First public commit.

lenser/iigs/viewer/iigs.s

@@ -0,0 +1,116 @@
+; lenser -- Apple IIGS Super Hi-Res boot loader + viewer (8-bit, no GS/OS).
+;
+; The Disk II boot ROM (slot 6) loads track 0 sector 0 to $0800 and JMPs $0801.
+; This re-entrant loader reads the 32K SHR block (pixels $2000-$9CFF, SCBs
+; $9D00, palettes $9E00) into main RAM $2000-$9FFF, then copies it to AUX RAM
+; with SHR shadowing enabled -- so the writes mirror into bank $E1 (the SHR
+; screen) -- and turns SHR on (NEWVIDEO bit 7).  All 8-bit, no 65C816 needed.
+
+        * = $0800
+        .byte $01               ; ROM scratch (boot sector byte 0)
+entry:                          ; $0801, re-entered after every ROM read
+        lda dpage
+        cmp #$a0
+        bcs done                ; loaded $2000..$9FFF -> show it
+        lda psec
+        cmp #$10
+        bcc readit              ; sectors left on this track
+        jsr seeknext            ; finished a track -> step to the next
+        lda #$00
+        sta psec
+readit:
+        lda psec
+        sta $3d                 ; desired sector
+        lda curtrk
+        sta $41                 ; desired track
+        lda #$00
+        sta $26                 ; buffer lo
+        lda dpage
+        sta $27                 ; buffer hi
+        inc psec
+        inc dpage
+        ldx $2b                 ; slot*16 (set by boot ROM)
+        jmp $c65c               ; slot 6 ROM read; reads sector then JMP $0801
+
+done:
+        ; Turn on Super Hi-Res (bit 7) AND linearize (bit 6) FIRST.  With these
+        ; set, CPU writes to bank $E1 $2000-$9FFF are de-interleaved by hardware
+        ; into the megaII layout the video reads -- so we can copy our linear SHR
+        ; block straight in and it displays correctly.
+        lda $c029
+        ora #$c0
+        sta $c029
+        ; copy 32K from bank $00 $2000 -> bank $E1 $2000 (65C816 block move)
+        clc
+        xce                     ; -> native mode
+        rep #$30                ; 16-bit A/X/Y
+.al
+.xl
+        lda #$7fff              ; count-1 (32768 bytes)
+        ldx #$2000              ; source offset
+        ldy #$2000              ; dest offset
+        .byte $54, $e1, $00     ; MVN src bank $00 -> dest bank $E1
+        sep #$30                ; 8-bit
+.as
+.xs
+        sec
+        xce                     ; -> emulation mode
+        lda $c034               ; border colour is the low nibble of $C034
+        and #$f0
+        sta $c034               ; black border
+hang:
+        jmp hang
+
+; advance the head one track (two half-steps), phase-overlap step.
+seeknext:
+        inc curtrk
+        lda #$00
+        sta $0a
+        jsr onestep
+        jsr onestep
+        lda halftrk
+        and #$03
+        asl
+        ora $2b
+        tax
+        lda $c080,x
+        rts
+onestep:
+        lda halftrk
+        clc
+        adc #$01
+        and #$03
+        asl
+        ora $2b
+        tax
+        lda $c081,x
+        ldx $0a
+        lda ontable,x
+        jsr wait
+        lda halftrk
+        and #$03
+        asl
+        ora $2b
+        tax
+        lda $c080,x
+        ldx $0a
+        lda offtable,x
+        jsr wait
+        inc halftrk
+        inc $0a
+        rts
+ontable:  .byte $13,$0a,$08,$06
+offtable: .byte $46,$1a,$10,$0c
+wait:
+        tay
+w1:     ldx #$00
+w2:     dex
+        bne w2
+        dey
+        bne w1
+        rts
+
+psec:    .byte $01
+dpage:   .byte $20
+halftrk: .byte $00
+curtrk:  .byte $00