JECXZ_JMP
exit
main ENDP
To test, enter a large number like 4 billion. Although the sum is far beyond the 32-bit maximum 0FFFFFFFFh, with only remainder left in EAX as (1+2+...+n) MOD 4294967295, it doesn't matter to our benchmark test. The following is the result from my Intel Core i7, 64-bit BootCamp:
Probably, the result will be slightly different on different systems. The test executable is available for try at LoopTest.EXE. Basically, using a conditional jump to construct your loop is more efficient than using the LOOP instruction directly. You can read "Intel® 64 and IA-32 Architectures Optimization Reference Manual" to find why. Also I would like to thank Mr. Daniel Pfeffer for his nice comments about optimizations that you can read in Comments and Discussions at the end.
Finally, I present above unmentioned macro as below. Again, it contains some Irvine32 library procedure calls. The source code in this section can be downloaded at Loop Test ASM Project. To understand further, please see the links in References
mCallSumProc MACRO SumProc:REQ
push ecx
call GetMseconds
mov esi,eax
call SumProc
mWrite "&SumProc: "
call WriteDec
call crlf
call GetMseconds
sub eax,esi
call WriteDec
mWrite <' millisecond(s) used', 0Dh,0Ah, 0Dh,0Ah >
pop ecx
ENDM
About procedure
Similar to functions in C/C++, we talk about some basics in assembly language's procedure.
21. Making a clear calling interface
When design a procedure, we hope to make it as reusable as possible. Make it perform only one task without others like I/O. The procedure's caller should take the responsibility to do input and putout. The caller should communicate with the procedure only by arguments and parameters. The procedure should only use parameters in its logic without referring outside definitions, without any:
- Global variable and array
- Global symbolic constant
Because implementing with such a definition makes your procedure un-reusable.
Recalling previous five FibonacciByXXX procedures, we use register ECX as both argument and parameter with the return value in EAX to make a clear calling interface:
FibonacciByXXX
Now the caller can do like
call FibonacciByXXX
To illustrate as a second example, let’s take a look again at calling Search2DAry in the previous section. The register arguments ESI and EAX are prepared so that the implementation of Search2DAry doesn’t directly refer to the global array, ary2D.
... ...
NUM_COL = 5
NUM_ROW = 2
.code
main PROC
mov esi, OFFSET ary2D
mov eax, 31h
call Search2DAry
exit
main ENDP
Search2DAry PROC
mov ecx,NUM_ROW
... ...
mov ecx,NUM_COL
... ...
Unfortunately, the weakness is its implementation still using two global constants NUM_ROW and NUM_COL that makes it not being called elsewhere. To improve, supplying other two register arguments would be an obvious way, or see the next section.
22. INVOKE vs. CALL
Besides the CALL instruction from Intel, MASM provides the 32-bit INVOKE directive to make a procedure call easier. For the CALL instruction, you only can use registers as argument/parameter pair in calling interface as shown above. The problem is that the number of registers is limited. All registers are global and you probably have to save registers before calling and restore after calling. The INVOKE directive gives the form of a procedure with a parameter-list, as you experienced in high level languages.
When consider Search2DAry with a parameter-list without referring the global constants NUM_ROW and NUM_COL, we can have its prototype like this
Search2DAry PROTO, pAry2D: PTR BYTE, val: BYTE, nRow: WORD, nCol: WORD
Again, as an exercise, you can try to implement this for a fix. Now you just do
INVOKE Search2DAry, ary2D, 31h, NUM_ROW, NUM_COL
Likewise, to construct a parameter-list procedure, you still need to follow the rule without referring global variables and constants. Besides, also attention to:
- The entire calling interface should only go through the parameter list without referring any register values set outside the procedure.
23. Call-by-Value vs. Call-by-Reference
Also be aware of that a parameter-list should not be too long. If so, use an object parameter instead. Suppose that you fully understood the function concept, call-by-value and call-by-reference in high level languages. By learning the stack frame in assembly language, you understand more about the low-level function calling mechanism. Usually for an object argument, we prefer passing a reference, an object address, rather than the whole object copied on the stack memory.
To demonstrate this, let’s create a procedure to write month, day, and year from an object of the Win32 SYSTEMTIME structure.
The following is the version of call-by-value, where we use the dot operator to retrieve individual WORD field members from the DateTime object and extend their 16-bit values to 32-bit EAX:
WriteDateByVal PROC, DateTime:SYSTEMTIME
movzx eax, DateTime.wMonth
movzx eax, DateTime.wDay
movzx eax, DateTime.wYear
ret
WriteDateByVal ENDP
The version of call-by-reference is not so straight with an object address received. Not like the arrow ->, pointer operator in C/C++, we have to save the pointer (address) value in a 32-bit register like ESI. By using ESI as an indirect operand, we must cast its memory back to the SYSTEMTIME type. Then we can get the object members with the dot:
WriteDateByRef PROC, datetimePtr: PTR SYSTEMTIME
mov esi, datetimePtr
movzx eax, (SYSTEMTIME PTR [esi]).wMonth
movzx eax, (SYSTEMTIME PTR [esi]).wDay
movzx eax, (SYSTEMTIME PTR [esi]).wYear
ret
WriteDateByRef ENDP
You can watch the stack frame of argument passed for two versions at runtime. For WriteDateByVal, eight WORD members are copied on the stack and consume sixteen bytes, while for WriteDateByRef, only need four bytes as a 32-bit address. It will make a big difference for a big structure object, though.
24. Avoid multiple RET
To construct a procedure, it’s ideal to make all your logics within the procedure body. Preferred is a procedure with one entrance and one exit. Since in assembly language programming, a procedure name is directly represented by a memory address, as well as any labels. Thus directly jumping to a label or a procedure without using CALL or INVOKE would be possible. Since such an abnormal entry would be quite rare, I am not to going to mention here.
Although multiple returns are sometimes used in other language examples, I don’t encourage such a pattern in assembly code. Multiple RET instructions could make your logic not easy to understand and debug. The following code on the left is such an example in branching. Instead, on the right, we have a label QUIT at the end and jump there making a single exit, where probably do common chaos to avoid repeated code.
