;-------------------------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---------------------------