mirror of
https://codeberg.org/cryu/micro-sci-a2-controller
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364 lines
14 KiB
ArmAsm
364 lines
14 KiB
ArmAsm
; Disk ][ controller card "BOOT0" code, found in the slot ROM. Reads the
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; BOOT1 code from track 0, sector 0, and jumps to it.
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;
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; Copyright Apple Computer Inc.
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;
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; Written by [a genius...Woz?]
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;
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; Extracted from AppleWin at $C600.
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;
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; Project created by Andy McFadden, using 6502bench SourceGen v1.5
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; Last updated 2020/01/15
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;
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; Converted for use with Micro-SCI repro project 2023 Chris RYU
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STACK := $0100 ;{addr/256}
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.ifdef SECSIZE_16
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TABLE_ENTRY := $02d6
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.else
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TABLE_ENTRY := BOOT1
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.endif
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TWOS_BUFFER := $0300 ;{addr/86} ;holds the 2-bit chunks
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CONV_TAB := $0356 ;{addr/128} ;6+2 conversion table
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BOOT1 := $0800 ;{addr/256} ;buffer for next stage of loader
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IWM_PH0_OFF := $c080 ;stepper motor control
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IWM_PH0_ON := $c081 ;stepper motor control
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IWM_MOTOR_ON := $c089 ;starts drive spinning
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IWM_SEL_DRIVE_1 := $c08a ;selects drive 1
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IWM_Q6_OFF := $c08c ;read
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IWM_Q7_OFF := $c08e ;WP sense/read
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MON_WAIT := $fca8 ;delay for (26 + 27*Acc + 5*(Acc*Acc))/2 cycles
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PRERR := $ff2d ; 13-sector only
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MON_IORTS := $ff58 ;JSR here to find out where one is
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data_ptr := $26 ;{addr/2} ;pointer to BOOT1 data buffer
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slot_index := $2b ;{addr/1} ;slot number << 4
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bits := $3c ;{addr/1} ;temp storage for bit manipulation
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sector := $3d ;{addr/1} ;sector to read
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.ifdef SECSIZE_13
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found_track := $2a ;{addr/1} ;track found
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signature := $b5
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.else
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found_track := $40 ;{addr/1} ;track found
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signature := $96
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.endif
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track := $41 ;{addr/1} ;track to read
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.org $c600
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ENTRY: ldx #$20 ;20/00/03 is the controller signature
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;
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; Generate a decoder table for 6+2 encoded data.
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;
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; This stores the values $00-$3f in a table on page 3. The byte values that
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; will be decoded are non-consecutive, so the decoder entries occupy various
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; locations from $36c to $3d5. Nearby bytes are left unchanged.
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;
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; We want 64 values that have the high bit set and don't have two consecutive 0
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; bits. This is required by the disk hardware. There are 70 possible values,
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; so we also mandate that there are two adjacent 1 bits, excluding bit 7. (Note
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; that $D5 and $AA, used to identify sector headers, do not meet these criteria,
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; which means they never appear in the encoded data.)
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;
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; In the code below, ASL+BIT+BCS test checks for adjacent 1s: if no two are
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; adjacent, the BIT will be zero. If the high bit is set, ASL will set the
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; carry.
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;
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; When we ORA the original and shifted values together, if there were three
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; adjacent 0s, there will still be at least two adjacent 0s. We EOR to invert
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; the bits, and then look for two adjacent 1s. We do this by just shifting
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; right until a 1 shifts into the carry, and if the A-reg is nonzero we know
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; there were at least two 1 bits. We need to ignore the bits on the ends:
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; nonzero high bit was handled earlier, and the low bit can false-positive
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; because ASL always shifts a 0 in (making it look like a 0 in the low bit is
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; adjacent to another 0), so we just mask those off with the AND.
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;
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; For example, we want to decode $A6 to $07. Y=$07 when X=$26...
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; TXA --> 0010 0110
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; ASL --> 0100 1100 C=0 (high bit is clear)
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; BIT --> Z=0 (only possible with adjacent bits)
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; ORA --> 0110 1110 (adjacent 0s become visible)
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; EOR --> 1001 0001 (turn them into 1s)
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; AND --> 0001 0000 (ignore the hi/lo)
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; LSR --> 0000 1000, repeat until A=0 C=1
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;
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ldy #$00
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.ifdef SECSIZE_16
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ldx #$03
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CreateDecTabLoop:
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stx bits
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txa
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asl A ;shift left, putting high bit in carry
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bit bits ;does shifted version overlap?
