; 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