MultiRetEx PROC
jx NEXTx
ret
NEXTx:
jy NEXTy
ret
NEXTy:
ret
MultiRetEx ENDP
| | SingleRetEx PROC
jx NEXTx
jmp QUIT
NEXTx:
jy NEXTy
jmp QUIT
NEXTy:
QUIT:
ret
SingleRetEx ENDP
|
Object data members
Similar to above SYSTEMTIME structure, we can also create our own type or a nested:
Rectangle STRUCT
UpperLeft COORD <>
LowerRight COORD <>
Rectangle ENDS
.data
rect Rectangle { {10,20}, {30,50} }
The Rectangle type contains two COORD members, UpperLeft and LowerRight. The Win32 COORD contains two WORD (SHORT), X and Y. Obviously, we can access the object rect’s data members with the dot operator from either direct or indirect operand like this
mov rect.UpperLeft.X, 11
mov esi,OFFSET rect
mov (Rectangle PTR [esi]).UpperLeft.Y, 22
mov esi,OFFSET rect.LowerRight
mov (COORD PTR [esi]).X, 33
mov esi,OFFSET rect.LowerRight.Y
mov WORD PTR [esi], 55
By using the OFFSET operator, we access different data member values with different type casts. Recall that any operator is processed in assembling at static time. What if we want to retrieve a data member’s address (not value) at runtime?
25. Indirect operand and LEA
For an indirect operand pointing to an object, you can’t use the OFFSET operator to get the member's address, because OFFSET only can take an address of a variable defined in the data segment.
There could be a scenario that we have to pass an object reference argument to a procedure like WriteDateByRef in the previous section, but want to retrieve its member’s address (not value). Still use the above rect object for an example. The following second use of OFFSET is not valid in assembling:
mov esi,OFFSET rect
mov edi, OFFSET (Rectangle PTR [esi]).LowerRight
Let’s ask for help from the LEA instruction that you have seen in FibonacciByRegLEA in the previous section. The LEA instruction calculates and loads the effective address of a memory operand. Similar to the OFFSET operator, except that only LEA can obtain an address calculated at runtime:
mov esi,OFFSET rect
lea edi, (Rectangle PTR [esi]).LowerRight
mov ebx, OFFSET rect.LowerRight
lea edi, (Rectangle PTR [esi]).UpperLeft.Y
mov ebx, OFFSET rect.UpperLeft.Y
mov esi,OFFSET rect.UpperLeft
lea edi, (COORD PTR [esi]).Y
I purposely have EBX here to get an address statically and you can verify the same address in EDI that is loaded dynamically from the indirect operand ESI at runtime.
About system I/O
From Computer Memory Basics, we know that I/O operations from the operating system are quite slow. Input and output are usually in the measurement of milliseconds, compared with register and memory in nanoseconds or microseconds. To be more efficient, trying to reduce system API calls is a nice consideration. Here I mean Win32 API call. For details about the Win32 functions mentioned in the following, please refer to MSDN to understand.
26. Reducing system I/O API calls
An example is to output 20 lines of 50 random characters with random colors as below:
We definitely can generate one character to output a time, by using SetConsoleTextAttribute and WriteConsole. Simply set its color by
INVOKE SetConsoleTextAttribute, consoleOutHandle, wAttributes
Then write that character by
INVOKE WriteConsole,
consoleOutHandle,
OFFSET buffer,
1,
OFFSET bytesWritten,
0
When write 50 characters, make a new line. So we can create a nested iteration, the outer loop for 20 rows and the inner loop for 50 columns. As 50 by 20, we call these two console output functions 1000 times.
However, another pair of API functions can be more efficient, by writing 50 characters in a row and setting their colors once a time. They are WriteConsoleOutputAttribute and WriteConsoleOutputCharacter. To make use of them, let’s create two procedures:
ChooseColor PROC
ChooseCharacter PROC
We call them in a loop to prepare a WORD array bufColor and a BYTE array bufChar for all 50 characters selected. Now we can write the 50 random characters per line with two calls here:
INVOKE WriteConsoleOutputAttribute,
outHandle,
ADDR bufColor,
MAXCOL,
xyPos,
ADDR cellsWritten
INVOKE WriteConsoleOutputCharacter,
outHandle,
ADDR bufChar,
MAXCOL,
xyPos,
ADDR cellsWritten
Besides bufColor and bufChar, we define MAXCOL = 50 and the COORD type xyPos so that xyPos.y is incremented each row in a single loop of 20 rows. Totally we only call these two APIs 20 times.
About PTR operator
MASM provides the operator PTR that is similar to the pointer * used in C/C++. The following is the PTR specification:
- type PTR expression
Forces the expression to be treated as having the specified type. - [[ distance ]] PTR type
Specifies a pointer to type.
This means that two usages are available, such as BYTE PTR or PTR BYTE. Let's discuss how to use them.
27. Defining a pointer, cast and dereference
The following C/C++ code demonstrates which type of Endian is used in your system, little endian or big endian? As an integer type takes four bytes, it makes a pointer type cast from the array name fourBytes, a char address, to an unsigned int address. Then it displays the integer result by dereferencing the unsigned int pointer.
int main()
{
unsigned char fourBytes[] = { 0x12, 0x34, 0x56, 0x78 };
unsigned int *ptr = (unsigned int *)fourBytes;
printf("1. Directly Cast: n is %Xh\n", *ptr);
return 0;
}
As expected in x86 Intel based system, this verifies the little endian by showing 78563412 in hexadecimal. We can do the same thing in assembly language with DWORD PTR, which is just similar to an address casting to 4-byte DWORD, the unsigned int type.
.data
fourBytes BYTE 12h,34h,56h,78h
.code
mov eax, DWORD PTR fourBytes
There is no explicit dereference here, since DWORD PTR combines four bytes into a DWORD memory and lets MOV retrieve it as a direct operand to EAX. This could be considered equivalent to the (unsigned int *) cast.
Now let's do another way by using PTR DWORD. Again, with the same logic above, this time we define a DWORD pointer type first with TYPEDEF:
DWORD_POINTER TYPEDEF PTR DWORD
This could be considered equivalent to defining the pointer type as unsigned int *. Then in the following data segment, the address variable dwPtr takes over the fourBytes memory. Finally in code, EBX holds this address as an indirect operand and makes an explicit dereference here to get its DWORD value to EAX.
.data
fourBytes BYTE 12h,34h,56h,78h
dwPtr DWORD_POINTER fourBytes
.code
mov ebx, dwPtr
mov eax, [ebx]
To summarize, PTR DWORD indicates a DWORD address type to define(declare) a variable like a pointer type. While DWORD PTR indicates the memory pointed by a DWORD address like a type cast.
