[Coco] ASM Coders - Round two!

[Robert Gault]

William Astle has given you the correct answer but I'll expand the 
explanation.

  You have chosen to move the hi-res graphics screen into your 64K 
address space so that it starts in the MMU block controlled by $FFA1; 
$2000 - $3FFF. That is perfectly OK but if you wish to address the 
entire screen in one contiguous block of memory, you need to consider 
exactly where it is going.
  I said in a previous message on this snip of code, that the screen is 
$7D00 bytes long; $60000 - $67D00. (The total assigned Coco3 space is 
$60000 - $68000.) You are moving it to start at $72000 and end at 
$72000+$7D00=$79D00. So where is that in your 64K address space? It 
starts in free ram but ends in "ROM" address space which on the Coco3 is 
normally RAM.
  Your counter is initialized to $3E80. So only memory from $2000 - 
$5E80 gets changed. This is also OK. The problem is that you have taken 
a previous suggestion that killing the IRQs was not needed and applied 
the suggestion to new code. Let's see what that means.
  Actually including LDA #$32, STA $FFA3 does nothing bad. That's 
because it changes memory at $6000-$7FFF. The only trouble will come 
when you include LDA #$33, STA $FFA4  because that will remove the first 
MMU block of Extended Basic code, $8000 - $9FFF. This will crash the 
Coco3 if the interrupts are still engaged.

  But that still does not address your question which implies that just 
changing *LDA #$32, *STA $FFA3  to  LDA #$32, STA $FFA3  causes a change 
  in what you see on the screen or possibly more trouble. That's because 
it won't do anything! You are either confused about what happens when 
you change $FFA3 or have not written your question accurately.
  In either case, there can be garbage in the lower half of the screen 
because you are clearing $3E80/$7D00 = 0.5 or half of the screen. Unless 
you have preset the graphics screen to a specific pattern, it will 
contain garbage which may or may not be random.

  So the full answer to clearing the complete 320x200x16 screen is your 
counter must start at $7D00. If you want the entire screen mapped into 
your address space, you must turn off the interrupts, swap the MMU 
blocks, send data to the blocks, restore the MMU blocks, and restart the 
IRQs.
  You don't have to map the entire screen at one time nor do you need to 
restart IRQs. But you must give more thought to the side effects of your 
code.