Re: A86: ASM Programming Test: div 3

To: assembly-86@lists.ticalc.org
Subject: Re: A86: ASM Programming Test: div 3
From: rabidcow@juno.com (Joshua J Seagoe)
Date: Thu, 10 Dec 1998 16:48:31 -0800
Delivered-To: assembly-86-outgoing@towerguard.unix.edu.sollentuna.se
Reply-To: assembly-86@lists.ticalc.org


why are you dividing then?  use fixed point to multiply by .3333...
you can do 16 bit left shifts
256*.333=$00.56=%00000000.01010110 (rounded up)

;de=number
;h.l=de/3
 push de
 ld h,d
 ld l,e
 add hl,hl
 add hl,hl	;*$0004
 add hl,de	;*$0005
 ld d,h
 ld e,l
 add hl,hl
 add hl,hl
 add hl,hl
 add hl,hl	;*$0010
 add hl,de	;*$0055 
 pop de
 add hl,de	;*$0056

this will work up to $02fa, which gives 762/3=255.something instead of
254.whatever
l is the fractional part, h is the integer part of the answer

-josh

On Wed, 9 Dec 1998 11:43:59 -0600 "David Phillips" <electrum@tfs.net>
writes:
>
>I spent about a day trying to figure this out.  It's not possible.  
>To
>divide by shifting requires at least a 16 or 32 bit fixed-point 
>number, and
>it still comes out a little too bit too small (reference: "More Tricks 
>of
>the Game Programming Gurus").  For example, to divide x by 3 in C (the 
>">>"
>operator means shift right N places...same as the assembler uses):
>
>x_div_3 = (x >> 2) + (x >> 4) + (x >> 6) + (x >> 8)
>
>I ended up using a little subtraction loop to divide a number (code 
>cut from
>program):
>
> ld a,(MapY)    ; load y coord
> ld b,3      ; need to divide by 3
> ld c,0      ; it starts at 1, always add once
>DoRadarL:
> inc c      ; increase quotient
> sub b      ; subtract divisor
> jr nc,DoRadarL    ; loop until it's negative
>
>The dividend is in A, the divisor in B and C is the quotient.  It 
>just
>increments the quotient and decrements the dividend by the divisor 
>until the
>dividend is less than 0.  If you add the divisor to the dividend at 
>the end,
>then you will have the remainder as well.
>
>-----Original Message-----
>From: Dux Gregis <assets@eden.rutgers.edu>
>To: assembly-86@lists.ticalc.org <assembly-86@lists.ticalc.org>
>Date: Wednesday, December 02, 1998 3:26 PM
>Subject: Re: A86: ASM Programming Test
>
>
>>
>>How about for the challenge write a call that divides the accumulator 
>by 3?
>
>

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