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So I tried to do a program that does the modulo. I couldn't do so.
I tried using idiv or div but I just couldn't think off a way to do so. I will really appreciate if you can show me how to perform the modulo operation.

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1 Answers1

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Modulo operator is % or \ in most assemblers. The modulo operation represents the remainder after dividend is integer-divided by divisor.

For instance when dividend is 13 and divisor is 5, then the modulo result is 13 % 5 = 3. Unsigned division is provided in x86 by the instruction DIV and it depends on CPU mode:

; 8bit, works with dividend 0 .. (2^16 - 1)      64 KiB
MOV AX, 13  ; dividend
MOV CL,  5  ; divisor
DIV CL      ; Remainder 3 is now in AH, quotient 2 is in AL.

; 16bit, works with dividend 0 .. (2^32 - 1)      4 GiB
MOV AX, 13  ; lower 16 bits of dividend
MOV DX,  0  ; higher 16 bits of dividend
MOV CX,  5  ; divisor
DIV CX      ; Remainder 3 is now in DX, quotient 2 is in AX.

; 32bit, works with dividend 0 .. (2^64 - 1)     16 EiB
MOV EAX, 13  ; lower 32 bits of dividend
MOV EDX,  0  ; higher 32 bits of dividend
MOV ECX,  5  ; divisor
DIV ECX      ; Remainder 3 is now in EDX, quotient 2 is in EAX.

; 64bit, works with dividend 0 .. (2^128 - 1)   256 ???
MOV RAX, 13  ; lower 64 bits of dividend
MOV RDX,  0  ; higher 64 bits of dividend
MOV RCX,  5  ; divisor
DIV RCX      ; Remainder 3 is now in RDX, quotient 2 is in RAX.

It is rather more complicated with signed numbers using signed division IDIV, see the article Modulo on Wikipedia.

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    Just to be clear, `idiv` gives you the signed remainder just as easily, with `cdq` / `idiv` for 32-bit operand size for example (*[When and why do we sign extend and use cdq with mul/div?](https://stackoverflow.com/q/36464879)*). But if you want modulo in the sense of Euclidean or floored division, not truncating (quotient rounds toward zero) where `-1 % 2 == -1`, then you'd want more instructions. – Peter Cordes Dec 10 '22 at 18:33
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    Following up on @PeterCordes comment, remainder is zero or has the same sign as dividend. Mathematical modulo is zero or has the same sign as the divisor. – rcgldr Dec 10 '22 at 18:59
  • Right, but a lot of people misuse the terminology and are fine with remainder. It's worth at least mentioning that remainder is easy, because a lot of people would search on "modulo" for an operation like the C `%` operator. (Of course, people should just ask a compiler if it's something C can do directly. https://godbolt.org/z/YKhvT3jGn – Peter Cordes Dec 10 '22 at 19:11
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    Just a tip, if you have to do modulo 2 (or some power of 2), just AND with that power of 2 minus 1. So for example, `n % 2` is the same as `n & 1`, `n % 4` is the same as `n & 3`, etc. Not only do you get the same result with `and`, it's also much faster. Unfortunately it only works with powers of 2. – puppydrum64 Dec 12 '22 at 17:32