> > > LDY #$FF ;2
> > > LDX #0 ;2
> > >B1 STX 0,Y ;2
> > > DEY ;1
> > > BNE B1 ;2
> >
> > You could change the first two lines of this to LDX #0 and TXY to save one
> > byte. It'll still work.
>
> Yes it will, but not with exactly the same result; the original doesn't
> clear address 0 (it clears 255 bytes of zero page). Your method will clear
> address 0, clearing all of zero page, which is more correct anyway :)
Well, not really more correct. Address 0 would be clearing VSYNC. This
doesn't need to be done during initialization b/c it is done later. I
wouldn't do it in a normal program, because it could confuse the TV
alittle to get 2 VSYNC's in such a short time. But as Erik points out,
here it can be used to save a byte, which is the goal.
> I don't have the original email handy, but I hope somewhere there the stack
> pointer was setup!!
No it was not. There was no point in doing so, as that requires more
code, and the stack is never used in the code.(no jsr's, etc)
> I dug up my earlier posting regarding an efficient startup.
> Here's my best...
>
>
> ; CLEARS ALL VARIABLES, STACK
> ; INIT STACK POINTER
> ; ALSO CLEARS TIA REGISTERS
> ; DOES THIS BY "WRAPPING" THE STACK - UNUSUAL
>
> LDX #0
> TXS
> PHA ; BEST WAY TO GET SP=$FF, X=0
Wow! that's interesting, I never would have thought of that. And I guess
it doesn't really matter that the stack starts out 'incorrect' because you
just do that first 'push' onto the VSYNC, which doesn't really matter...
> TXA
> CLEAR PHA
> DEX
> BNE CLEAR
>
> ; 9 BYTES TOTAL FOR CLEARING STACK, MEMORY
> ; STACK POINTER NOW $FF, A=X==0
Wow, and because PHA is only one byte, it is more efficient than doing the
index addressing STA/STX as I used. novel use of the push operation...
Here, I don't need the stack, so I should be able to remove that. Your
code manages to setup the stack as well in the same 9 bytes I used to just
clear the TIA.
How about this, assumeing that you don't need the stack:
LDY #0 ;2
TYA ;1 ;a-la Erik's suggestion,
clear
PHA ;1 ;a-la Andrew's suggestion,
DEY ;1
BNE clear ;2
Oh, boy, that's good. Down to 7 bytes. Quite an improvement from my 9.
And I think this should work because, although I have not setup the stack
pointer, it will loop all the way around, and to all 256 bytes, is this
correct?
Mark
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