[Coco] ASM Coders - please help!

[David Roper]

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

and

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

CLRSCRN ldy HSCLEN
        ldx HRESSCRN
CLRLOOP clr ,X+
        leay -1,Y
        bne CLRLOOP
ENDLOOP jmp ENDLOOP
   
HRESSCRN equ $2000
HSCLEN   equ $2000

         end START

So!

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,
David