[Coco] ASM Coders - please help!

[Robert Gault]

David Roper wrote:

><snip>

> 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

> 

> 

> 


Here's the problem. Note the ldx and ldy values which are equivalent to
    ldy  $2000
    ldx  $2000
That is not what you wanted or need. What you actually will get is 
unknown since the contents of $2000-$2001 will vary depending on what is 
present on the hi-res graphics screen.

You really need
    ldy  #$2000
    ldx  #$2000
because regX is a pointer to the start of the screen and regY is a counter.

And by the way, the value for regY is not large enough to clear the 
entire screen if that was intended. A 320x200x16 screen requires $7D00 
bytes.