; vcsdrive - Simple Atari VCS driving game
;
; Copyright (C) 2024  Ian Cowburn (ianc@noddybox.demon.co.uk)
;
; This program is free software: you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation, either version 3 of the License, or
; (at your option) any later version.
;
; This program is distributed in the hope that it will be useful,
; but WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
; GNU General Public License for more details.
;
; You should have received a copy of the GNU General Public License
; along with this program.  If not, see <http://www.gnu.org/licenses/>.
;
; -------------------------------------------------------------------------
;
; Code
;
        processor       6502
        option          output-file,vcsdrive.bin

        include         vcs.asm

bgcolour equ $81

        org             $f000

        ; Set up stack pointer
start:
        sei
        cld
        ldx             #$ff
        txs

        ; Initialise page zero
        lda             #0
.zero:
        sta             $0,x
        dex
        bne             zero

        ; Start frame
next_frame:
        lda             #2
        ldy             bgcolour
        sta             VBLANK
        sty             COLUBK
        iny
        sta             VSYNC
        sty             COLUBK
        iny
        sta             WSYNC
        sty             COLUBK
        iny
        sta             WSYNC
        sty             COLUBK
        iny
        sta             WSYNC
        sty             COLUBK
        iny
        lda             #0
        sta             VSYNC

        ; Do VBLANK
        ldx             #37
.vblank:
        sta             WSYNC
        dex
        bne             vblank
        lda             #0
        sta             VBLANK

        ; Draw 192 visible scan lines
        ldx             #192
.scanlines:
        sty             COLUBK
        sta             WSYNC
        iny
        dex
        bne             scanlines

        ; Complete the frame
        lda             #2
        sta             VBLANK
        ldx             #30
.complete:
        sty             COLUBK
        iny
        sta             WSYNC
        dex
        bne             complete

        dec             bgcolour
        jmp             next_frame

        ; Set start adresses and pad ROM
        org             $fffc
        word            start
        word            start