28. Using PTR in a procedure
To define a procedure with a parameter list, you might want to use PTR in both ways. The following is such an example to increment each element in a DWORD array:
IncrementArray PROC, pAry:PTR DWORD, count:DWORD
mov edi,pAry
mov ecx,count
L1:
inc DWORD PTR [edi]
add edi, TYPE DWORD
loop L1
ret
IncrementArray ENDP
As the first parameter pAry is a DWORD address, so PTR DWORD is used as a parameter type. In the procedure, when incrementing a value pointed by the indirect operand EDI, you must tell the system what the type(size) of that memory is by using DWORD PTR.
Another example is the earlier mentioned WriteDateByRef, where SYSTEMTIME is a Windows defined structure type.
WriteDateByRef PROC, datetimePtr: PTR SYSTEMTIME
mov esi, datetimePtr
movzx eax, (SYSTEMTIME PTR [esi]).wMonth
... ...
ret
WriteDateByRef ENDP
Likewise, we use PTR SYSTEMTIME as the parameter type to define datetimePtr. When ESI receives an address from datetimePtr, it has no knowledge about the memory type just like a void pointer in C/C++. We have to cast it as a SYSTEMTIME memory, so as to retrieve its data members.
Signed and Unsigned
In assembly language programming, you can define an integer variable as either signed as SBYTE, SWORD, and SDWORD, or unsigned as BYTE, WORD, and DWORD. The data ranges, for example of 8-bit, are
BYTE: 0 to 255 (00h to FFh), totally 256 numbersSBYTE: half negatives, -128 to -1 (80h to FFh), half positives, 0 to 127 (00h to 7Fh)
Based on the hardware point of view, all CPU instructions operate exactly the same on signed and unsigned integers, because the CPU cannot distinguish between signed and unsigned. For example, when define
.data
bVal BYTE 255
sbVal SBYTR -1
Both of them have the 8-bit binary FFh saved in memory or moved to a register. You, as a programmer, are solely responsible for using the correct data type with an instruction and are able to explain a results from the flags affected:
- The carry flag
CF for unsigned integers - The overflow flag
OF for signed integers
The following are usually several tricks or pitfalls.
29. Comparison with conditional jumps
Let's check the following code to see which label it jumps:
mov eax, -1
cmp eax, 1
ja L1
jmp L2
As we know, CMP follows the same logic as SUB while non-destructive to the destination operand. Using JA means considering unsigned comparison, where the destination EAX is FFh, i.e. 255, while the source is 1. Certainly 255 is bigger than 1, so that makes it jump to L1. Thus, any unsigned comparisons such as JA, JB, JAE, JNA, etc. can be remembered as A(Above) or B(Below). An unsigned comparison is determined by CF and the zero flag ZF as shown in the following examples:
| CMP if | Destination | Source | ZF(ZR) | CF(CY) |
| Destination<Source | 1 | 2 | 0 | 1 |
| Destination>Source | 2 | 1 | 0 | 0 |
| Destination=Source | 1 | 1 | 1 | 0 |
Now let's take a look at signed comparison with the following code to see where it jumps:
mov eax, -1
cmp eax, 1
jg L1
jmp L2
Only difference is JG here instead of JA. Using JG means considering signed comparison, where the destination EAX is FFh, i.e. -1, while the source is 1. Certainly -1 is smaller than 1, so that makes JMP to L2. Likewise, any signed comparisons such as JG, JL, JGE, JNG, etc. can be thought of as G(Greater) or L(Less). A signed comparison is determined by OF and the sign flag SF as shown in the following examples:
| CMP if | Destination | Source | SF(PL) | OF(OV) |
| Destination<Source: (SF != OF) | -2 | 127 | 0 | 1 |
| -2 | 1 | 1 | 0 |
| Destination>Source: (SF == OF) | 127 | 1 | 0 | 0 |
| 127 | -1 | 1 | 1 |
| Destination = Source | 1 | 1 | ZF=1 |
30. When CBW, CWD, or CDQ mistakenly meets DIV...
As we know, the DIV instruction is for unsigned to perform 8-bit, 16-bit, or 32-bit integer division with the dividend AX, DX:AX, or EDX:EAX respectively. As for unsigned, you have to clear the upper half by zeroing AH, DX, or EDX before using DIV. But when perform signed division with IDIV, the sign extension CBW, CWD, and CDQ are provided to extend the upper half before using IDIV.
For a positive integer, if its highest bit (sign bit) is zero, there is no difference to manually clear the upper part of a dividend or mistakenly use a sign extension as shown in the following example:
mov eax,1002h
cdq
mov ebx,10h
div ebx
This is fine because 1000h is a small positive and CDQ makes EDX zero, the same as directly clearing EDX. So if your value is positive and its highest bit is zero, using CDQ and
XOR EDX, EDX
are exactly the same.
However, it doesn’t mean that you can always use CDQ/CWD/CBW with DIV when perform a positive division. For an example of 8-bit, 129/2, expecting quotient 64 and remainder 1. But, if you make this
mov al, 129
cbw
mov bl,2
div bl
Try above in debug to see how integer division overflow happens as a result. If really want to make it correct as unsigned DIV, you must:
mov al, 129
XOR ah, ah
mov bl,2
div bl
On the other side, if really want to use CBW, it means that you perform a signed division. Then you must use IDIV:
mov al, 129
cbw
mov bl,2
idiv bl
As seen here, 81h in signed byte is decimal -127 so that signed IDIV gives the correct quotient and remainder as above
31. Why 255-1 and 255+(-1) affect CF differently?
To talk about the carry flag CF, let's take the following two arithmetic calculations:
mov al, 255
sub al, 1
mov bl, 255
add bl, -1
From a human being's point of view, they do exactly the same operation, 255 minus 1 with the result 254 (FEh). Likewise, based on the hardware point, for either calculation, the CPU does the same operation by representing -1 as a two's complement FFh and then add it to 255. Now 255 is FFh and the binary format of -1 is also FFh. This is how it has been calculated:
1111 1111
+ 1111 1111
-------------
1111 1110
Remember? A CPU operates exactly the same on signed and unsigned because it cannot distinguish them. A programmer should be able to explain the behavior by the flag affected. Since we talk about the CF, it means we consider two calculations as unsigned. The key information is that -1 is FFh and then 255 in decimal. So the logic interpretation of CF is
- For
sub al, 1, it means 255 minus 1 to result in 254, without need of a borrow, so CF = 0 - For
add bl, -1, it seems that 255 plus 255 is resulted in 510, but with a carry 1,0000,0000b (256) out, 254 is a remainder left in byte, so CF = 1
From hardware implementation, CF depends on which instruction used, ADD or SUB. Here MSB (Most Significant Bit) is the highest bit.
