[Coco] ASM Coders - please help!

Gene Heskett gene.heskett at verizon.net
Tue Aug 29 05:52:29 EDT 2006

On Monday 28 August 2006 22:36, David Roper wrote:
>Hi there,
>I've been trying to figure out how to perform a "simple" task or two
>using 6809 on a 512k Coco3 - I want to avoid making bios calls if
>possible, simply because later down the track I'll need every ounce of
>speed I can get out of this thing (and specific-code is always faster
>than generalised code)
>I'm trying to
>a) Set the equivalent of HSCREEN 2
>b) Clear (some of) the graphics screen - in the example below, 8K's
>worth (so I can visually see what's been done)
>Eventually I'd like to plot a pixel, but that's an extension of b)
>This is the code I've got so far (in Rainbow IDE format) cobbled
>together from various sources (coco3.com forum, coco3 secrets revealed,
> etc)
>        org $0300
>START   lda #%01001100 Coco3, MMU on, DRAM constant, standard SCS 16K
>internal 16K external ROM
>        sta $FF90
>        lda #%10000000 graphics
>        sta $FF98
>        lda #%00111110 200 lines of 320H by 16 colors
>        sta $FF99
>        clr $FF9C clear vertical scroll
>        ldd #$C000 screen start divided by 8
>        std $FF9D vertical offset
>        clr $FF9F clear horizontal scroll
>        orcc #$50
>        lda #$30 first block of graphics screen at $60000
>        sta $FFA1 mapped to $2000
>        andcc #$AF
>        ldx HRESSCRN
>CLRLOOP clr ,X+
>        leay -1,Y
>        bne CLRLOOP
>HRESSCRN equ $2000
>HSCLEN   equ $2000

Humm, maybe this s/b HSCLEN equ $1FFF?  You may be going one byte too far, 
but I haven't actually ran this in my mind either.
>         end START
>What I end up seeing (in MESS, at least) is a screen of alternating
>vertical black and green lines.. and not much else
>I'm missing something, or messed something up, or generally on the wrong
>track *somewhere*, and I'm sure someone out there will have much more
>luck explaing where, and why I've gone wrong!
>Any takers to place me under their wing?
>Kind regards,

Cheers, Gene
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Copyright 2006 by Maurice Eugene Heskett, all rights reserved.

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