Z80 Routines:Math:Division

Introduction

All these routines use the restoring division algorithm, adapted to the z80 architecture to maximize speed. They can easily be unrolled to gain some speed.

8/8 division

The following routine divides d by e and places the quotient in d and the remainder in a

div_d_e:
   xor	a
   ld	b, 8

_loop:
   sla	d
   rla
   cp	e
   jr	c, $+4
   sub	e
   inc	d
   
   djnz	_loop
   
   ret
 

16/8 division

The following routine divides hl by c and places the quotient in hl and the remainder in a

div_hl_c:
   xor	a
   ld	b, 16

_loop:
   add	hl, hl
   rla
   jr	c, $+5
   cp	c
   jr	c, $+4

   sub	c
   inc	l
   
   djnz	_loop
   
   ret
 

16/16 division

The following routine divides ac by de and places the quotient in ac and the remainder in hl

div_ac_de:
   ld	hl, 0
   ld	b, 16

_loop:
   sll	c
   rla
   adc	hl, hl
   sbc	hl, de
   jr	nc, $+4
   add	hl, de
   dec	c
   
   djnz	_loop
   
   ret
 

24/8 division

The following routine divides ehl by d and places the quotient in ehl and the remainder in a

div_ehl_d:
   xor	a
   ld	b, 24

_loop:
   add	hl, hl
   rl	e
   rla
   jr	c, $+5
   cp	d
   jr	c, $+4

   sub	d
   inc	l
   
   djnz	_loop
   
   ret
 

32/8 division

The following routine divides dehl by c and places the quotient in dehl and the remainder in a

div_dehl_c:
   xor	a
   ld	b, 32

_loop:
   add	hl, hl
   rl	e
   rl	d
   rla
   jr	c, $+5
   cp	c
   jr	c, $+4

   sub	c
   inc	l
   
   djnz	_loop
   
   ret
 

32/16 division

The following routine divides acix by de and places the quotient in acix and the remainder in hl

Div32By16:
; IN:	ACIX=dividend, DE=divisor
; OUT:	ACIX=quotient, DE=divisor, HL=remainder, B=0
	ld	hl,0
	ld	b,32
Div32By16_Loop:
	add	ix,ix
	rl	c
	rla
	adc	hl,hl
	jr	c,Div32By16_Overflow
	sbc	hl,de
	jr	nc,Div32By16_SetBit
	add	hl,de
	djnz	Div32By16_Loop
	ret
Div32By16_Overflow:
	or	a
	sbc	hl,de
Div32By16_SetBit:
	.db	$DD,$2C		; inc ixl, change to inc ix to avoid undocumented
	djnz	Div32By16_Loop
	ret
 

Rounded 16/8 division

The following routine divides hl by c and places the rounded quotient in hl and twice the prerounded remainder in a.

RoundHL_Div_C:
   xor	a
   ld	b, 16

_loop:
   add	hl, hl
   rla
   cp	c
   jr	c, $+4
   sub	c
   inc	l   
   djnz	_loop
;This part is the rounding
   add a,a
   cp	c
   ret	c
   inc hl
   ret