singrdk/base/Kernel/Native/ix86/_ulldvrm.asm

;*******************************************************************************
;ulldvrm.asm - unsigned long divide and remainder routine
;
;   Copyright (c) Microsoft Corporation. All rights reserved.
;
;Purpose:
;   defines the unsigned long divide and remainder routine
;       __aulldvrm
;
;Revision History:
;   10-06-98  SMK   Initial version.
;
;*******************************************************************************

include hal.inc

;***
;ulldvrm - unsigned long divide and remainder
;
;Purpose:
;   Does a unsigned long divide and remainder of the arguments.  Arguments
;       are not changed.
;
;Entry:
;   Arguments are passed on the stack:
;       1st pushed: divisor (QWORD)
;       2nd pushed: dividend (QWORD)
;
;Exit:
;   EDX:EAX contains the quotient (dividend/divisor)
;       EBX:ECX contains the remainder (divided % divisor)
;   NOTE: this routine removes the parameters from the stack.
;
;Uses:
;   ECX
;
;Exceptions:
;
;*******************************************************************************

__aulldvrm PROC NEAR

    push    esi

; Set up the local stack and save the index registers.  When this is done
; the stack frame will look as follows (assuming that the expression a/b will
; generate a call to aulldvrm(a, b)):
;
;       -----------------
;       |       |
;       |---------------|
;       |       |
;       |--divisor (b)--|
;       |       |
;       |---------------|
;       |       |
;       |--dividend (a)-|
;       |       |
;       |---------------|
;       | return addr** |
;       |---------------|
;   ESP---->|      ESI  |
;       -----------------
;

DVND    equ [esp + 8]   ; stack address of dividend (a)
DVSR    equ [esp + 16]  ; stack address of divisor (b)

;
; Now do the divide.  First look to see if the divisor is less than 4194304K.
; If so, then we can use a simple algorithm with word divides, otherwise
; things get a little more complex.
;

    mov eax,HIWORD(DVSR) ; check to see if divisor < 4194304K
    or  eax,eax
    jnz short L1    ; nope, gotta do this the hard way
    mov ecx,LOWORD(DVSR) ; load divisor
    mov eax,HIWORD(DVND) ; load high word of dividend
    xor edx,edx
    div ecx     ; get high order bits of quotient
    mov ebx,eax     ; save high bits of quotient
    mov eax,LOWORD(DVND) ; edx:eax <- remainder:lo word of dividend
    div ecx     ; get low order bits of quotient
    mov esi,eax     ; ebx:esi <- quotient

;
; Now we need to do a multiply so that we can compute the remainder.
;
    mov     eax,ebx         ; set up high word of quotient
    mul dword ptr LOWORD(DVSR) ; HIWORD(QUOT) * DVSR
    mov ecx,eax         ; save the result in ecx
    mov     eax,esi         ; set up low word of quotient
    mul dword ptr LOWORD(DVSR) ; LOWORD(QUOT) * DVSR
    add edx,ecx     ; EDX:EAX = QUOT * DVSR
    jmp short L2    ; complete remainder calculation

;
; Here we do it the hard way.  Remember, eax contains DVSRHI
;

L1:
    mov ecx,eax     ; ecx:ebx <- divisor
    mov ebx,LOWORD(DVSR)
    mov edx,HIWORD(DVND) ; edx:eax <- dividend
    mov eax,LOWORD(DVND)
L3:
    shr ecx,1       ; shift divisor right one bit; hi bit <- 0
    rcr ebx,1
    shr edx,1       ; shift dividend right one bit; hi bit <- 0
    rcr eax,1
    or  ecx,ecx
    jnz short L3    ; loop until divisor < 4194304K
    div ebx     ; now divide, ignore remainder
    mov esi,eax     ; save quotient

;
; We may be off by one, so to check, we will multiply the quotient
; by the divisor and check the result against the orignal dividend
; Note that we must also check for overflow, which can occur if the
; dividend is close to 2**64 and the quotient is off by 1.
;

    mul dword ptr HIWORD(DVSR) ; QUOT * HIWORD(DVSR)
    mov ecx,eax
    mov eax,LOWORD(DVSR)
    mul esi     ; QUOT * LOWORD(DVSR)
    add edx,ecx     ; EDX:EAX = QUOT * DVSR
    jc  short L4    ; carry means Quotient is off by 1

;
; do long compare here between original dividend and the result of the
; multiply in edx:eax.  If original is larger or equal, we are ok, otherwise
; subtract one (1) from the quotient.
;

    cmp edx,HIWORD(DVND) ; compare hi words of result and original
    ja  short L4    ; if result > original, do subtract
    jb  short L5    ; if result < original, we are ok
    cmp eax,LOWORD(DVND) ; hi words are equal, compare lo words
    jbe short L5    ; if less or equal we are ok, else subtract
L4:
    dec esi     ; subtract 1 from quotient
    sub eax,LOWORD(DVSR) ; subtract divisor from result
    sbb edx,HIWORD(DVSR)
L5:
    xor ebx,ebx     ; ebx:esi <- quotient

L2:
;
; Calculate remainder by subtracting the result from the original dividend.
; Since the result is already in a register, we will do the subtract in the
; opposite direction and negate the result.
;

    sub eax,LOWORD(DVND) ; subtract dividend from result
    sbb edx,HIWORD(DVND)
    neg edx     ; otherwise, negate the result
    neg eax
    sbb edx,0

;
; Now we need to get the quotient into edx:eax and the remainder into ebx:ecx.
;
    mov ecx,edx
    mov     edx,ebx
    mov     ebx,ecx
    mov     ecx,eax
    mov     eax,esi
;
; Just the cleanup left to do.  edx:eax contains the quotient.
; Restore the saved registers and return.
;

    pop esi

    ret 16

__aulldvrm ENDP

    end