Re: A85: 8bit division routine

To: assembly-85@lists.ticalc.org
Subject: Re: A85: 8bit division routine
From: Humberto Yeverino <humberto@engr.csufresno.edu>
Date: Sun, 29 Nov 1998 23:47:45 -0800 (PST)
Delivered-To: assembly-85-outgoing@towerguard.unix.edu.sollentuna.se
In-Reply-To: <Pine.A41.3.96.981129194622.76324B-100000@acs2.acs.ucalgary.ca>
Reply-To: assembly-85@lists.ticalc.org


On Sun, 29 Nov 1998, Erik Huizing wrote:
> has anyone got a small 8 bit division routine?
> or something that's hard-coded to divide by 3
> 
An 8-bit division routine:

;E = A/D
;A = A%D

Div8:
  ld e,a
  cp 0
  ret z
  ld a,e	;Check for divide by zero
  ld e,0
  ld b,1
Div8Loop1:
  bit 7,d
  jr z,Div8Set	;if bit 7 = 1 then Ready 
  inc b
  sla d
  jr Div8Loop1	;shift D until bit 7 is 1, keep track of shifts in B
Div8Set:
  cp d
  jr c,Div8NoSub
  sub d
Div8NoSub:
  ccf
  rr e
  srl d
  djnz Div8Set
  ret

Hard Coded to use only 3.. Same except for setup.. which is a lot smaller

;E = A/3
;A = A%3

DivBy3:
  ld e,0
  ld d,%11000000
  ld b,7
DivBy3Set:
  cp d
  jr c,NoSub
  sub d
NoSub:
  ccf
  rr e
  srl d
  djnz DivBy3Set
  ret

AS if anyone cares, here's how it works:  Check if D =0, if you don't,
you'll get an infinite loop.  After that it works the same as long
division in binary, you find the answer starting with the most signifcant
bit which is determined by a comparison ..

         00110010
         ________
00000011|10011000
         11000000  Too Big -> Bit 6=0
        - 1100000  Smaller -> Bit 5=1 and do a subtraction
        =  111000
        -  110000  Smaller -> Bit 4=1 and do a subtraction
        =    1000
            11000  Too Big -> Bit 3=0
             1100  Too Big -> Bit 2=0
        -     110  Smaller -> Bit 1=1 and do a subtraction
        =      10
               11  Too Big -> Bit 0=0

So 10011000 / 11 = 110010 and the remainder is 10

Anybody read this far?
If you did I'll be posting a demo of my game (barely playable and ugly) on
the tyrant web page, as a backup to make it easy to look at then throw
away.  Have fun.

-Humberto Yeverino Jr.


Official Tyrant Home Page:                              
  http://www.engr.csufresno.edu/~humberto/tyrant.html

References:

A85: 8bit division routine

From: Erik Huizing <ehuizing@ucalgary.ca>