C02/vcs/include/k2line1.a02

;k2line.a02 - Two Line Kernal Assembly Language File for C02
;Requires Constants PLYR0H and PLYR1H,
;Variables PLYR0D and PLYR1D, and Pointers PLYR0P and PLYR1P

PLYR0H  EQU $D0 ;Player 0 Height
PLYR1H  EQU $D1 ;Player 1 Height
PLYR0D  EQU $D2 ;Player 0 Draw Counter 
PLYR1D  EQU $D3 ;Player 1 Draw Pointer
PLYR0P  EQU $D4 ;Player 0 Graphics Pointer
PLYR1P  EQU $D6 ;Player 1 Graphics Pointer
PLYR0T  EQU $D8 ;Player 0 Pointer Offset
PLYR1T  EQU $DA ;Player 1 Pointer Offset
SCNLNS  EQU $DF ;Number of Scanlines

;setpl0() - Set Player 0 Data
;Args: A = Player 0 Height
;      Y,X = Pointer to Player 0 Graphics Data
SETPL0: STA PLYR0H    ;Save Player 0 Height
        DEC PLYR0H    ;and Decrement
        STX PLYR0P    ;Save Player 0 Pointer
        STY PLYR0P+1
        RTS

;setpl1() - Set Player 1 Data
;Args: A = Player 1 Height
;      Y,X = Pointer to Player 1 Graphics Data
SETPL1: STA PLYR1H    ;Save Player 1 Height
        DEC PLYR1H    ;and Decrement
        STX PLYR1P    ;Save Player 1 Pointer
        STY PLYR1P+1
        RTS

;prppl0() - Prep Player Variables
;Args:  A = Player 0 Y-Position
PRPPL0: LDX #1        ;Divide Y-Position by 2
        SEC
        SBC #1
        LSR           ;and Set Vertical Delay if No Remainder
        STA TEMP0 
        BCS PRPPN0
        STX VDELP0
PRPPN0: LDA SCNLNS    ;PLYR0D = SCNLNS + PLYR0H - PLYR0Y
        CLC
        ADC PLYR0H
        SEC
        SBC TEMP0
        STA PLYR0D
        LDA PLYR0P    ;PLYR0T = PLYR0P + PLYR0H - PLYR0Y - 1
        CLC
        ADC PLYR0H
        SEC
        SBC TEMP0
        STA PLYR0T
        LDA PLYR0P+1
        SBC #0
        STA PLYR0T+1
        RTS

;prppl1() - Prep Player 1 Variables
;Args:  A = Player 1 Y Position
PRPPL1: LDX #1
        CLC           ;Increment Y-Position
        ADC #1        ;  to compensate for priming of GRP1
        LSR           ;Divide by 2 
        STA TEMP1     ;  and set Vertical Delay if No Remainder
        BCS PRPPN1
        STX VDELP1
PRPPN1: LDA SCNLNS    ;PLYR1D = SCNLNS + PLYR1H - PLYR1Y
        CLC
        ADC #1
        ADC PLYR1H
        SEC
        SBC TEMP1
        STA PLYR1D
        LDA PLYR1P    ;PLYR1T = PLYR1P + PLYR1H - PLYR1Y - 1
        CLC
        ADC PLYR1H
        SEC
        SBC TEMP1
        STA PLYR1T
        LDA PLYR1P+1
        SBC #
        STA PLYR1T+1
        RTS

;prpdsp() - Prep Display
PRPDSP: LDA #0
        STA WSYNC     ;Wait for Horizontal Sync
        STA HMOVE     ;Move Objects
        STA VDELP0    ;Clear Player 0 Vertical Delay
        STA VDELP1    ;Clear Player 0 Vertical Delay
        RTS

;dsplns() - Display Two Line Kernal
DSPLNS: LDY SCNLNS     ; (13)  Set Number of Lines to Draw
        LDA PLYR1H     ; 2  9 - Get Player 1 Height
        DCP PLYR1D     ; 5 14 - Decrement counter and compare with height
        BCS DSPLNP     ; 2 16 - (3 18) If not on current scanline
        LDA #0         ; 2 18 -   Load Blank Line  
        BYTE $2C       ; 4 22 - Else (BIT trick)
DSPLNP: LDA (PLYR1T),Y ; 5 22 -   Load Player 0 Graphics Line
        STA GRP1       ; 3 25 - Set Player 0 Graphics Data
        DEY
DSPLNL: LDA PLYR0H     ;2 21 - Height of Player 0 (Zero Based)
        DCP PLYR0D     ;5 26 - Decrement counter and compare with height
        BCS DSPLN0     ;2 28 - (3 23) If Player 0 not on Current Scanline
        LDA #0         ;2 30 -   Load Blank Line  
        BYTE $2C       ;4 34 - Else (BIT trick)
DSPLN0: LDA (PLYR0T),Y ;5 49 -   Load Player 0 Graphics Line
        STA WSYNC      ;3 52 - End of Line 2
        STA GRP0       ;3  3 - Set Player 0 Graphics Data
        LDX #%11111111 ;2  5 - Load Playfield Pattern
        STX PF0        ;3  8 - and Store 
        STX PF1        ;3 11
        STX PF2        ;3 14
        LDA PLYR1H     ;2 16 - Get Player 1 Height
        DCP PLYR1D     ;5 21 - Decrement counter and compare with height
        BCS DSPLN1     ;2 23 - (3 18) If not on current scanline
        LDA #0         ;2 25 -   Load Blank Line  
        BYTE $2C       ;4 29 - - Else (BIT trick)
DSPLN1  LDA (PLYR1T),Y ;5 34 - Load Player 1 Graphics Line
        STA WSYNC      ;3 37 - End of Line 1
        STA GRP1       ;3  3 - Set Player 0 Graphics Data
        LDX #0         ;2  5 - Load Playfield Pattern
        STX PF0        ;3  8 - and Store
        STX PF1        ;3 11
        STX PF2        ;3 14 
        DEY            ;2 16 - decrease the 2LK loop counter
        BPL DSPLNL     ;2 18 - (3 21) Branch if more lines to draw
        RTS