; lenser -- Apple II HGR boot loader + viewer (self-contained, no DOS)
;
; The Disk II boot ROM loads track 0 sector 0 to $0800 and JMPs $0801.  This
; re-entrant loader reads the 32 sectors of the 8K HGR bitmap into $2000-$3FFF
; (track 0 sectors 1-15, then tracks 1 and 2), seeking the head between tracks
; with the standard phase-overlap step, then switches on HGR and loops.  The ROM
; read at $Cn5C verifies the address field against BOTH the sector ($3D) and the
; track ($41), then reads into page $27 and JMPs back to $0801.
;
; assembled by apple/viewer/assemble.py via xa

        * = $0800
        .byte $01               ; ROM scratch (boot sector byte 0)
entry:                          ; $0801, (re)entered after every ROM read
        lda dpage
        cmp #$40
        bcs done                ; loaded $2000..$3FFF -> show it
        lda psec
        cmp #$10
        bcc readit              ; still 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 (ROM read verifies both)
        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:
        lda $c050               ; graphics
        lda $c052               ; full screen (not mixed)
        lda $c054               ; page 1
        lda $c057               ; hi-res
#include "awyt.i"

; advance the head one track (two half-steps) with the standard phase-overlap
; step.  energize the next phase while the current one is still on, then release
; the old phase, using on/off settle delays from an acceleration table.
seeknext:
        inc curtrk
        lda #$00
        sta $0a
        jsr onestep
        jsr onestep
        lda halftrk             ; release final phase before reading
        and #$03
        asl
        ora $2b
        tax
        lda $c080,x
        rts
onestep:
        lda halftrk             ; energize NEXT phase (old still on -> overlap)
        clc
        adc #$01
        and #$03
        asl
        ora $2b
        tax
        lda $c081,x
        ldx $0a
        lda ontable,x
        jsr wait
        lda halftrk             ; release OLD phase
        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

; ---- loader state (initial values; updated in place during the load) ----
psec:    .byte $01              ; next physical sector (track 0 starts at 1)
dpage:   .byte $20              ; next destination page ($2000)
halftrk: .byte $00              ; current half-track
curtrk:  .byte $00              ; current track