Re: In pursuit of Dead Wild Cat

[Thomas Harte]

I'm using a variant on that page's square table method for my 10 bit  
multiplies (though I always do (a + b)^2 - (a - b)^2), and something a  
lot like his divide. But my divide is a lot messier. Specifically (in  
pyz80 syntax):

;
;       FIXDIV, works out BC / DE and stores it to HL (all 8.8 fixed point)
;
;       clobbers: a
;
FIXDIV:
        ; check signs
        ld a, b
        and a
        jp p, @+check2

        ; bc is negative
        ld a, d
        and a
        jp p, @+divnegpos

        ; if we get to here then both are negative. Easy...
        ld hl, 0
        sbc hl, de
        ex de, hl
        ld hl, 0
        and a
        sbc hl, bc
        ld b, h
        ld c, l
        jr @+divpospos

@check2:
        ; bc is positive
        ld a, d
        and a
        jp m, @+divposneg       ;jump if de is negative too

@divpospos:
        ld hl, 0

@DIVLOOP1: EQU FOR 8
        sla b
        rl l                    ; lb <<= 1

	sbc hl, de		; find hl - de (nb: carry is definitely clear, so sbc is  
equivalent to sub)
	jr nc, @+noadd		; if hl was larger than de then obviously it was  
wrong as it's the remainder

        add hl, de              ; oh, hl was smaller - so add de back in again
@noadd:
        NEXT @DIVLOOP1

				; get to here and b is the lowest byte of the answer, which we  
just throw away
dtpos:
@DIVLOOP2: EQU FOR 8
        sl1 c
        adc hl, hl              ; hlc = (hlc << 1) | 1

        sbc hl, de              ; find hl - de (carry definitely clear)
	jr nc, @+noadd		; if hl was is larger than de then obviously it was  
wrong as it's the remainder

        add hl, de              ; oh, hl was smaller than de - so add de back 
in again
        dec c                   ; remove bottom bit of c
@noadd:
        NEXT @DIVLOOP2

                                ; c is now the middle byte of the answer ...

@DIVLOOP3: EQU FOR 8
        sl1 b

	add hl, hl		; double answer - NB carry from sl1 b ignored, so it's as  
though b = 0 at start of loop
	and a			; need to clear carry - add hl, hl may overflow from here on  
out
        sbc hl, de              ; find hl - de (sub hl, de would be better here)
	jr nc, @+noadd		; if hl was is larger than de then obviously it was  
wrong as it's the remainder

	add hl, de		; oh, hl was smaller than de - so add de back in again  
and remove bottom bit of c
        dec b                   ; remove bottom bit of c
@noadd:
        NEXT @DIVLOOP3

                                ; ... and b is low byte of the answer

        ld h, c                 ; put b and c into hl, but the other way round
        ld l, b
dlend:  RET

@divnegpos:
        ; negate bc
        ld hl, 0
        and a
        sbc hl, bc
        ld b, h
        ld c, l
        call @-divpospos
        ex de, hl
        ld hl, 0
        and a
        sbc hl, de
        ret

@divposneg:
        ; negate de
        ld hl, 0
        and a
        sbc hl, de
        ex hl, de
        call @-divpospos
        ex de, hl
        ld hl, 0
        and a
        sbc hl, de
        ret


On 19 May 2008, at 08:00, ellvis wrote:

Hi,

I am not really a coder, but you can always try to look at http://baze.au.com/misc/z80bits.html 
 It's a collection of z80 math routines, all commented, it's done by  
Baze/3SC - Spectrum demo coder.

Just for an inspiration. Anyway, it's really great to read such a  
stuff here on the list, pretty inspirative!

ellvis

Chris Pile wrote:
Re: code, I really don't think that cycle pinching is the main  
problem  — I think it's algorithmic. I'm going to need to think  
about that a  bit more. At the minute I'm a bit indoctrinated as  
to how I think a 3d  pipeline should go, and I may need to rethink  
that.
Sometimes you have to take the unconventional route when optimising  
for "old-skool"
hardware!  Textbook methods are fine if you have the horsepower -  
if not, then you're
going to have to look at pulling out all the tricks you can.  :-)

--
ellvis/ZeroTeam


web: http://zeroteam.sk
e-mail: [EMAIL PROTECTED]

Sinclair ZX Spectrum support since 1996