initial release

fstbt.s

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+;fast boot-loader in one sector
+;copyright (c) Peter Ferrie 2016
+;thanks to 4am for inspiration and testing
+;assemble using ACME
+!cpu 6502
+!to "fstbt",plain
+*=$800
+
+!byte 1
+
+        tay                     ;A is last read sector+1 on entry
+
+        ;2, 4, 6, 8, $0A, $0C, $0E
+        ;because PROM increments by one itself
+        ;and is too slow to read sectors in purely incremental order
+        ;so we offer every other sector for read candidates
+
+incsector
+        iny
+        cpy     #$10            ;set carry, needed for seek
+        bcc     incindex        ;cases 1-$0F
+        beq     sector1         ;finished with $0E
+                                ;next should be 1 for 1, 3, 5, 7... sequence
+
+        ;finished with $0F, now we are $11, so 16 sectors done
+
+        jsr     seek            ;returns A=0
+
+        ;back to 0
+
+        tay
+        !byte   $2C             ;mask LDY #1
+sector1
+        ldy     #1
+
+incindex
+        sty     $3d             ;set sector
+        iny                     ;prepare to be next sector in case of unallocated sector
+        inc     adrindex + 1    ;select next address
+
+adrindex
+        lda     adrtable - 1    ;15 entries in first row, 16 entries thereafter
+        beq     incsector       ;skip empty slots to allow sparse tracks
+        sta     $27             ;set high part of address
+        tay
+        iny
+        beq     jmpoep          ;#$FF means end of data
+
+        ;convert slot to PROM address
+
+        txa
+        lsr
+        lsr
+        lsr
+        lsr
+        ora     #$c0
+        pha
+        lda     #$5B            ;read-1
+        pha
+        rts                     ;return to PROM
+
+        ;requires carry set on entry
+        ;carry is set by cpy above
+        ;and by wait below
+
+seek
+        inc     $41             ;next track
+        jsr     seek1
+
+seek1                           ;phase on
+        inc     phase + 1
+
+phase
+        ldy     #0              ;self-modified
+        jsr     delay
+        clc                     ;phase off
+        ldy     phase + 1
+        dey
+
+delay
+        tya
+        and     #3
+        rol
+        ora     $2B             ;merge in slot
+        tay
+        lda     $C080, y
+        lda     #$30
+        jmp     $FCA8           ;common delay for all phases
+
+jmpoep
+        jmp     $1234
+
+adrtable
+;15 slots for track 0 (track 0 sector 0 is not addressible)
+;16 slots for all other tracks
+!byte $FF ;end of list

readme.md

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+fast boot-loader for Apple II that fits in a single sector.
+as fast as the ProDOS loader!