- For
ADD instruction, add bl, -1, directly use the carry out of the MSB, so CF = 1 - For
SUB instruction, sub al, 1, must INVERT the carry out of the MSB, so CF = 0
32. How to determine OF?
Now let's see the overflow flag OF, still with above two arithmetic calculations as this:
mov al, 255
sub al, 1
mov bl, 255
add bl, -1
Both of them are not overflow, so OF = 0. We can have two ways to determine OF, the logic rule and hardware implementation.
Logic viewpoint: The overflow flag is only set, OF = 1, when
- Two positive operands are added and their sum is negative
- Two negative operands are added and their sum is positive
For signed, 255 is -1 (FFh). The flag OF doesn't care about ADD or SUB. Our two examples just do -1 plus -1 with the result -2. Thus, two negatives are added with the sum still negative, so OF = 0.
Hardware implementation: For non-zero operands,
OF = (carry out of the MSB) XOR (carry into the MSB)
As seen our calculation again:
1111 1111
+ 1111 1111
-------------
1111 1110
The carry out of the MSB is 1 and the carry into the MSB is also 1. Then OF = (1 XOR 1) = 0
To practice more, the following table enumerates different test cases for your understanding:
Ambiguous "LOCAL" directive
As mentioned previously, the PTR operator has two usages such as DWORD PTR and PTR DWORD. But MASM provides another confused directive LOCAL, that is ambiguous depending on the context, where to use with exactly the same reserved word. The following is the specification from MSDN:
LOCAL localname [[, localname]]...
LOCAL label [[ [count ] ]] [[:type]] [[, label [[ [count] ]] [[type]]]]...
- In the first directive, within a macro,
LOCAL defines labels that are unique to each instance of the macro. - In the second directive, within a procedure definition (PROC),
LOCAL creates stack-based variables that exist for the duration of the procedure. The label may be a simple variable or an array containing count elements.
This specification is not clear enough to understand. In this section, I'll expose the essential difference in between and show two example using the LOCAL directive, one in a procedure and the other in a macro. As for your familiarity, both examples calculate the nth Fibonacci number as early FibonacciByMemory. The main point delivered here is:
- The variables declared by
LOCAL in a macro are NOT local to the macro. They are system generated global variables on the data segment to resolve redefinition. - The variables created by
LOCAL in a procedure are really local variables allocated on the stack frame with the lifecycle only during the procedure.
For the basic concepts and implementations of data segment and stack frame, please take a look at some textbook or MASM manual that could be worthy of several chapters without being talked here.
33. When LOCAL used in a procedure
The following is a procedure with a parameter n to calculate nth Fibonacci number returned in EAX. I let the loop counter ECX take over the parameter n. Please compare it with FibonacciByMemory. The logic is the same with only difference of using the local variables pre and cur here, instead of global variables previous and current in FibonacciByMemory.
FibonacciByLocalVariable PROC USES ecx edx, n:DWORD
LOCAL pre, cur :DWORD
mov ecx,n
mov eax,1
mov pre,0
mov cur,0
L1:
add eax, pre
mov edx, cur
mov pre, edx
mov cur, eax
loop L1
ret
FibonacciByLocalVariable ENDP
The following is the code generated from the VS Disassembly window at runtime. As you can see, each line of assembly source is translated into machine code with the parameter n and two local variables created on the stack frame, referenced by EBP:
231:
232: FibonacciByLocalVariable PROC USES ecx edx, n:DWORD
011713F4 55 push ebp
011713F5 8B EC mov ebp,esp
011713F7 83 C4 F8 add esp,0FFFFFFF8h
011713FA 51 push ecx
011713FB 52 push edx
233:
234:
235:
236: LOCAL pre, cur :DWORD
237:
238: mov ecx,n
011713FC 8B 4D 08 mov ecx,dword ptr [ebp+8]
239: mov eax,1
011713FF B8 01 00 00 00 mov eax,1
240: mov pre,0
01171404 C7 45 FC 00 00 00 00 mov dword ptr [ebp-4],0
241: mov cur,0
0117140B C7 45 F8 00 00 00 00 mov dword ptr [ebp-8],0
242: L1:
243: add eax,pre
01171412 03 45 FC add eax,dword ptr [ebp-4]
244: mov EDX, cur
01171415 8B 55 F8 mov edx,dword ptr [ebp-8]
245: mov pre, EDX
01171418 89 55 FC mov dword ptr [ebp-4],edx
246: mov cur, eax
0117141B 89 45 F8 mov dword ptr [ebp-8],eax
247: loop L1
0117141E E2 F2 loop 01171412
248:
249: ret
01171420 5A pop edx
01171421 59 pop ecx
01171422 C9 leave
01171423 C2 04 00 ret 4
250: FibonacciByLocalVariable ENDP
When FibonacciByLocalVariable running, the stack frame can be seen as below:
Obviously, the parameter n is at EBP+8. This
add esp, 0FFFFFFF8h
just means
sub esp, 08h
moving the stack pointer ESP down eight bytes for two DWORD creation of pre and cur. Finally the LEAVE instruction implicitly does
mov esp, ebp
pop ebp
that moves EBP back to ESP releasing the local variables pre and cur. And this releases n, at EBP+8, for STD calling convention:
ret 4
34. When LOCAL used in a macro
To have a macro implementation, I almost copy the same code from FibonacciByLocalVariable. Since no USES for a macro, I manually use PUSH/POP for ECX and EDX. Also without a stack frame, I have to create global variables mPre and mCur on the data segment. The mFibonacciByMacro can be like this:
mFibonacciByMacro MACRO n
LOCAL mPre, mCur, mL
.data
mPre DWORD ?
mCur DWORD ?
.code
push ecx
push edx
mov ecx,n
mov eax,1
mov mPre,0
mov mCur,0
mL:
add eax, mPre
mov edx, mCur
mov mPre, edx
mov mCur, eax
loop mL
pop edx
pop ecx
ENDM
If you just want to call mFibonacciByMacro once, for example
mFibonacciByMacro 12
You don't need LOCAL here. Let's simply comment it out:
mFibonacciByMacro accepts the argument 12 and replace n with 12. This works fine with the following Listing MASM generated:
mFibonacciByMacro 12
0000018C 1 .data
0000018C 00000000 1 mPre DWORD ?
