CODEWITHSUNDEEP
assemblyx86masmmasm32
anaya
anaya

Reputation: 13

MASM giving incorrect results upon entering negative decimals

So, I am writing a program that divides two negative integers and stores the quotient and remainder, but it displays incorrect results. Here is my code:

    mov eax, input1
    mov edx, 0             ; dividend high half = 0.  prefer  xor edx,edx
    mov ebx, input2         ; divisor can be any register or memory
    div ebx
    mov remainder, edx       
    mov edx, offset prompt
    call Writestring
    mov quotient, eax 
    call WriteDec
    mov edx, offset prompt1
    call Writestring
    mov edx, remainder
    call WriteDec
    call Crlf

Upvotes: 1

Views: 193

Answers (1)

Nate Eldredge
Nate Eldredge

Reputation: 58518

div is used for unsigned division. For signed division you want idiv. This takes a signed 64-bit dividend in edx:eax, so you don't want edx to be zero if the dividend is negative; rather, you want to sign extend eax into edx. This can be done with cdq.

So I think you want

    mov eax, input1
    cdq 
    mov ebx, input2
    idiv ebx
    mov remainder, edx  
    mov quotient, eax
    ...

A couple other remarks:

  • Where does WriteDec expect to find the number to be written? Check its documentation. You don't seem to be putting the results in any consistent register.

  • You are waiting until after calling Writestring to save the quotient from eax into memory. Are you sure Writestring won't overwrite eax? Many calling conventions would allow it to.

  • Your program will crash with a divide error if input2 is 0, or if input1 is 0x80000000 (the most negative 32-bit integer) and input2 is -1. You may want to do some input checking if you don't already have it elsewhere.

  • As your comment says, idiv can take its operand from register or memory. So there is no need to mov ebx, input2 when you can simply do idiv input2. (Though you may also want to design your program so that input2 lives in a register all along and never needs to be stored in memory in the first place.)

Upvotes: 3

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