Question

It’s my second day I’m learning NASM and assembly language at all, so I’ve decided to write a kind of a calculator. The problem is that when user enters the operation, the program doesn’t compare it.I mean compares but it doesn’t consider that strings are equal when they are. I’ve googled a lot, but no results. What might be my broblem? Here’s source

operV   resb    255  ; this is declaration of variable later used to store the input, in .bss of course

mov rax, 0x2000003         ;here user enters the operation, input is "+", or "-"
mov rdi, 0
mov rsi, operV
mov rdx, 255
syscall

mov rax, operV             ; here is part where stuff is compared
mov rdi, "+"         
cmp rax, rdi
je add

mov rdi, "-"
cmp rax, rdi
je substr
;etc...

Answer 1

You also press the enter key when you submit information like that. change rdi to “+”, 10 on linux, or “+”, 11 on windows

The answer came to me in a dream last night.

operV resd 4; Resd because registers are a dword in size, and we want the comparison to be as seamless ass possible, you can leave this as 255, but thats a little excessive as we only need 4

mov rax, 0x2000003
mov rdi, 0
mov rsi. operV
mov rdx, 4 ;You only need 4 bytes (1 dword) as this is all we are accepting (+ and line end, then empty space so it matches up with the register we are comparing too) you can leave this as 255 too, but again, we only need 4
syscall

mov rax, operV
mov rdi, "x", 10
cmp dword[rax], rdi ; You were comparing the pointer in rax with rdi, not the content pointed to by rax with rdi. now we are comparing the double word (4 bytes) at the location pointed too by rax (operV) and comparing them. Instead of comparing the location of operV with the thing you want.
je add

That’s the changes done 😛