00000190 00000000 1 mCur DWORD ?
00000000 1 .code
00000000 51 1 push ecx
00000001 52 1 push edx
00000002 B9 0000000C 1 mov ecx,12
00000007 B8 00000001 1 mov eax,1
0000000C C7 05 0000018C R 1 mov mPre,0
00000000
00000016 C7 05 00000190 R 1 mov mCur,0
00000000
00000020 1 mL:
00000020 03 05 0000018C R 1 add eax,mPre
00000026 8B 15 00000190 R 1 mov edx, mCur
0000002C 89 15 0000018C R 1 mov mPre, edx
00000032 A3 00000190 R 1 mov mCur, eax
00000037 E2 E7 1 loop mL
00000039 5A 1 pop edx
0000003A 59 1 pop ecx
Nothing changed from the original code with just a substitution of 12. The variables mPre and mCur are visible explicitly. Now let's call it twice, like
mFibonacciByMacro 12
mFibonacciByMacro 13
This is still fine for the first mFibonacciByMacro 12 but secondly, causes three redefinitions in preprocessing mFibonacciByMacro 13. Not only are data labels, i.e., variables mPre and mCur, but also complained is the code label mL. This is because in assembly code, each label is actually a memory address and the second label of any mPre, mCur, or mL should take another memory, rather than defining an already created one:
mFibonacciByMacro 12
0000018C 1 .data
0000018C 00000000 1 mPre DWORD ?
00000190 00000000 1 mCur DWORD ?
00000000 1 .code
00000000 51 1 push ecx
00000001 52 1 push edx
00000002 B9 0000000C 1 mov ecx,12
00000007 B8 00000001 1 mov eax,1
0000000C C7 05 0000018C R 1 mov mPre,0
00000000
00000016 C7 05 00000190 R 1 mov mCur,0
00000000
00000020 1 mL:
00000020 03 05 0000018C R 1 add eax,mPre
00000026 8B 15 00000190 R 1 mov edx, mCur
0000002C 89 15 0000018C R 1 mov mPre, edx
00000032 A3 00000190 R 1 mov mCur, eax
00000037 E2 E7 1 loop mL
00000039 5A 1 pop edx
0000003A 59 1 pop ecx
mFibonacciByMacro 13
00000194 1 .data
1 mPre DWORD ?
FibTest.32.asm(83) : error A2005:symbol redefinition : mPre
mFibonacciByMacro(6): Macro Called From
FibTest.32.asm(83): Main Line Code
1 mCur DWORD ?
FibTest.32.asm(83) : error A2005:symbol redefinition : mCur
mFibonacciByMacro(7): Macro Called From
FibTest.32.asm(83): Main Line Code
0000003B 1 .code
0000003B 51 1 push ecx
0000003C 52 1 push edx
0000003D B9 0000000D 1 mov ecx,13
00000042 B8 00000001 1 mov eax,1
00000047 C7 05 0000018C R 1 mov mPre,0
00000000
00000051 C7 05 00000190 R 1 mov mCur,0
00000000
1 mL:
FibTest.32.asm(83) : error A2005:symbol redefinition : mL
mFibonacciByMacro(17): Macro Called From
FibTest.32.asm(83): Main Line Code
0000005B 03 05 0000018C R 1 add eax,mPre
00000061 8B 15 00000190 R 1 mov edx, mCur
00000067 89 15 0000018C R 1 mov mPre, edx
0000006D A3 00000190 R 1 mov mCur, eax
00000072 E2 AC 1 loop mL
00000074 5A 1 pop edx
00000075 59 1 pop ecx
To rescue, let's turn on this:
LOCAL mPre, mCur, mL
Again, running mFibonacciByMacro twice with 12 and 13, fine this time, we have:
mFibonacciByMacro 12
0000018C 1 .data
0000018C 00000000 1 ??0000 DWORD ?
00000190 00000000 1 ??0001 DWORD ?
00000000 1 .code
00000000 51 1 push ecx
00000001 52 1 push edx
00000002 B9 0000000C 1 mov ecx,12
00000007 B8 00000001 1 mov eax,1
0000000C C7 05 0000018C R 1 mov ??0000,0
00000000
00000016 C7 05 00000190 R 1 mov ??0001,0
00000000
00000020 1 ??0002:
00000020 03 05 0000018C R 1 add eax,??0000
00000026 8B 15 00000190 R 1 mov edx, ??0001
0000002C 89 15 0000018C R 1 mov ??0000, edx
00000032 A3 00000190 R 1 mov ??0001, eax
00000037 E2 E7 1 loop ??0002
00000039 5A 1 pop edx
0000003A 59 1 pop ecx
mFibonacciByMacro 13
00000194 1 .data
00000194 00000000 1 ??0003 DWORD ?
00000198 00000000 1 ??0004 DWORD ?
0000003B 1 .code
0000003B 51 1 push ecx
0000003C 52 1 push edx
0000003D B9 0000000D 1 mov ecx,13
00000042 B8 00000001 1 mov eax,1
00000047 C7 05 00000194 R 1 mov ??0003,0
00000000
00000051 C7 05 00000198 R 1 mov ??0004,0
00000000
0000005B 1 ??0005:
0000005B 03 05 00000194 R 1 add eax,??0003
00000061 8B 15 00000198 R 1 mov edx, ??0004
00000067 89 15 00000194 R 1 mov ??0003, edx
0000006D A3 00000198 R 1 mov ??0004, eax
00000072 E2 E7 1 loop ??0005
00000074 5A 1 pop edx
00000075 59 1 pop ecx
Now the label names, mPre, mCur, and mL, are not visible. Instead, running the first of mFibonacciByMacro 12, the preprocessor generates three system labels ??0000, ??0001, and ??0002 for mPre, mCur, and mL. And for the second mFibonacciByMacro 13, we can find another three system generated labels ??0003, ??0004, and ??0005 for mPre, mCur, and mL. In this way, MASM resolves the redefinition issue in multiple macro executions. You must declare your labels with the LOCAL directive in a macro.
