Apple II display is 65 cycles per row
65*192=12480(screen)+ 4550 more for vblank = 17030 cycles
17030/1023000 = 60fps
Theory maximum screen update
completely unrolled 192*40*4 cycles = 30720 cycles
30720 / 1023000 = 33 fps
17030 cycles
; Total of 12480 cycles to draw screen
; Vertical blank = 4550 cycles (70 scan lines)
; Total of 17030 cycles to get back to where was
to fill the hi-res screen with a single color you need to update 7680
bytes. If you completely unroll the loop this takes 30720 cycles, or
roughly 33fps.
This plasma algorithm if drawing the whole screen (192 lines) manages
only 3fps. The video here I am skipping half the lines which gives
5.5fps. I have a version drawing every 4th line which still looks OK
that can do 9fps.
The code is already heavily optimized with lots of self modifying code
and table lookups. I'm losing some cycles to doing page flipping, and
part of the problem is the Apple II's bizzarre framebuffer layout. I
also waste cycles because I'm doing table lookups to get the dithered
patterns, and again due to Apple II oddness (3.5 colored pixels per
byte) I need separate lookups for odd/even columns
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