singrdk/base/boot/Singldr/uldiv.asm

    page    ,132
    title   uldiv - unsigned long divide routine
;***
;uldiv.asm - unsigned long divide routine
;
;   Copyright (c) Microsoft Corporation.  All rights reserved.
;
;Purpose:
;   defines the unsigned long divide routine
;   the following routines are created:
;       __aFuldiv   (large, medium models)
;       __aNuldiv   (small, compact models)
;
;*******************************************************************************

.xlist
include ulhelp.inc
.list

sBegin  code
    assumes cs,code
    assumes ds,data

page
;***
;uldiv - unsigned long divide
;
;Purpose:
;   Does a unsigned long divide of the arguments.  Arguments are
;   not changed.
;
;Entry:
;   Arguments are passed on the stack:
;       1st pushed: divisor (DWORD)
;       2nd pushed: dividend (DWORD)
;
;Exit:
;   DX:AX contains the quotient (dividend/divisor)
;   NOTE: this routine removes the parameters from the stack.
;
;Uses:
;   CX
;
;Exceptions:
;
;*******************************************************************************

    ASGN    uldiv

    push    bx
    push    si

; 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 uldiv(a, b)):
;
;       -----------------
;       |       |
;       |---------------|
;       |       |
;       |--divisor (b)--|
;       |       |
;       |---------------|
;       |       |
;       |--dividend (a)-|
;       |       |
;       |---------------|
;       | return addr** |
;       |---------------|
;   BP----->|    old BP |
;       |---------------|
;       |      BX   |
;       |---------------|
;   SP----->|      SI   |
;       -----------------
;
; ** - 2 bytes if small/compact model; 4 bytes if medium/large model

DVND    equ BPARGBAS[bp]    ; stack address of dividend (a)
DVSR    equ BPARGBAS+4[bp]  ; stack address of divisor (b)

;
; Now do the divide.  First look to see if the divisor is less than 64K.
; If so, then we can use a simple algorithm with word divides, otherwise
; things get a little more complex.
;

    mov ax,HIWORD(DVSR) ; check to see if divisor < 64K
    or  ax,ax
    jnz L1      ; nope, gotta do this the hard way
    mov cx,LOWORD(DVSR) ; load divisor
    mov ax,HIWORD(DVND) ; load high word of dividend
    xor dx,dx
    div cx      ; get high order bits of quotient
    mov bx,ax       ; save high bits of quotient
    mov ax,LOWORD(DVND) ; dx:ax <- remainder:lo word of dividend
    div cx      ; get low order bits of quotient
    mov dx,bx       ; dx:ax <- quotient hi:quotient lo
    jmp short L2    ; restore stack and return

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

L1:
    mov cx,ax       ; cx:bx <- divisor
    mov bx,LOWORD(DVSR)
    mov dx,HIWORD(DVND) ; dx:ax <- dividend
    mov ax,LOWORD(DVND)
L3:
    shr cx,1        ; shift divisor right one bit; hi bit <- 0
    rcr bx,1
    shr dx,1        ; shift dividend right one bit; hi bit <- 0
    rcr ax,1
    or  cx,cx
    jnz L3      ; loop until divisor < 64K
    div bx      ; now divide, ignore remainder
    mov si,ax       ; 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**32 and the quotient is off by 1.
;

    mul word ptr HIWORD(DVSR) ; QUOT * HIWORD(DVSR)
    xchg    cx,ax       ; "mov cx,ax" but only 1 byte
    mov ax,LOWORD(DVSR)
    mul si      ; QUOT * LOWORD(DVSR)
    add dx,cx       ; DX:AX = QUOT * DVSR
    jc  L4      ; carry means Quotient is off by 1

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

    cmp dx,HIWORD(DVND) ; compare hi words of result and original
    ja  L4      ; if result > original, do subtract
    jb  L5      ; if result < original, we are ok
    cmp ax,LOWORD(DVND) ; hi words are equal, compare lo words
    jbe L5      ; if less or equal we are ok, else subtract
L4:
    dec si      ; subtract 1 from quotient
L5:
    xor dx,dx       ; dx:ax <- quotient
    xchg    ax,si       ; "mov ax,si" but only 1 byte

;
; Just the cleanup left to do.  dx:ax contains the quotient.
; Restore the saved registers and return.
;

L2:

    pop si
    pop bx

cEnd    <nolocals>
return  8

sEnd

    end