[Coco] ASM Coders - please help!

[David Roper]

Robert Gault wrote:

> 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.

>

Thanks, Robert!

So the code I should be using (given that I'm using EQU's) is

ldy #HSCLEN
ldx #HRESSCRN

is that right?

Yes, I'm aware that it's not enough to clear the entire screen - if it's 
enough to get -something- plotting, I'm happy!

Apart from that, does everything else seem ok?

Kind regards,
David