First public commit.

lenser/apple/viewer/dhgr.s

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+; lenser -- Apple //e Double Hi-Res boot loader + viewer (self-contained)
+;
+; Loads 16K to main $2000-$5FFF with ordinary reads (main half at $2000, aux half
+; at $4000), then block-copies $4000-$5FFF into auxiliary $2000-$3FFF (RAMWRT on
+; only for that clean copy, never during the boot-ROM reads), then turns on DHGR.
+; ROM read at $C65C reads sector $3D into page $27 and re-enters $0801.
+;
+; assembled by apple/viewer/assemble.py via xa
+
+        * = $0800
+        .byte $01
+entry:                          ; $0801, re-entered after each ROM read
+        lda dpage
+        cmp #$60                ; loaded $2000-$5FFF (64 pages)?
+        bcs done
+        lda psec
+        cmp #$10
+        bcc readit
+        jsr seeknext
+        lda #$00
+        sta psec
+readit:
+        lda psec
+        sta $3d                 ; desired sector
+        lda curtrk
+        sta $41                 ; desired track (the ROM read verifies BOTH)
+        lda #$00
+        sta $26
+        lda dpage
+        sta $27
+        inc psec
+        inc dpage
+        ldx $2b
+        jmp $c65c
+
+done:
+        ; copy main $4000-$5FFF -> aux $2000-$3FFF
+        lda #$00
+        sta $06
+        lda #$40
+        sta $07                 ; src = $4000
+        lda #$00
+        sta $08
+        lda #$20
+        sta $09                 ; dst = $2000
+        sta $c005               ; RAMWRT on (writes go to aux)
+        ldx #$20                ; 32 pages
+cpl:
+        ldy #$00
+cp1:
+        lda ($06),y
+        sta ($08),y
+        iny
+        bne cp1
+        inc $07
+        inc $09
+        dex
+        bne cpl
+        sta $c004               ; RAMWRT off
+        ; turn on Double Hi-Res
+        lda $c050               ; graphics
+        lda $c052               ; full screen
+        lda $c054               ; page 1
+        lda $c057               ; hi-res
+        sta $c00d               ; SET80VID (write-triggered switch -- must STA)
+        sta $c05e               ; SETDHIRES (write-triggered)
+#include "awyt.i"
+
+; advance the head one track (two half-steps) with the standard phase-overlap
+; seek.  energize the NEXT phase while the current one is still on (this pulls
+; the head smoothly), then release the old phase, using on/off settle delays
+; from an acceleration table indexed by step number ($0a).  the final phase is
+; released before reading.
+seeknext:
+        inc curtrk              ; now on the next track
+        lda #$00
+        sta $0a                 ; step index for the timing table
+        jsr onestep
+        jsr onestep
+        lda halftrk             ; release final phase before the read
+        and #$03
+        asl
+        ora $2b
+        tax
+        lda $c080,x
+        rts
+onestep:
+        lda halftrk             ; energize NEXT phase (halftrk+1), old still on
+        clc
+        adc #$01
+        and #$03
+        asl
+        ora $2b
+        tax
+        lda $c081,x
+        ldx $0a
+        lda ontable,x
+        jsr wait
+        lda halftrk             ; release OLD phase (halftrk)
+        and #$03
+        asl
+        ora $2b
+        tax
+        lda $c080,x
+        ldx $0a
+        lda offtable,x
+        jsr wait
+        inc halftrk
+        inc $0a
+        rts
+; Apple seek timing tables (head accelerates over a move, slow first step).
+ontable:  .byte $13,$0a,$08,$06,$05,$04,$04,$03
+offtable: .byte $46,$1a,$10,$0c,$0a,$09,$08,$08
+wait:                           ; delay loop ~ proportional to A
+        tay
+w1:
+        ldx #$00
+w2:
+        dex
+        bne w2
+        dey
+        bne w1
+        rts
+
+psec:    .byte $01
+dpage:   .byte $20
+halftrk: .byte $00
+curtrk:  .byte $00