8bitlenser/lenser/viewer/cart.s

; lenser -- C64 cartridge viewer (runs from ROM at $8000), fully self-contained.
;
; Unlike the disk viewer (a PRG at $0801 with the picture appended), this runs
; from a 16K cartridge.  It copies the image data block -- appended after this
; code in ROM -- down to $2000 (same layout the disk viewer wants, bitmap at
; $2000, screen $3F40, and for multicolor colram $4328 + bg $4710), programs the
; VIC, and holds the picture.  It uses NO KERNAL/IRQ services (a 16K cart hides
; BASIC and the autostart entry runs before the KERNAL fully initialises), so it
; polls the hardware directly -- CIA1 for the keyboard, the raster for timing.
;
; #defines set by viewer/assemble.py --
;   MCMODE   0 = hires, 1 = multicolor
;   NPAGES   number of 256-byte pages of image data to copy
;   WAITMODE 0 forever / 1 until a key / 2 about WAITSECS seconds
;   WAITSECS, RATE (frames per second, 50 PAL / 60 NTSC)
;
; assembled by viewer/assemble.py via xa

        * = $8000
        .word cold              ; cold-start vector
        .word cold              ; NMI / warm vector
        .byte $c3,$c2,$cd,$38,$30   ; CBM80 autostart signature

SRC = $fb
DST = $fd
CNT = $02               ; 16-bit frame countdown (seconds mode)

cold:
        sei
        ldx #$ff
        txs
        cld

        ; map the standard memory layout so BOTH halves of the 16K cart are
        ; visible -- ROML $8000-$9FFF and ROMH $A000-$BFFF (needs HIRAM=1) -- plus
        ; I/O at $D000.  Set the data latch ($01) BEFORE the DDR ($00): on a cart
        ; cold-start the latch may be low while the DDR is all-inputs (so the port
        ; reads the correct map via pull-ups). Enabling the DDR first would drive
        ; that low latch onto the port, bank ROML out, and crash the next fetch
        ; (this is exactly what failed in VICE).
        lda #$37
        sta $01                 ; LORAM+HIRAM+CHAREN = ROML, ROMH, I/O, KERNAL
        lda #$2f
        sta $00                 ; now enable the port outputs

        ; VIC bank 0 ($0000-$3FFF) so the VIC sees our bitmap at $2000
        lda #$3f
        sta $dd02               ; CIA2 port A bits 0-5 = output
        lda #$c7
        sta $dd00               ; bank 0 (bits 0-1 = 11)

        lda #$0b
        sta $d011               ; blank during setup

        ; ---- copy NPAGES of image data from ROM (datasrc) to $2000 ----
        lda #<datasrc
        sta SRC
        lda #>datasrc
        sta SRC+1
        lda #$00
        sta DST
        lda #$20
        sta DST+1
        ldx #NPAGES
        ldy #$00
dcopy:
        lda (SRC),y
        sta (DST),y
        iny
        bne dcopy
        inc SRC+1
        inc DST+1
        dex
        bne dcopy

        ; ---- copy screen RAM $3F40 -> $0400 ----
        lda #$40
        sta SRC
        lda #$3f
        sta SRC+1
        lda #$00
        sta DST
        lda #$04
        sta DST+1
        jsr copy1024

#if MCMODE == 1
        ; ---- copy colour RAM $4328 -> $D800 ----
        lda #$28
        sta SRC
        lda #$43
        sta SRC+1
        lda #$00
        sta DST
        lda #$d8
        sta DST+1
        jsr copy1024
#endif

        ; ---- program the VIC-II ----
        lda #$00
        sta $d020               ; border black
#if MCMODE == 1
        lda $4710
        sta $d021               ; background colour
        lda #$d8
        sta $d016               ; multicolor on
#endif
#if MCMODE == 0
        lda #$c8
        sta $d016               ; hires
#endif
        lda #$18
        sta $d018               ; screen $0400, bitmap $2000
        lda #$3b
        sta $d011               ; bitmap mode, display on

        ; ---- hold the picture ----
#if WAITMODE == 0
forever:
        jmp forever
#endif

#if WAITMODE == 1
        lda #$ff
        sta $dc02               ; CIA1 port A = output (column select)
        lda #$00
        sta $dc03               ; CIA1 port B = input  (row read)
        sta $dc00               ; drive all keyboard columns low
kwait:
        lda $dc01               ; rows; $FF = no key, any key pulls a row low
        cmp #$ff
        beq kwait
        jmp $fce2               ; reset (a 16K cart can't return to BASIC)
#endif

#if WAITMODE == 2
        lda #<(WAITSECS*RATE)
        sta CNT
        lda #>(WAITSECS*RATE)
        sta CNT+1
sloop:
sw1:
        lda $d012
        cmp #$fa
        bne sw1                 ; wait until raster reaches line 250
sw2:
        lda $d012
        cmp #$fa
        beq sw2                 ; wait until it leaves -> one frame elapsed
        lda CNT
        bne sdec
        dec CNT+1
sdec:
        dec CNT
        lda CNT
        ora CNT+1
        bne sloop
        jmp $fce2               ; reset
#endif

copy1024:
        ldx #4
        ldy #0
c1k:
        lda (SRC),y
        sta (DST),y
        iny
        bne c1k
        inc SRC+1
        inc DST+1
        dex
        bne c1k
        rts

datasrc:
        ; image data block appended here by the packager