[Coco] ASM Coders - Round two!

[William Astle]

David Roper wrote:
> a) To achieve 32k of contiguous memory (whether the physical ram is
> contiguous or not is irrelevent, as long as it's mapped to be
> contiguous), where can you recommend I map this to?

Once you've disabled interrupts, you should be able to use any mapping
you like. You might want to take a memory map like follows:

$0000-$7FFF: Hires screen
$8000-$9FFF: heat effects
$A000-$FDFF: available for other uses

To make this work, you will need to load your code somewhere, say $6000,
then play a few games with the MMU to move it out of the way of your new
memory map. I'll take a stab at how to do this off the cuff (Warning:
Mailer Code (tm)):

	ORG $6000
START	ORCC #$50
	LDA #$3B
	STA $FFA5
	JMP LAUNCH+$4000
LAUNCH	LDD #$3031
	STD $FFA0
	LDD #$3233
	STD $FFA2
	* now initialize the graphics screen, etc.
	END START

Theoretically, you can then do LOADM:EXEC in sequence and the program
will launch. (You can make it autolaunch with LOADM but that's an
exercise for another day. You can even make the whole thing launch from
a floppy with the DOS command.)

What that code does is as follows:

1. disable interrupts
2. puts the block normally at $6000 into place at $A000 (it will still
be at $6000)
3. jumps to the next instruction, but in the instance of the block at $A000
4. puts the hires screen blocks in starting at $0000 in the logical space

Why it should work:

The absolute origin means we know where we are in the memory space and
the rest of the code assumes the standard BASIC memory mapping. We know
that the offset from the base address of the block is the same no matter
where in memory the block resides. That means if we are at $600A and we
have a mirror of the block starting at $A000, the value at $A00A is the
same as at $600A as far as the CPU is concerned. Neat trick, eh?

Why put the screen at $0000?

This way you never need to add an offset after calculating the pixel
byte location and it should save you a few cycles if you are plotting
pixels, etc., rather a lot.

Additional notes:

You can then choose any RAM block other than $30,$31,$32,$33,$3B for the
heat data and put it at $8000 by storing the number at $FFA4. You might
use, say, $3A (the block ordinarily at $4000-$5FFF), for example.

As you can see, the MMU can make your brain explode as you try to work
out just what is going on.

> 
> b) can i simply disable interrupts and leave them disabled for the
> entire code, without side effects?

You should be able to since you don't need the system to do anything
else while your demo is running.

-- 
William Astle
finger lost at l-w.net for further information

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