;Atart 2600 Program Stub - C02 Assembly Language File

;Preset Total Number of Kernal Lines
KNLLNS  SET 192   ;192 Lines on NTSC Television

;Variables Used By Other Libraries
TEMP0   EQU $E0   ;Temporary Storage
TEMP1   EQU $E1
TEMP2   EQU $E2
TEMP3   EQU $E3

;Initialization Code
; From CLEAN_START macro by Andrew Davie
; Standardised STArt-up code, clears STAck, all TIA registers and RAM to 0
; Sets STAck pointer to $FF, and all registers to 0
; Sets decimal mode off, sets interrupt flag (kind of un-necessary)
; Use as very first section of code on boot (ie: at reset)
; Code written to minimise total ROM usage - uses weird 6502 knowledge :)
START:  SEI
        CLD
        LDX #0
        TXA
        TAY
STARTC: DEX
        TXS
        PHA
        BNE STARTC

        JSR SETUP   ;Execute Setup Code

MAIN:   LDA #2      ;Vertical Sync On Bit Mask
        LDX #49     ;41 scanlines * 76 cycles / 64
        STA WSYNC   ;Wait for SYNC (halts CPU until end of scanline)
        STA VSYNC   ;Turn On Vertical Sync
        STX TIM64T  ;Set timer to go off in 41 scanlines 
        STA WSYNC   ;First Scanline of VSYNC
        STA WSYNC   ;Second Scanline of VSYNC
        LDA #0      ;Vertical Sync Off Bit Mask
        STA WSYNC   ;Third  Scanline of VSYNC
        STA VSYNC   ;Turn Off Vertical Sync
        JSR VRTBLK  ;Execute Vertical Blank Code
        STA WSYNC   ;Wait For Horizontal Sync
        STA HMOVE   ;Execute Horizontal Moves
        JSR CLRHMX  ;Clear Horizontal Move Registers
WTVBLK: STA WSYNC   ;Wait for End of Vertical Blank
        LDA INTIM   ;Check Timer
        BNE WTVBLK  ;Loop If Not Zero
        STA VBLANK  ;Turn off Vertical Blank
        JSR KERNEL  ;Execute Kernel Code
        STA WSYNC   ;Wait for End of Scanline
        LDA #2      ;Vertical Blank On Bit Mask
        STA VBLANK  ;Turn on Vertical Blank
        LDA #32     ;27 scanlines * 76 cycles / 64
        STA TIM64T  ;Set Timer for 27 scanlines
        JSR OVRSCN  ;Execute Overscan Code
WTOSCN: STA WSYNC   ;Wait for End of Overscan
        LDA INTIM   ;Check Time
        BNE WTOSCN  ;Loop If Not Zero
        BEQ MAIN    ;Loop Back to Vertical Sync

IRQBRK: RTI         ;Catch 6502 Break Instructions

CLRHMR: NOP         ; 2  8 (+6 for calling JSR) 
        JSR CLRHMX  ;12 20
        LDA #0      ; 2 22 - Clear Horizontal Move Registers
        LDX #4      ; 2 24
CLRHML  STA HMP0,X  ;      - Safe to Clear 24 Cycles after HMOVE
        DEX
        BPL CLRHML
CLRHMX: RTS