Category : Assembly Language Source Code
Archive   : ASM_SUBR.ZIP
Filename : MBINDIV

;-------------------------mbindiv routine begins--------------------------+
; from BLUEBOOK OF ASSEMBLY ROUTINES FOR IBM PC & XT.
; page : 122
;
; NAME MBINDIV
;
; ROUTINE FOR Multidigit Binary DIVISION
;
; FUNCTION: This routine divides two multidigit binary numbers, producing
; both a quotient and remainder.
;
; INPUT: Upon entry DS:SI points to the divisor; DS:BX points to where
; the dividend is upon entry and where the remainder will be upon exit,
; and DS:DI points to the location where the quotient will be upon exit.
; The size of these multidigit numbers is controlled by the constant
; ISIZE. The divisor and quotient contain 16*ISIZE number of
; bits and the dividend and remainder have double that precision.
; Both the divisor and the quotient are stored in ISIZE number of 16-bit
; words of memory and the dividend and remainder are stored in 2*ISIZE
; number of 16-bit words of memory.
;
; OUTPUT: Upon exit DS:BX points to where the quotient is stored and
; DS:DI points to where the remainder is stored.
;
; REGISTERS USED: No registers are modified.
;
; SEGMENTS REFERENCED: Upon entry the data segment must contain
; storage for the multidigit numbers described above.
;
; ROUTINES CALLED: None
;
; SPECIAL NOTES: None
;
; LOCAL SUBROUTINE TO COMPARE DIVISOR AGAINST DIVIDEND
;
divcmp proc near
;
push si ; save registers
push di
push cx
;
std ; backward direction
add si,4*isize-2 ; point to end of temp divisor
add di,4*isize-2 ; point to end of quotient
mov cx,2*isize ; count for double precision
;
repz cmpsw ; compare 'digit' by 'digit'
;
pop cx ; restore registers
pop di
poop si
ret ; return from divcmp
;
; LOCAL SUBROUTINE FOR ARITHMETIC SHIFT DIVISOR LEFT
;
divsal proc near
;
push si ; save registers
push cx
;
mov cx,2*isize ; set counter
clc ; clear carry in
;
divsal1:
rcl word ptr [si],1 ; shift one word by one bit
inc si ; point to next word
inc si
loop divsal1 ; loop through entire divisor
;
pop cx ; restore registers
pop si
ret ; return from divsal
;
divsal endp
;
; LOCAL SUBROUTINE FOR LOGICAL SHIFT DIVISOR RIGHT
;
divslr proc near
;
push si ; save registers
push cx
;
add si,4*isize-2 ; poin to end of temp divisor
mov cx,2*isize ; count for double precision
clc ; clear carry in
;
divslr1:
rcr word ptr [si],1 ; shift one word by one bit
inc si ; point to next word
inc si
loop divslr1 ; loop through entire divisor
;
pop cx ; restore registers
pop si
ret ; return from divslr
;
divslr endp
;
; LOCAL SUBROUTINE TO SUBTRACT SHIFTED DIVISOR FROM DIVIDEND
;
divsub proc near
;
push si ; save registers
push di
push cx
;
clc ; clear carry in
mov cx,2*isize ; set count fop double precision
;
divsub1:
mov ax,[si] ; get word from shifted divisor
inc si ; point to next word
inc si
sbb [di],ax ; subtract from word of dividend
inc di ; point to next word
inc di
loop divsub1 ; loop through all words
;
pop cx
pop di
pop si
;
ret ; return from divsub
;
divsub endp
;
; LOCAL SUBROUTINE TO SHIFT QUOTIENT LEFT
;
quotshl proc near
;
push bx ; save registers
push cx
;
mov cx,isize ; count for single precision
;
quotshl1:
rcl word ptr [bx],1 ; shift word of quotient left once
inc bx ; point to next word
inc bx
loop quotshl1 ; loop through entire quotient
;
pop cx ; restore registers
pop bx
ret ; retirn from quotshl
;
quotshl endp
;
; ROUTINE TO DIVIDE MULTIDIGIT BINARY NUMBERS
;
mbindiv proc far
;
push si ; save registers
push di
push bx
push cx
push ax
;
; put single precision divisor into double precision location
push di ; save dividend pointer
lea di,tempdiv ; point to temporary divisor
mov cx,isize ; for a count of isize
cld ; forward direction
rep movsw ; make the transfer
;
; clear upper part of double precision location
mov ax,0 ; zero word
mov cx,isize ; for a count of isize
rep stosw ; clear the rest of the words
;
; restore dividend pointer and point to temp divisor
pop di ; restore dividend pointer
lea si,tempdiv ; point SI to temporary divisor
;
; initialize shift count
mov cx,1 ; initial count of one
;
; normalize divisor
mbindiv1:
test msbdiv,8000h ; test msb of divisor
jnz mbindiv2 ; exit if normalized
call divsal ; arithmetic shift if not
inc cx ; count the shift
jmp mbindiv1 ; keep on looping until normalized
;
; compare, subtract, shift loop
mbindiv2:
call divcmp ; compare divisor against dividend
ja mbindiv3 ; skip if too large
call divsub ; subtract if OK
stc ; new bit of quotient is 1
jmp mbindiv4 ; jump to end of loop
;
mbindiv3:
clc ; new bit of quotient is 0
;
mbindiv4:
call quotshl ; shift bit into the quotient
call divslr ; logical shift divisor right once
loop mbindiv2 ; loop for next digit
;
pop ax ; restore registers
pop cx
pop bx
pop di
pop si
;
mbindiv endp
;-------------------------mbindiv routine ends---------------------------