Both stored procedures are part of the system and you can not change them...
sp_readerrorlog call xp_readerrorlog
create proc sys.sp_readerrorlog(
@p1 int = 0,
@p2 int = NULL,
@p3 nvarchar(4000) = NULL,
@p4 nvarchar(4000) = NULL)
as
begin
if (not is_srvrolemember(N'securityadmin') = 1)
begin
raiserror(15003,-1,-1, N'securityadmin')
return (1)
end
if (@p2 is NULL)
exec sys.xp_readerrorlog @p1
else
exec sys.xp_readerrorlog @p1,@p2,@p3,@p4
end
xp_readerrorlog is not a real stored procedure and sits inside the xpstar.dll, and called by a specific engine created for extended stored procedures...
Also notice that both procedures are NON-DOCUMENTED, so you can use them only at your own risk and Microsoft will not help you out with them...