First public commit.

lenser/a2600/viewer/a2600.s

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+; 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