; lenser -- Atari 2600 (VCS) image viewer kernel (6502, "racing the beam").
;
; No framebuffer -- per visible scanline the kernel feeds the TIA a 40-pixel
; asymmetric playfield (left 20 + right 20, rewritten mid-line) plus a shared
; playfield colour (COLUPF) and two backgrounds -- COLUBK is set for the left
; half during HBLANK then rewritten mid-line for the right half, so each
; scanline shows 3 colours (left bg + right bg + foreground).  The nine 192-byte
; data tables are page-aligned (set by the cartridge builder) so LDA tab,Y never
; crosses a page and the kernel stays cycle-exact.
;
; #defines from the wrapper -- PF0L,PF1L,PF2L,PF0R,PF1R,PF2R,BKL,BKR,PFT (table
;   bases), WAITMODE (0 forever / 1 fire / 2 seconds), WAITSECS.
;
; assembled by a2600/viewer/assemble.py via xa

VSYNC  = $00
VBLANK = $01
WSYNC  = $02
NUSIZ0 = $04
COLUPF = $08
COLUBK = $09
CTRLPF = $0A
PF0    = $0D
PF1    = $0E
PF2    = $0F
INPT4  = $3C
FRLO   = $80            ; frame counter (seconds mode)
FRHI   = $81
PARITY = $82            ; interlace frame parity (IL mode)

        * = $f000
start:
        sei
        cld
        ldx #0
        txa
clr:
        sta $00,x
        inx
        bne clr                 ; clear $00-$FF (RAM + TIA)
        ldx #$ff
        txs

frame:
        lda #2
        sta VSYNC
        sta WSYNC
        sta WSYNC
        sta WSYNC               ; 3 VSYNC lines
        lda #0
        sta VSYNC
        lda #2
        sta VBLANK
        sta CTRLPF              ; reflect off (we draw both halves explicitly)
        lda #0
        sta CTRLPF
#if IL
        ; temporal interlace -- alternate the two 4K banks (frame A / frame B)
        ; each frame; the kernel is identical in both banks so execution
        ; continues seamlessly and only the data tables differ.
        lda PARITY
        eor #1
        sta PARITY
        beq selbank0
        lda $1ff9               ; F8 hotspot -> select bank 1 (AD F9 1F)
        jmp bankdone
selbank0:
        lda $1ff8               ; F8 hotspot -> select bank 0 (AD F8 1F)
bankdone:
#endif
        ldx #36
vbl:
        sta WSYNC
        dex
        bne vbl
        lda #0
        sta VBLANK

        ; ---- visible image, 192 cycle-exact scanlines ----
        ldy #0
kloop:
        sta WSYNC
        lda BKL,y               ; left background (HBLANK)
        sta COLUBK
        lda PFT,y               ; shared foreground
        sta COLUPF
        lda PF0L,y
        sta PF0
        lda PF1L,y
        sta PF1
        lda PF2L,y
        sta PF2
        lda PF0R,y              ; right playfield PF0 (early, before mid-line)
        sta PF0
        lda BKR,y               ; right background (lands at the half boundary)
        sta COLUBK
        lda PF1R,y
        sta PF1
        lda PF2R,y
        sta PF2
        iny
        cpy #192
        bne kloop

        lda #0
        sta PF0
        sta PF1
        sta PF2
        sta COLUBK

        ; ---- hold / exit ----
#if WAITMODE == 1
        lda INPT4
        bmi over                ; bit7 set = fire not pressed
        jmp start               ; pressed -> reset (re-display)
#endif
#if WAITMODE == 2
        inc FRLO
        bne fr_ok
        inc FRHI
fr_ok:
        lda FRHI
        cmp #>(WAITSECS*60)
        bcc over
        lda FRLO
        cmp #<(WAITSECS*60)
        bcc over
        jmp start
#endif
over:
        lda #2
        sta VBLANK
        ldx #30
ovl:
        sta WSYNC
        dex
        bne ovl
        jmp frame