[Coco] ASM Coders - please help!

David Roper dave at ebonhost.com
Tue Aug 29 03:57:09 EDT 2006

William Astle wrote:
> David Roper wrote:
>>          orcc #$50
>>        lda #$30 first block of graphics screen at $60000
>>        sta $FFA1 mapped to $2000
>>        andcc #$AF
> Since someone else already corrected the X/Y loading problem, I'll 
> comment on the above:
> The CC operations are actually spurious and not needed. You only need 
> to disable interrupts if you are switching the stack and/or the 
> interrupt handlers out of the logical address space. Since you're 
> moving only $2000-$3FFF, you're not going to be touching either unless 
> someone has loaded something odd before you run your code.
> Also, note that you are reenabling interrupts immediately after 
> swapping the memory block. That's rather pointless as it turns the two 
> CC operations into essentially NOPs because there is no possible 
> danger to switching the MMU with interrupts enabled except as noted in 
> my previous paragraph.
> Also, from an "optimizing the initialization code" point of view, I'd 
> probably use U for the index register and X for the counter. Saves 1 
> byte of code. (LDU opcode is 1 byte, LDY opcode is 2). Of course, 
> optimizing that case is rather pointless. Of course, if you were using 
> U for something else before, that would be a different story. :)
Hi William,

Thankyou for your comments - this sort of information is invaluable, and 
almost impossible to find in books!

Do you mind if I fire other questions towards you when and if they arise?

Once I've had a chance to rework my code, I'll post an updated copy, 
just for the record :P

Once again, thanks William - and Robert too!

Kind regards,

More information about the Coco mailing list