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beq reject ;no, doesn't have two adjacent 1s
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ora bits ;merge
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eor #$ff ;invert
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and #$7e ;clear hi and lo bits
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check_dub0: bcs reject ;initial hi bit set *or* adjacent 0 bits set
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lsr A ;shift right, low bit into carry
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bne check_dub0 ;if more bits in byte, loop
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tya ;we have a winner, store Y to memory
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sta CONV_TAB,x ;actual lookup will be on bytes with
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;hi bit set
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iny ; so they'll read from CONV_TAB-128
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reject: inx ;try next candidate
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bpl CreateDecTabLoop
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.else
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CreateDecTabLoop:
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LDA #$03
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STA bits
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CLC
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DEY
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TYA
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LC60B: BIT bits
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BEQ CreateDecTabLoop
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ROL bits
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BCC LC60B
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CPY #$D5 ; $D5 is reserved to indicate header
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BEQ CreateDecTabLoop
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DEX
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TXA
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STA TABLE_ENTRY,Y
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BNE CreateDecTabLoop
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.endif
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;
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; Prep the hardware.
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;
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jsr MON_IORTS ;known RTS
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tsx
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lda STACK,x ;pull hi byte of our address off stack
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.ifdef SECSIZE_13
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pha
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.endif
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asl A ;(we assume no interrupts have hit)
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asl A ;multiply by 16
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asl A
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asl A
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sta slot_index ;keep this around
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tax
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.ifdef SECSIZE_13
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lda #$d0 ; ck fixme
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pha
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.endif
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lda IWM_Q7_OFF,x ;set to read mode
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lda IWM_Q6_OFF,x
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lda IWM_SEL_DRIVE_1,x ;select drive 1
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lda IWM_MOTOR_ON,x ;spin it up
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;
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; Blind-seek to track 0.
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;
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ldy #80 ;80 phases (40 tracks)
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seek_loop: lda IWM_PH0_OFF,x ;turn phase N off
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tya
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and #$03 ;mod the phase number to get 0-3
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asl A ;double it to 0/2/4/6
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ora slot_index ;add in the slot index
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tax
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lda IWM_PH0_ON,x ;turn on phase 0, 1, 2, or 3
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lda #86
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jsr MON_WAIT ;wait 19664 cycles
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dey ;next phase
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bpl seek_loop
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.ifdef SECSIZE_13
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lda #>TWOS_BUFFER
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sta data_ptr+1 ;A-reg is 0 when MON_WAIT returns
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lda #<TWOS_BUFFER
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sta data_ptr ;so we're looking for T=0 S=0
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sta sector
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.else
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sta data_ptr ;A-reg is 0 when MON_WAIT returns
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sta sector ;so we're looking for T=0 S=0
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sta track
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lda #>BOOT1 ;write the output here
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sta data_ptr+1
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.endif
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;
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; Sector read routine.
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;
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; Read bytes until we find an address header (D5 AA 96) or data header (D5 AA
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; AD), depending on which mode we're in.
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;
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; This will also be called by the BOOT1 code read from the floppy disk.
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;
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; On entry:
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; X: slot * 16
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; $26-27: data pointer
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; $3d: desired sector
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; $41: desired track
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;
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ReadSector: clc ;C=0 to look for addr (C=1 for data)
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ReadSector_C: php
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@rdbyte1: lda IWM_Q6_OFF,x ;wait for byte
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bpl @rdbyte1 ;not yet, loop
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@check_d5: eor #$d5 ;is it $d5?
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bne @rdbyte1 ;no, keep looking
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@rdbyte2: lda IWM_Q6_OFF,x ;grab another byte
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bpl @rdbyte2
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cmp #$aa ;is it $aa?
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bne @check_d5 ;no, check if it's another $d5
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nop ;(?)
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@rdbyte3: lda IWM_Q6_OFF,x ;grab a third byte
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bpl @rdbyte3
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cmp #signature ;is it $96?
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beq FoundAddress ;winner
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plp ;did we want data?
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bcc ReadSector ;nope, keep looking
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eor #$ad ;yes, see if it's data prologue
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beq FoundData ;got it, read the data (note A-reg = 0)
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bne ReadSector ;keep looking
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;
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; Read the sector address data. Four fields, in 4+4 encoding: volume, track,
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; sector, checksum.
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;
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FoundAddress: ldy #$03 ;sector # is the 3rd item in header
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.ifdef SECSIZE_13
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hdr_loop: sty found_track ;store $96, then volume, then track
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.else
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hdr_loop: sta found_track ;store $96, then volume, then track
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.endif
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@rdbyte1: lda IWM_Q6_OFF,x ;read first part
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bpl @rdbyte1
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rol A ;first byte has bits 7/5/3/1
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sta bits
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@rdbyte2: lda IWM_Q6_OFF,x ;read second part
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bpl @rdbyte2
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and bits ;merge them
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dey ;is this the 3rd item?