However, by the name LOCAL, the directive sounds misleading, because the system generated ??0000, ??0001, etc. are not limited to a macro's context. They are really global in scope. To verify, I purposely initialize mPre and mCur as 2 and 3:
LOCAL mPre, mCur, mL
.data
mPre DWORD 2
mCur DWORD 3
Then simply try to retrieve the values from ??0000 and ??0001 even before calling two mFibonacciByMacro in code
mov esi, ??0000
mov edi, ??0001
mFibonacciByMacro 12
mFibonacciByMacro 13
To your surprise probably, when set a breakpoint, you can enter &??0000 into the VS debug Address box as a normal variable. As we can see here, the ??0000 memory address is 0x0116518C with DWORD values 2, 3, and so on. Such a ??0000 is allocated on the data segment together with other properly named variables, as shown string ASCII beside:
To summarize, the LOCAL directive declared in a macro is to prevent data/code labels from being globally redefined.
Further, as an interesting test question, think of the following multiple running of mFibonacciByMacro which is working fine without need of a LOCAL directive in mFibonacciByMacro. Why?
mov ecx, 2
L1:
mFibonacciByMacro 12
loop L1
Calling an assembly procedure in C/C++ and vice versa
Most assembly programming courses should mention an interesting topic of mixed language programming, e.g., how C/C++ code calls an assembly procedure and how assembly code calls a C/C++ function. But probably, not too much would be involved, especially for manual stack frame manipulation and name decoration. Here in first two sections, I'll give a simple example of C/C++ code calling an assembly procedure. I'll show C and STD calling conventions, using procedures either with advanced parameter lists or directly dealing with stack frame and name mangling.
The logic just calculates x-y, like 10-3 to show 7 resulted:
int someFunction(int x, int y)
{
return x-y;
}
cout << "Call someFunction: 10-3 = " << someFunction(10, 3) << endl;
When calling an assembly procedure from a C/C++ function, both must be consistent to use the same calling and naming conventions, so that a linker can resolve references to the caller and its callee. As for Visual C/C++ functions, C calling convention can be designated by the keyword __cdecl that should be default in a C/C++ module. And STD calling convention can be designated by __stdcall. While on the assembly language side, MASM also provides reserved words C and stdcall correspondingly. In an assembly language module, you can simply use the .model directive to declare all procedures follow C calling convention like this:
.model flat, C
But you also can override this global declaration by indicating an individual procedure as a different calling convention like:
ProcSTD_CallWithParameterList PROC stdcall, x:DWORD, y:DWORD
The following sections suppose that you have basic knowledge and understanding about above.
35. Using C calling convention in two ways
Let's first see a high level procedure with a parameter list easily from the following. I purposely leave blank for the calling convention attribute field in the .model directive, but I have PROC C to define it as C calling convention:
.386P
.model flat
.code
ProcC_CallWithParameterList PROC C, x:DWORD, y:DWORD
mov eax, x
sub eax, y
ret
ProcC_CallWithParameterList endp
The procedure ProcC_CallWithParameterList simply does subtraction x-y and returns the difference in EAX. In order to call it from a function in a .CPP file, I must have an equivalent C prototype declared in the .CPP file accordingly, where __cdecl is default:
extern "C" int ProcC_CallWithParameterList(int, int);
Then call it in main() like
cout << "C-Call With Parameters: 10-3 = " << ProcC_CallWithParameterList(10, 3) << endl;
Using the language attribute C to declare ProcC_CallWithParameterList makes a lot hidden behind the scene. Please recall what happens to the C calling convention __cdecl. The main point I want show here is
| Convention | | Implementation required |
| Argument passing | | From right to left |
| Stack maintenance | | Caller pops arguments from the stack |
| Name decoration | | Underscore character (_) prefixed to the function name |
Based on these specifications, I can manually create this procedure to fit C calling convention:
_ProcC_CallWithStackFrame PROC near
push ebp
mov ebp,esp
mov eax,[ebp+8]
sub eax,[ebp+12]
pop ebp
ret
_ProcC_CallWithStackFrame endp
As seen here, an underscore is prepended as _ProcC_CallWithStackFrame and two arguments x and y passed in reverse order with the stack frame looks like this:
Now let's verify that two procedures work exactly the same by C++ calls
extern "C" {
int ProcC_CallWithParameterList(int, int);
int ProcC_CallWithStackFrame(int, int);
}
int main()
{
cout << "C-Call With Parameters: 10-3 = " << ProcC_CallWithParameterList(10, 3) << endl;
cout << "C-Call With Stack Frame: 10-3 = " << ProcC_CallWithStackFrame(10, 3) << endl;
}
36. Using STD calling convention in two ways
Now we can take a look at STD call in the similar way. The following is simply a parameter list procedure with the language attribute stdcall defined for PROC:
ProcSTD_CallWithParameterList PROC stdcall, x:DWORD, y:DWORD
mov eax, x
sub eax, y
ret
ProcSTD_CallWithParameterList endp
Except for the calling conventions, no difference between ProcSTD_CallWithParameterList and ProcC_CallWithParameterList. In order to call ProcSTD_CallWithParameterList from a C function, the prototype should be like this:
extern "C" int __stdcall ProcSTD_CallWithParameterList(int, int);
Notice that __stdcall is a must to declare this time. Likewise, using stdcall to declare ProcSTD_CallWithParameterList also hides a lot details. Please recall what happens to the STD calling convention __stdcall. The main point to talk is
| Convention | | Implementation required |
| Argument passing | | From right to left |
| Stack maintenance | | Called function itself pops arguments from the stack |
| Name decoration | | Underscore character (_) prefixed to the function name. The name is followed by the at sign (@) and the byte count in decimal of the argument list |
Based on these specifications, I can manually create this procedure to fit STD calling convention.
_ProcSTD_CallWithStackFrame@8 PROC near
push ebp
mov ebp,esp
mov eax,[ebp+8]
sub eax,[ebp+12]
pop ebp
ret 8
_ProcSTD_CallWithStackFrame@8 endp
Although the stack frame is the same with two arguments x and y passed in reverse order, one difference is _ProcSTD_CallWithStackFrame@8 suffixed by the number eight, 8 bytes of two int type arguments. Another is ret 8 that is for this procedure itself to release the stack argument memory.
