AMD x86 manual Example 8 Remainder

22007E/0 — November 1999 AMD Athlon™ Processor x86 Code Optimization

MOV	ECX, EAX	;save quotient
IMUL	EDI, EAX	;quotient * divisor hi-word
		; (low only)

MUL	DWORD PTR [ESP+20];quotient * divisor lo-word
ADD	EDX, EDI		;EDX:EAX = quotient * divisor
SUB	EBX, EAX		;dividend_lo – (quot.*divisor)_lo
MOV	EAX, ECX		;get quotient
MOV	ECX, [ESP+16]		;dividend_hi
SBB	ECX, EDX	;subtract divisor *		quot. from dividend
SBB	EAX, 0	;adjust quotient if		remainder negative
XOR	EDX, EDX	;clear hi-word of quot(EAX<=FFFFFFFFh)
POP	EDI	;restore EDI as per		calling convention
POP	EBX	;restore EBX as per		calling convention
RET		;done, return to caller

_ulldiv ENDP

Example 8 (Remainder):

;_ullrem divides two unsigned 64-bit integers, and returns

;the remainder.

;INPUT: [ESP+8]:[ESP+4] dividend

;[ESP+16]:[ESP+12] divisor

;OUTPUT: EDX:EAX remainder of division

;

;DESTROYS: EAX,ECX,EDX,EFlags

_ullrem	PROC
PUSH	EBX	;save EBX as per calling convention
MOV	ECX, [ESP+20]	;divisor_hi
MOV	EBX, [ESP+16]	;divisor_lo
MOV	EDX, [ESP+12]	;dividend_hi
MOV	EAX, [ESP+8]	;dividend_lo
TEST	ECX, ECX	;divisor > 2^32–1?
JNZ	$r_big_divisor ;yes, divisor > 32^32–1
CMP	EDX, EBX	;only one division needed? (ECX = 0)
JAE	$r_two_divs	;need two divisions
DIV	EBX	;EAX = quotient_lo
MOV	EAX, EDX	;EAX = remainder_lo
MOV	EDX, ECX	;EDX = remainder_hi = 0
POP	EBX	;restore EBX as per calling convention
RET		;done, return to caller

Efficient 64-Bit Integer Arithmetic 89