Hello all. Sorry I've been away for a while.

Recently, I've been doing a lot of programming on the C64 and I've run across a section that I want to optimize in my program.

My initial thoughts tell me I should be using some lookup table but I'm struggling with the implementation.

Let's say, for example, that I have the following:



$10 and $11 contain the address of the C64's SCREEN RAM. And, for the sake of example, the Y register contains an offset to some column. In other words, that code above would plot the character #240 roughly in the middle of the screen.

Now, what I want to do next is add 9 more locations (total of 10) that would be something like 16, 40, 56, 80, 96, etc. All 8-bit numbers.

It's easy enough to do something like this:



That certainly works. But looks messy. Especially since I need to do it 10 times per frame.

I hope I'm making sense.

So, any tips would be appreciated.

Thanks!!

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Cat; the other white meat.

For a start, you can replace LDA #0 : ADC $11 : STA $11 with BCC *+4 : INC $11 which is both smaller and faster, though the cycle counts might be less predictable since they are now data dependent.

If all of the offsets are within 255 bytes of the base, then you could use the Y register for that instead of addition, and add the value of the Y register to the base just once. Then you only need LDY# instead of addition sequences.

Unless I'm missing something obvious, can't you do something like this?



then use something like



to use it?

Hmm. Let me see if I can try some of those suggestions.

Yeah, the INC is definitely better than the ADC #00 because I'm not counting cycles here.
I'll try that (Offsets, X) too.

Thanks!!

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Cat; the other white meat.

I think this can all be done simply with a table.

LDX #0
LOOP EQU *
LDY COLUMNS,X
LDA VALUES,X
STA ($10),Y
INX
CPX #?? ; # of values in table
BNE LOOP

COLUMNS DFB #16,#40,#56,#80,#96
VALUES DFB #240, ... ??