Situation such: there is an instruction

it is compiled in following bytes

Under the documentation "IA32" it is a command

byte 3С it ModR/M ([--][--]), it indicate that is SIB byte follow, and SIB byte indicate by table 2-3:
[EAX] and r32 = EBX

It follows from this that the instruction should look so:

Can help to understand it?

fasm syntax is made to be simple.

exit the strange adressing representations from early ages of computing science.

then:

cmp byte[eax+ebx],0

the result of the compilation is only fucntion of useN

if use16, then, will be prefixed for 32 bits interpretation under 16 bit mode
if use32, then, will be like you found, and if use64, then, will be about SIB.

Yes, I should specify at once that at me use32

I not absolutely understand as to use it SIB in byte. For example if instead of 03 would be С6, this command would look so?



is this correct?

This is correct.

Download OllyDbg (www.ollydbg.de), load any EXE file into it and start playing with the opcodes: just press any key (while the code window has focus) and type DB 80, 3C, 0C6, 00. Press enter to disassemble it and see the result.