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.ifdef SECSIZE_13
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bne @rdbyte1 ;nope, keep going
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.else
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bne hdr_loop ;nope, keep going
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.endif
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plp ;pull this off to keep stack in balance
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cmp sector ;is this the sector we want?
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bne ReadSector ;no, go back to looking for addresses
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.ifdef SECSIZE_16
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lda found_track
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cmp track ;correct track?
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bne ReadSector ;no, try again
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.endif
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bcs ReadSector_C ;correct T/S, find data (branch-always)
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;
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; Read the 6+2 encoded sector data.
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;
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; Values range from $96 - $ff. They must have the high bit set, and must not
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; have three consecutive zeroes.
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;
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; The data bytes are written to disk with a rolling XOR to compute a checksum,
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; so we read them back the same way. We keep this in the A-reg for the
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; duration. The actual value is always in the range [$00,$3f] (6 bits).
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;
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; On entry:
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; A: $00
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;
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FoundData:
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.ifdef SECSIZE_13
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ldy #$9a
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.else
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ldy #86 ;read 86 bytes of data into $300-355
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.endif
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read_twos_loop:
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sty bits ;each byte has 3 sets of 2 bits, encoded
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@rdbyte1: ldy IWM_Q6_OFF,x
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bpl @rdbyte1
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eor TABLE_ENTRY,y ;$02d6 + $96 = $36c, first table entry
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ldy bits
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dey
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.ifdef SECSIZE_13
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sta TABLE_ENTRY,y
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.else
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sta TWOS_BUFFER,y ;store these in our page 3 buffer
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.endif
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bne read_twos_loop
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;
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read_sixes_loop:
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sty bits ;read 256 bytes of data into $800
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@rdbyte2: ldy IWM_Q6_OFF,x ;each byte has the high 6 bits, encoded
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bpl @rdbyte2
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.ifdef SECSIZE_13
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eor TABLE_ENTRY,y
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.else
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eor CONV_TAB-128,y
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.endif
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ldy bits
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sta (data_ptr),y ;store these in the eventual data buffer
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iny
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bne read_sixes_loop
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;
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@rdbyte3: ldy IWM_Q6_OFF,x ;read checksum byte
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bpl @rdbyte3
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.ifdef SECSIZE_13
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eor TABLE_ENTRY,y ;does it match?
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.else
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eor CONV_TAB-128,y ;does it match?
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.endif
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another: bne ReadSector ;no, try to find one that's undamaged
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.ifdef SECSIZE_13
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GRP := $33
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RTS
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DENIB: TAY
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LC6D2: LDX #0
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LC6D4: LDA BOOT1,Y
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LSR
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ROL TWOS_BUFFER+4*GRP,X
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LSR
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ROL TWOS_BUFFER+3*GRP,X
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STA bits
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LDA (data_ptr),Y
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ASL
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ASL
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ASL
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ORA bits
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STA (data_ptr),Y
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INY
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INX
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CPX #GRP
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BNE LC6D4
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DEC found_track
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BNE LC6D2
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CPY TWOS_BUFFER
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BNE ERROR
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JMP TWOS_BUFFER+1
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ERROR: JMP PRERR
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.byte $FF
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.else
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;
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; Decode the 6+2 encoding. The high 6 bits of each byte are in place, now we
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; just need to shift the low 2 bits of each in.
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;
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ldy #$00 ;update 256 bytes
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init_x: ldx #86 ;run through the 2-bit pieces 3x (86*3=258)
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decode_loop: dex
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bmi init_x ;if we hit $2ff, go back to $355
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lda (data_ptr),y ;foreach byte in the data buffer...
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lsr TWOS_BUFFER,x ; grab the low two bits from the stuff
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; at $300-$355
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rol A ; and roll them into the low two bits
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; of the byte
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lsr TWOS_BUFFER,x
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rol A
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sta (data_ptr),y
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iny
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bne decode_loop
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;
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; Advance the data pointer and sector number, and check to see if the sector
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; number matches the first byte of BOOT1. If it does, we're done. If not, go
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; read the next sector.
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;
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inc data_ptr+1
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inc sector
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lda sector ;sector we'd read next
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cmp BOOT1 ;is next sector < BOOT1?
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ldx slot_index
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bcc another ;yes, go get another sector
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;(note branch x2)
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; All done, jump to BOOT1 ($0801).
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jmp BOOT1+1
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.byte 0, 0, 0, 0, 0 ;spare bytes
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.endif
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