Frantic reboot?!?
I was reviewing the source for Frantic this weekend and while I was going over the time-critical kernel loop:
KernelLoop: ; 70 lda #<DS_COLUP0 ; 2 72 sta WSYNC KernelLoopNoWSYNC: beq RepositionP0 ; 2 2 sta COLUP0 ; 3 5 - before 25 lda #<DS_COLUP1 ; 2 7 beq RepositionP1 ; 2 9 sta COLUP1 ; 3 12 - before 25 stx ENAM0 ; 3 15 - before 25 lda #<DS_GRP0 ; 2 17 sta GRP0 ; 3 20 - before 25 sty ENAM1 ; 3 23 - before 25 lda #<DS_PF2L ; 2 25 sta PF2 ; 3 28 - before 38 lda #<DS_GRP1 ; 2 30 - any sta GRP1 ; 3 33 - any sty PF0 ; 3 36 - PF0R, 28-49 lda #<DS_PF1R ; 2 38 sta PF1 ; 3 41 - PF1R, 39-54 lda #<AMPLITUDE ; 2 43 - any sta AUDV0 ; 3 46 - any lda #<DS_PF2R ; 2 48 sta PF2 ; 3 51 - PF2R, 50-65 ldy DS_PF0R_M1 ; 4 55 lda #<DS_PF1L ; 2 57 ldx DS_PF0L_M0 ; 4 61 - 0 triggers end-of-kernel stx PF0 ; 3 64 - PF0L, after 55 sta PF1 ; 3 67 - PF1L, 66 - 28 bne KernelLoop ; 3 70 - 2 69 not taken jmp KernelDone ; 3 72
I noticed a few things that made me go hmmm 
I did a bit of rewrite and came up with this:
KernelLoopNoWSYNC:
; at this point the registers hold the following:
; A - graphics for player 0
; X - enable for missile 0
; Y - enable for missile 1 & PF0 for right side of screen
; PF0 and PF1 have already been updated for left side of room
; GRP1 (on VDEL) has been preloaded with player 1 graphics
sta GRP0 ; 3 3 - before 25 - updates GRP1 too via VDEL
lda #<DS_COLUP0 ; 2 5
sta COLUP0 ; 3 8 - before 25
lda #<DS_COLUP1 ; 2 10
sta COLUP1 ; 3 13 - before 25
stx ENAM0 ; 3 16 - before 25
sty ENAM1 ; 3 19 - before 25
lda #<DS_EVENT_BL ; 2 21 - bit 7 triggers kernel event
sta ENABL ; 3 24 - before 25
bmi KernelEvent ; 2 26 - 3 27 if taken
lda #<DS_PF2L ; 2 28
sta PF2 ; 3 31 - PF2L, before 38
sty PF0 ; 3 34 - PF0R, 28-49
ldy DS_PF0R_M1 ; 4 38 - any after sty PF0, loads for next line
lda #<DS_PF1R ; 2 40
sta PF1 ; 3 43 - PF1R, 39-54
lda #<AMPLITUDE ; 2 45 - any
sta AUDV0 ; 3 48 - any
lda #<DS_PF2R ; 2 50
sta PF2 ; 3 53 - PF2R, 50-65
ldx DS_PF0L_M0 ; 4 57 - 0 triggers end-of-kernel
stx PF0 ; 3 60 - PF0L, after 55
lda #<DS_GRP1 ; 2 62
sta GRP1 ; 3 65 - any, on VDEL
lda #<DS_PF1L ; 2 67
sta.w PF1 ; 4 71 - PF1L, 66 - 28
lda #<DS_GRP0 ; 2 73
jmp KernelLoopNoWSYNC ; 3 76/0
KernelEvent: ; 3 27
; Y has PF0 for right of screen, needs to be written 28-49 if not end-of-kernel
; A holds the Kernel Event
; IF A{6} = 1 then reposition Player 0
; ELIF A{5} = 1 then reposition Player 1
; ELSE exit display kernel loop
;
; have to update PF2 before figuring out which Kernel Event, otherwise
; PF2 is updated too late in the Reposition Player 1 routines
ldx DS_PF2L ; 4 31
stx PF2 ; 3 34 - PF2L, before 38
ASL ; 2 36
BPL CheckRepo1 ; 2 38
; start of Reposition Player 0 routine
CheckRepo1: ; 3 39
ASL ; 2 41
BPL KernelDone ; 2 43
; start of Reposition Player 1 routine
KernelDone: ; 3 44
; show lives/score
Namely what I noticed in the original routine was a WSYNC and 3 conditional branches, 2 of which occur during the premium horizontal retrace time (the first 22 cycles of the line).
The WSYNC is easy to get rid of, but eliminating it wouldn't buy enough time for anything useful. The 3 branches are for the reposition of player 0, reposition of player 1 and end-of-kernel. I wondered if I could combine the 3 conditional branches and free up enough time to enable the ball. The ball needs to be set during the horizontal retrace to prevent mid-screen shearing.
To do this I utilized an additional datastream, DF4DATA, that I aliased as DS_EVENT_BL (to make reading the code easier). The bits (76543210) in the datastream bytes are:
7 - event triggered
6 - reposition player 0 event
5 - reposition player 1 event
1 - ball enable flag
The end-of-kernel event is triggered with bit 7 on, and bits 5 & 6 both off.
ball = dotted blue line, missile 0 = green/white lines, missile 1 = red lines
Next is to review and rewrite the reposition routines. There's a chance timing won't work out due to the extra cycles needed for controlling the ball, but if it does then Frantic's going to be rebooted so I can show 2 humanoid shots and 4 robot shots!
ROM
Source
I also did some minor updates to my jEdit mode file for Atari programming:
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