Now put all together, we can verify four procedures getting called by C++ with the same results:
extern "C" {
int ProcC_CallWithParameterList(int, int);
int ProcC_CallWithStackFrame(int, int);
int __stdcall ProcSTD_CallWithParameterList(int, int);
int __stdcall ProcSTD_CallWithStackFrame(int, int);
}
int main()
{
cout << "C-Call With Parameters: 10-3 = " << ProcC_CallWithParameterList(10, 3) << endl;
cout << "C-Call With Stack Frame: 10-3 = " << ProcC_CallWithStackFrame(10, 3) << endl;
cout << "STD-Call With Parameters: 10-3 = " << ProcSTD_CallWithParameterList(10, 3) << endl;
cout << "STD-Call With Stack Frame: 10-3 = " << ProcSTD_CallWithStackFrame(10, 3) << endl;
}
37. Calling cin/cout in an assembly procedure
This section will answer an opposite question, how to call C/C++ functions from an assembly procedure. We really need such a technique to make use of ready-made high level language subroutines for I/O, floating point data, and math function processing. Here I simply want to perform a subtraction task in an assembly procedure, together with input and output by calling cin and cout like this:
I use C calling convention for both calls and in order to do this, let's make three C prototypes:
extern "C" {
int ReadFromConsole(unsigned char);
void DisplayToConsole(char*, int);
void DoSubtraction();
}
It's trivial defining first two functions to be called in DoSubtraction, while DoSubtraction is supposed to call in main():
int ReadFromConsole(unsigned char by)
{
cout << "Enter " << by <<": ";
int i;
cin >> i;
return i;
}
void DisplayToConsole(char* s, int n)
{
cout << s << n <<endl <<endl;
}
int main()
{
DoSubtraction();
}
Now is time to implement the assembly procedure DoSubtraction. Since DoSubtraction will call two C++ functions for I/O, I have to make their equivalent prototypes acceptable and recognized by DoSubtraction:
ReadFromConsole PROTO C, by:BYTE
DisplayToConsole PROTO C, s:PTR BYTE, n:DWORD
Next, simply fill the logic to make it work by invoking ReadFromConsole and DisplayToConsole:
DoSubtraction PROC C
.data
text2Disp BYTE 'X-Y =', 0
diff DWORD ?
.code
INVOKE ReadFromConsole, 'X'
mov diff, eax
INVOKE ReadFromConsole, 'Y'
sub diff, eax
INVOKE DisplayToConsole, OFFSET text2Disp, diff
ret
DoSubtraction endp
Finally, all source code in above three sections is available for download at CallingAsmProcInC, with main.cpp, subProcs.asm, and VS project.
About ADDR operator
In 32-bit mode, the INVOKE, PROC, and PROTO directives provide powerful ways for defining and calling procedures. Along with these directives, the ADDR operator is an essential helper for defining procedure parameters. By using INVOKE, you can make a procedure call almost the same as a function call in high-level programming languages, without caring about the underlying mechanism of the runtime stack.
Unfortunately, the ADDR operator is not well explained or documented. The MASM simply said it as an address expression (an expression preceded by ADDR). The textbook [1], mentioned a little more here:
The ADDR operator, also available in 32-bit mode, can be used to pass a pointer argument when calling a procedure using INVOKE. The following INVOKE statement, for example, passes the address of myArray to the FillArray procedure:
INVOKE FillArray, ADDR myArray
The argument passed to ADDR must be an assembly time constant. The following is an error:
INVOKE mySub, ADDR [ebp+12]
The ADDR operator can only be used in conjunction with INVOKE. The following is an error:
mov esi, ADDR myArray
All these sound fine, but are not very clear or accurate, and even not conceptually understandable in programming. ADDR not only can be used at assembly time with a global variable like myArray to replace OFFSET, it also can be placed before a stack memory, such as a local variable or a procedure parameter. The following is actually possible without causing an assembly error:
INVOKE mySub, ADDR [ebp+12]
Don't do this, just because unnecessary and somewhat meaningless. The INVOKE directive automatically generates the prologue and epilogue code for you with EBP and pushes arguments in the format of EBP offset. The following sections show you how smart is the ADDR operator, with different interpretations at assembly time and at runtime.
38. With global variables defined in data segment
Let's first create a procedure to perform subtraction C=A-B, with all three address parameters (call-by-reference). Obviously, we have to use indirect operand ESI and dereference it to receive two values from parA and parB. The out parameter parC saves the result back to the caller:
SubWithADDR PROC, parA:PTR BYTE, parB:PTR BYTE, parC:PTR BYTE
mov esi, parA
mov al, [esi]
mov esi, parB
sub al, [esi]
mov esi, parC
mov [esi], al
ret
SubWithADDR ENDP
And define three global variables in the DATA segment:
.data
valA BYTE 7
valB BYTE 3
valC BYTE 0
Then directly pass these global variables to SubWithADDR with ADDR as three addresses:
INVOKE SubWithADDR, ADDR valA, ADDR valB, ADDR valC
mov bl, valC
Now let's generate the code Listing by use the option "Listing All Available Information" as below:
The Listing simply shows three ADDR operators replaced by OFFSET:
INVOKE SubWithADDR, ADDR valA, ADDR valB, ADDR valC
0000005B 68 00000002 R * push OFFSET valC
00000060 68 00000001 R * push OFFSET valB
00000065 68 00000000 R * push OFFSET valA
0000006A E8 FFFFFF91 * call SubWithADDR
0000006F 8A 1D 00000002 R mov bl, valC
This is logically reasonable, since valA, valB, and valC are created statically at assembly time and the OFFSET operator must be applied at assembly time accordingly. In such a case, where we can use ADDR, we also can use OFFSET instead. Let's try
INVOKE SubWithADDR, ADDR valA, OFFSET valB, OFFSET valC
and regenerate the Listing here to see actually no essential differences:
INVOKE SubWithADDR, ADDR valA, OFFSET valB, OFFSET valC
0000005B 68 00000002 R * push dword ptr OFFSET FLAT: valC
00000060 68 00000001 R * push dword ptr OFFSET FLAT: valB
00000065 68 00000000 R * push OFFSET valA
0000006A E8 FFFFFF91 * call SubWithADDR
0000006F 8A 1D 00000002 R mov bl, valC
39. With local variables created in a procedure
In order to test ADDR applied to a local variable, we have to create another procedure where three local variables are defined:
WithLocalVariable PROC
LOCAL locA, locB, locC: BYTE
mov locA, 8
mov locB, 2
INVOKE SubWithADDR, ADDR locA, ADDR locB, ADDR locC
mov cl, locC
ret
WithLocalVariable ENDP
Notice that locA, locB, and locC are the memory of BYTE type. To reuse SubWithADDR by INVOKE, I need to prepare values like 8 and 2 to the input arguments locA and locB, and let locC to get back the result. I have to apply ADDR to three of them to satisfy the calling interface of SubWithADDR prototype. Now simply do the second test:
call WithLocalVariable
At this moment, the local variables are created on the stack frame. This is the memory dynamically created at runtime. Obviously, the assembly time operator OFFSET cannot be assumed by ADDR. As you might think, the instruction LEA should be coming on duty (LEA mentioned already: 11. Implementing with plus (+) instead of ADD and 21. Making a clear calling interface).
