First public commit.

lenser/apple/viewer/hgr.s

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+; 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