汇编x86中的模

Question

所以我试着做一个做模的程序。我不能这样做。
我尝试使用idiv或div，但我只是想不出一种方法来做到这一点。如果你能告诉我怎么做模运算，我将非常感激。

Answer 1

模算子%或在大多数汇编程序中。模运算表示余数在股息被除数整除.

如被除数为13，除数为5，则取模结果为13 % 5 = 3。无符号除法在x86中由指令DIV提供，它取决于CPU模式:

; 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.

使用有符号除法IDIV处理有符号数要复杂得多，参见维基百科上的Modulo文章。