107 lines
3.1 KiB
ArmAsm
107 lines
3.1 KiB
ArmAsm
;license:BSD-3-Clause
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;fast boot-loader in one sector
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;copyright (c) Peter Ferrie 2016
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;thanks to 4am for inspiration and testing
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;assemble using ACME
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!cpu 6502
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!to "fstbt",plain
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*=$800
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enable_banked = 0 ;set to bank number (1 or 2) to enable reading into banked RAM
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!byte 1
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tay ;A is last read sector+1 on entry
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!if enable_banked > 0 {
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lda $C081 ;bank in ROM
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}
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;check array before checking sector number
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;allows us to avoid a redundant seek if all slots are full in a track,
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;and then the list ends
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incindex
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inc adrindex + 1 ;select next address
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adrindex
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lda adrtable - 1 ;15 entries in first row, 16 entries thereafter
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cmp #$C0
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beq jmpoep ;#$C0 means end of data
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sta $27 ;set high part of address
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lda #1 ;preload in case we are finished with the first round
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;2, 4, 6, 8, $0A, $0C, $0E
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;because PROM increments by one itself
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;and is too slow to read sectors in purely incremental order
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;so we offer every other sector for read candidates
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iny
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cpy #$10
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bcc setsector ;cases 1-$0F
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beq sector1 ;finished with $0E
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;next should be 1 for 1, 3, 5, 7... sequence
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;finished with $0F, now we are $11, so 16 sectors done
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jsr seek ;returns A=0
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;back to 0
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sector1
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tay
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setsector
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sty $3D ;set sector
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iny ;prepare to be next sector in case of unallocated sector
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lda $27
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beq incindex ;empty slot, back to the top
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;convert slot to PROM address
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txa
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jsr $F87B ;4xlsr
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ora #$C0
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pha
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lda #$5B ;read-1
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pha
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!if enable_banked > 0 {
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writeenable
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lda $C093-(enable_banked*8)
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lda $C093-(enable_banked*8)
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;write-enable RAM and bank it in so read can decode
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}
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rts ;return to PROM
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seek
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inc $41 ;next track
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asl $40 ;carry clear, phase off
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jsr seek1 ;returns carry set, not useful
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clc ;carry clear, phase off
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seek1
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jsr delay ;returns with carry set, phase on
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inc $40 ;next phase
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delay
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lda $40
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and #3
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rol
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ora $2B ;merge in slot
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tay
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lda $C080, y
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lda #$30
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jmp $FCA8 ;common delay for all phases
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jmpoep
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!if enable_banked > 0 {
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jsr writeenable ;bank in our RAM, write-enabled
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}
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jmp $1234 ;arbitrary entry-point to use after read completes
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;set to the value that you need
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adrtable
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;15 slots for track 0 (track 0 sector 0 is not addressable)
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;16 slots for all other tracks, fill with addresses, 0 to skip any sector
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!byte $C0 ;end of list
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