>LF805: DEY ;14> BPL LF805 ;20>
NOP ;2 ;loop totals 36 cycles>> STA
RESP0 ;P0 at pixel 108?> STA RESP1 ;P1 at pixel
117?> LDA #$F0 ;shift P0 one posn to the right (109?)
>;//>;If the TIA only processes RESP0 every 15 pixels as has been
stated,>;then the above numbers would not make sense to me. P0 would end
up >at>;121, and P1 would be 120... ???>;//I'm pretty sure the TIA
processes a write to RESP0 as soon as it gets it (when the instruction
completes), but it can take up to five cycles (fifteen pixels) to write
the register. A standard STA RESP0 takes three cycles, but STA allows
indexed indirect addressing, which takes five. In this case, the players
end up at pixels 109 and 117 as stated in your comments. >LF825: LDY
$93 ; (61) +3 ;current line> LDA ($F9),Y
; p0 p1 (64) +5 ;indirect indexed> STA GRP0 ;
F9 (69) +3 > STA WSYNC ; Cycle count begins>
LDA ($F7),Y ; (0) +5 > STA GRP1 ;
F7 (5) +3> LDA ($F5),Y ; (8) +5>
STA GRP0 ; F5 (13) +3> LDA ($F3),Y ;
(16) +5> STA $94 ; (21) +3>
LDA ($F1),Y ; (24) +5> TAX ;
(29) +2 ;preload X> LDA ($EF),Y ;
(31) +5> TAY ; (36) +2 ;and Y>
LDA $94 ; (38) +3 ;and A> STA GRP1
; F3 (41) +3 ;reload GRP at>pixel
;123?> STX
GRP0 ; F1 (44) +3> STY GRP1 ; EF
(47) +3 ;reload GRP at>pixel
;141?> STA
GRP0 ; F3 (50) +3> DEC $93 ;
(53) +5> BPL LF825 ; Branch taken, so (58) +3> ;
End six-digit loopIs this right: $EF => ones digit, $F1 => tens digit,
$F3 => hundreds digit, ... , $F9 => hundred-thousands digit? And the
digits begin at pixels 109, 117, 125, 133, 141, and 149 (cycles 36, 39,
41, 44, 47, and 49.) Writes to registers take effect when the
instruction completes, so your "Reload GRP1 at 123" actually executes at
132, in time for the hundreds digit... the next STX GRP0 finishes at 141,
just in time for the tens digit, and your "Reload GRP1 at 141" takes
effect at 150, apparently one pixel too late, but my guess is that the
leftmost bit of the displayed byte is always 0 (to provide at least a
one-pixel gap between digits) so it still works.I see what's going on
here, especially the tricky STA $94 and then loading it again because we
need to use A to load the other bytes because LDX and LDY don't permit
indexed indirect addressing. I don't quite understand what the last STA
GRP0 is for, though (at cycle (50)+3 ) .. it completes at cycle 53 =
pixel 159, which is after the score is done displaying on that line. I
also don't see how the STA GRP0 at cycle (69)+3 helps either.. it looks
like its preloading GRP0 for the hundred-thousands digit on the next
scanline (its executing before WSYNC), but GRP0 gets rewritten with the
thousands digit ($F5) at cycle (13)+3, pixel 48 of the next line.. that's
within horizontal blank, before the hundred-thousands digit gets
displayed.I'm pretty sure the last digit of Defender's score is always
0.. it doesn't look like it's hardcoded to display 0, though. I forget
at the moment if it displays a hundred-thousands digit (the code would
seem to indicate it doesn't), and I don't feel like playing the game that
long just to see =) Other games like Missile Command and Millipede
display six unrestricted digits, though, so it's gotta be possible.One
more thing - you list BPL as taking three cycles if the branch is taken..
my 6502 reference says conditional branches take 4 cycles, though...? PC
Atari's debugger adds nine pixels for a taken branch, so I guess it is
three, but I'd like to know for sure.>;//>;Wow. The above loop just drew
the score with no cycles to spare.>;//That says it all =) I'll try
disassembling a couple other games to see if I can figure anything else
out.
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