on 2/17/01 8:24 AM, Eckhard Stolberg at Eckhard_Stolberg@xxxxxx wrote:
>> No, when the instruction completes is what you want. That way, you can
>> extend
>> the STA GRP0 instruction itself by a cycle (using absolute rather than
>> zero
>> page addressing) and have it do what you want. Maybe that's what sta.w
>> GRP0
>> does. Is it?
>
> Yes, it is. ".w" forces a word sized operand for an instruction.
Okay, I'm going to have to plead ignorance; could you explain what that
means in a bit more detail? I guess I'm ignorant to why the magic of zero
page works... though I'm going to hazard a guess and you can tell me if/how
I'm wrong.
Since zero page only includes the first $FF addresses, it can dispense with
bothering with the first byte of the address (in a two byte addressing the
Atari uses). This implicit first byte is essentially $00, but not having to
explicitly deal with it saves the processor a cycle. When you want
something from an address like $FF00, there's no way to use the shortcut.
By using sta.w, you force grabbing the address with two bytes (a word?
Sounds more like a snack-sized Snicker bar, but that's harder to say), eg
making $A1 (just as an example) into "$00A1" and forcing the processor to
use up the cycle to deal with $00, so to speak.
That didn't make much sense the way I wrote it, but that sounds right. Am I
close?
Thanks,
Ruffin Bailey
-
Archives (includes files) at http://www.biglist.com/lists/stella/archives/
Unsub & more at http://www.biglist.com/lists/stella/
