;Atari 2600 Timer Bar Kernel

;Timer Variables
TMRPF0  EQU $F0 ;Timer Playfield Leftmost Byte
TMRPF1  EQU $F1
TMRPF2  EQU $F2
TMRPF3  EQU $F3
TMRPF4  EQU $F4
TMRPF5  EQU $F5 ;Timer Playfield Rightmost Byte

;tmprep() - Prepare Timer Bar
TMPREP: LDA #%11111111
        STA TMRPF0
        STA TMRPF1
        STA TMRPF2
        STA TMRPF3
        STA TMRPF4
        STA TMRPF5
        RTS

;tmtick() - Tick Down Timer Bar
;Returns: A=$FF If Timer has Not Expired
;           $00 If Timer has Expired
TMTICK: LDA TMRPF0    ;Check Leftmost Nybble of Timer Bar
        AND #$F0      ;If 0
        BEQ TMTICX    ;  Return FALSE
        LSR TMRPF5    ;PF2 right side, reversed bits so shift right
        ROL TMRPF4    ;PF1 right side, normal bits so shift left
        ROR TMRPF3    ;PF0 right side, reversed bits so shift right
        LDA TMRPF3    ;only upper nybble used, so we need to put bit 3 into C
        LSR
        LSR
        LSR
        LSR
        ROR TMRPF2    ;PF2 left side, reversed bits so shift right
        ROL TMRPF1    ;PF1 left side, normal bits so shift left
        ROR TMRPF0    ;PF0 left side, reversed bits so shift right
        LDA #$FF      ;Return TRUE 
TMTICX: RTS

;tmdisp() - Display Timer Bar
;Args: A = Timer Bar Color
TMDISP: STA COLUPF      ;      - Set Playfield Color
        LDX #0          ;      - Offset for Writes
        STX CTRLPF      ;      - Set Playfield to Defaults    
        LDY #1          ;      - Use Two Scanlines        
TMDISL: STA WSYNC       ;      - Wait for Beginning of Line
        LDA TMRPF0,X    ; 4  4 - Copy Data to Playfield Left
        STA PF0,X       ; 5  9 - Indexed Mode Uses Extra Cycles
        LDA TMRPF1,X    ; 4 13
        STA PF1,X       ; 5 18
        LDA TMRPF2,X    ; 4 22
        STA PF2,X       ; 5 27
        JSR TMDISX      ;12 39 - Wait 12 Cycles
        LDA TMRPF3      ; 3 42 - Copy Data To Playfield Right
        STA PF0         ; 3 45
        LDA TMRPF4      ; 3 50
        STA PF1         ; 3 53
        LDA TMRPF5      ; 3 56
        STA PF2         ; 3 59
        DEY             ; 2 62
        BPL TMDISL      ; 2 64 (3 65)
        STA WSYNC       ; 3 67
        LDA #0          ; 2  2
        STA PF0         ; 3  5
        STA PF1         ; 3  8
        STA PF2         ; 3 11
        STA COLUPF      ; 3 14
        STA WSYNC       ; 3 20 - Gap between timer and Arena
TMDISX: RTS