Isolating remainder in nasm program

问题

I'm currently writing a NASM program on x86 Linux and I'm trying to perform a calculation that divides the first command line arg (a year) by the first leap year check. I want to check if the remainder is 0 or not but I'm struggling with how to do that. I know the div command stores the answer in a certain register and a remainder in another but right now I'm just using test. Here's the code

        global  main
        extern  puts
        extern  printf
        extern  atoi

        section  .text

main:
        sub     rsp, 8
        cmp     rdi, 2
        jne     error1              ; jump if aguments != 1
        mov     rdi, [rsi+8]
        call    atoi
        test    rdi, fourTest
        jnz     notLeapYear
        jmp     done
testTwo:
        jmp     done

notLeapYear:
        mov     edi, nLeap
        call    puts
        jmp     done
error1:
        mov     edi, badArgs
        call    puts
        jmp     done  

done:
        add     rsp, 8
        ret

badArgs:
        db      "Requires exactly one argument", 5, 0

nLeap:
        db      "Not a leap year", 5, 0

        section  .data
fourTest:      dq       4
hundTest:      dq       100
fHundTest:     dq       400

I believe I need to change the test rdi, fourTest to using div but don't know how to isolate the remainder and determine if I should jump to the next test or if I should jump to is not a leap year.

回答1:

First, return values from functions go into eax, so that's where the atoi result will be. Then use div like this:

xor edx,edx
div 4

Now the result of the division will be in eax, and the remainder will be in edx.

来源：https://stackoverflow.com/questions/36901647/isolating-remainder-in-nasm-program