Wow exactly, the operator ADDR is now cleaver enough to choose LEA this time. To be readable, I want to avoid using Listing to see 2s complement offset to EBP. Instead, check the Disassembly intuitive display at runtime here. The code shows three ADDR operators replaced by three LEA instructions, working with EBP on the stack as follows:
43: WithLocalVariable PROC
00401046 55 push ebp
00401047 8B EC mov ebp,esp
00401049 83 C4 F4 add esp,0FFFFFFF4h
44: LOCAL locA, locB, locC: BYTE
45:
46:
47:
48:
49:
50:
51: mov locA, 8
0040104C C7 45 FC 08 00 00 00 mov dword ptr [ebp-4],8
52: mov locB, 2
00401053 C7 45 F8 02 00 00 00 mov dword ptr [ebp-8],2
53: INVOKE SubWithADDR, ADDR locA, ADDR locB, ADDR locC
0040105A 8D 45 F7 lea eax,[ebp-9]
0040105D 50 push eax
0040105E 8D 45 F8 lea eax,[ebp-8]
00401061 50 push eax
00401062 8D 45 FC lea eax,[ebp-4]
00401065 50 push eax
00401066 E8 C5 FF FF FF call 00401030
54: mov cl, locC
0040106B 8A 4D F7 mov cl,byte ptr [ebp-9]
55: ret
0040106E C9 leave
0040106F C3 ret
56: WithLocalVariable ENDP
where the hexadecimal 00401030 is SubWithADDR's address. Because of the LOCAL directive, MASM automatically generates the prologue and epilogue with EBP representations. To view EBP offset instead of variable names like locA, locB, and locC, just uncheck the Option: Show symbol names:
40. With arguments received from within a procedure
The third test is to make ADDR apply to arguments. I create a procedure WithArgumentPassed and call it like:
INVOKE WithArgumentPassed, 9, 1, OFFSET valC
Reuse the global valC here with OFFSET, since I hope to get the result 8 back. It's interesting to see how to push three values in the Listing:
INVOKE WithArgumentPassed, 9, 1, OFFSET valC
0000007B 68 00000002 R * push dword ptr OFFSET FLAT: valC
00000080 6A 01 * push +000000001h
00000082 6A 09 * push +000000009h
00000084 E8 FFFFFFB7 * call WithArgumentPassed
The implementation of WithArgumentPassed is quite straight and simply reuse SubWithADDR by passing arguments argA and argB prefixed with ADDR to be addresses, while ptrC already a pointer without ADDR:
WithArgumentPassed PROC argA: BYTE, argB: BYTE, ptrC: PTR BYTE
INVOKE SubWithADDR, ADDR argA, ADDR argB, ptrC
mov esi, ptrC
mov dl, [esi]
ret
WithArgumentPassed ENDP
If you are familiar with the concepts of stack frame, imagine the behavior of ADDR that must be very similar to the local variables, since arguments are also dynamically created memory on the stack at runtime. The following is the generated Listing with two ADDR operators replaced by LEA. Only difference is the positive offset to EBP here:
00000040 WithArgumentPassed PROC argA: BYTE, argB: BYTE, ptrC: PTR BYTE
00000040 55 * push ebp
00000041 8B EC * mov ebp, esp
INVOKE SubWithADDR, ADDR argA, ADDR argB, ptrC
00000043 FF 75 10 * push dword ptr ss:[ebp]+000000010h
00000046 8D 45 0C * lea eax, byte ptr ss:[ebp]+00Ch
00000049 50 * push eax
0000004A 8D 45 08 * lea eax, byte ptr ss:[ebp]+008h
0000004D 50 * push eax
0000004E E8 FFFFFFAD * call SubWithADDR
00000053 8B 75 10 mov esi, ptrC
00000056 8A 16 mov dl, [esi]
ret
00000058 C9 * leave
00000059 C2 000C * ret 0000Ch
0000005C WithArgumentPassed ENDP
Because of WithArgumentPassed PROC with a parameter-list, MASM also generates the prologue and epilogue with EBP representations automatically. Three address arguments pushed in the reverse order are EBP plus 16 (ptrC), plus 12 (argB), and plus 8 (argA).
Finally, all source code in above three sections available to download at TestADDR, with TestADDR.asm, TestADDR.lst, and TestADDR.vcxproj.
Summary
I talked so much about miscellaneous features in assembly language programming. Most of them are from our class teaching and assignment discussion [1]. The basic practices are presented here with short code snippets for better understanding without irrelevant details involved. The main purpose is to show assembly language specific ideas and methods with more strength than other languages.
As noticed, I haven’t given a complete test code that requires a programming environment with input and output. For an easy try, you can go [2] to download the Irvine32 library and setup your MASM programming environment with Visual Studio, while you have to learn a lot in advance to prepare yourself first. For example, the statement exit mentioned here in main is not an element in assembly language, but is defined as INVOKE ExitProcess,0 there.
Assembly language is notable for its one-to-one correspondence between an instruction and its machine code as shown in several Listings here. Via assembly code, you can get closer to the heart of the machine, such as registers and memory. Assembly language programming often plays an important role in both academic study and industry development. I hope this article could serve as an useful reference for students and professionals as well.
References
History
- January 28, 2019 -- Added: About ADDR operator, three sections
- January 22, 2017 -- Added: Calling an assembly procedure in C/C++ and vice versa, three sections
- January 11, 2017 -- Added: FOR/WHILE loop and Making loop more efficient, two sections
- December 20, 2016 -- Added: Ambiguous "LOCAL" directive, two sections
- November 28, 2016 -- Added: Signed and Unsigned, four sections
- October 30, 2016 -- Added: About PTR operator, two sections
- October 16, 2016 -- Added: Little-endian, two sections
- October 11, 2016 -- Added: the section, Using INC to avoid PUSHFD and POPFD
- October 2, 2016 -- Added: the section, Using atomic instructions
- August 1, 2016 -- Original version posted
