;
;	File:		SonyRead.a
;
;	Contains:	This file contains disk driver routines used to read
;				sector information from twiggy diskettes.
;
;	Written by:	Larry Kenyon
;
;	Copyright:	© 1982-1992 by Apple Computer, Inc.  All rights reserved.
;
;	Change History (most recent first):
;
;	   <SM6>	12/14/92	RC		Restore Pre-PDM D2 with Horror rollin
;	   <SM4>	 12/7/92	rab		Roll in Horror changes. Comments followÉ
;		<H5>	 6/19/92	CMP		Patched SetUpPoll to set A6 to point to a bogus set of
;									"registers" so that no SCC polling is done if there is no
;									PollProc installed.  This is because on DB Lite with LocalTalk
;									connected to the internal serial port, we get so much poll data
;									that we corrupt the heap.
;		<H4>	11/14/91	SWC		Converted the SCC polling code to a macro (in SonyHdr.a) so it's
;									easier to overpatch. Also cleaned up comments.
;		 <6>	 7/14/92	CSS		Fixed the comment below so an exact version of this
;									file could be copied into SuperMario.
;		 <5>	 4/27/92	JSM		Get rid of conditionals: hasHarpoVIA is always false,
;									supportsMFM and forROM are always true. This file now has no
;									conditionals.
;		 <4>	 1/21/91	SWC		Cleaned up header comments.
;		 <3>	 9/21/90	BG		Removed <2>. 040s are behaving more reliably now.
;		 <2>	 6/18/90	CCH		Added NOPs for flaky 68040's.
;	   <2.4>	 5/23/89	GGD		No changes to this file, entire Sony Driver is checked out and
;									in as a group.
;	   <2.3>	 4/29/89	GGD		No changes to this file, entire Sony Driver is checked out and
;									in as a group.
;	   <2.2>	 4/10/89	gmr		No changes to this file, entire Sony Driver is checked out and
;									in as a group.
;	   <2.1>	 2/21/89	GGD		Added extra VIA accesses in read code to allow time for a new
;									bit to shift in, and clear the data latch, so that a previous
;									data byte doesn't get interpreted as a new byte on fast
;									processors (25mhz). Changed bit number of SCC/VIA direction bit
;									save in chipState.
;	   <2.0>	12/15/88	GGD		Changed some machine based conditionals to feature based.
;									Modified nibble timeout code in RdAddr to be processor speed
;									independent.
;	   <1.1>	11/11/88	CCH		Fixed Header.
;	   <1.0>	 11/9/88	CCH		Adding to EASE.
;	   <1.9>	 9/29/88	GGD		No changes to this file, entire Sony Driver is checked out and
;									in as a group.
;	   <1.8>	 9/19/88	GGD		No changes to this file, entire Sony Driver is checked out and
;									in as a group.
;	   <1.7>	 8/16/88	GGD		Changed usages of absolute VIA/SCC hardware addresses in
;									SetUpPoll, to use low mem hw base addresses instead. Changed
;									vBufA to vBufD for HcMac. Saved and changed VIA Buf B Direction
;									bit in SetUpPoll for HcMac.
;	   <1.7>	  8/9/88	Saved		and changed VIA Buf B Direction bit in SetUpPoll for HcMac.
;	   <1.7>	  8/5/88	GGD		Changed usages of absolute VIA/SCC hardware addresses in
;									SetUpPoll, to use low mem hw base addresses instead. Changed
;									vBufA to vBufD for HcMac.
;	   <1.6>	 7/15/88	GGD		No changes to this file, entire Sony Driver is checked out and
;									in as a group.
;	   <1.5>	 6/15/88	GGD		No changes to this file, entire Sony Driver is checked out and
;									in as a group.
;	   <1.4>	 5/25/88	GGD		No changes to this file, entire Sony Driver is checked out and
;									in as a group.
;	   <1.3>	 5/24/88	GGD		No changes to this file, entire Sony Driver is checked out and
;									in as a group.
;	   <1.2>	  5/3/88	GGD		No changes, entire sony driver checked out and in as a group.
;	   <1.1>	 4/18/88	GGD		Merged in MFM support <C437> by Steve Christensen Renamed
;									RdVerify to RVerify to eleminate the name conflict with the
;									RdVerify equate in SysEqu.a (removes an ASM warning)
;	   <1.0>	 2/12/88	BBM		Adding file for the first time into EASEÉ
;	  <C965>	11/13/87	MSH		Time-out loop in RdAddr was too short because we are so fast.
;	  <C907>	 10/8/87	MSH		Port to HcMac (Laguna).
;	  <C437>	11/21/86	SWC		Patched RdAddr and RdData to jump to their MFM counterparts if
;									we're running in MFM mode.
;	  <A351>	 11/5/86	TJ		Text cleanup
;	  <A302>	10/30/86	TJ		Removed IWM absolute addresses.
;	   <C55>	 6/25/86	RDC		Increased no-nibble timeout for NuMac
;				 11/2/85	LAK		Use NewIntf instead of Sides when adjusting MustFndCnt.
;				 7/29/85	RDC		Changed interrupt level settings to use equates
;				 7/24/85	LAK		Convert back to WorkShop.
;				 1/15/85	JTC		convert to MDS.
;				10/26/84	LAK		Patched RdAddr to compensate for missed nibbles after write
;									(during erase) and increased no-nibble timeout.
;				 8/18/83	LAK		Addresses of AdrMks and DtaMks tables are passed thru vectors.
;				 8/11/83	LAK		Sets head select according to SideTrack.
;				  8/4/83	LAK		Added verify-only read (does it if lowmem var DskVerify is
;									non-zero).
;				 4/17/83	LAK		Rewrote both RdAddr and RdData to incorporate SCC channel A
;									polling scheme.
;				12/17/82	LAK		Changed nibble "must-find" count to 1500 from 3000.
;				11/18/82	LAK		Added head select setup and a search for some nibbles to avoid
;									long delays on blank tracks
;

			title	'File: SonyRead.a'
			eject

;_______________________________________________________________________
;
; Routine:	RdAddr,SetUpPoll,RdAddrSetup
; Arguments:	  A5.L (input)	-- ptr to 6522 A-reg (has head sel, wait/req)
;		  A6.L (input)	-- ptr to  SCC A-channel data register
;		  A2.L (input)	-- pointer to AdrMks table
;		  D0.W (output) --
;		   0-1499 = no error, 1500 - nybbles before address mark
;			 -66 = no transitions found
;			 -67 = no address mark found  (no header read)
;			 -68 = bad	data nybble (header read aborted)
;			 -69 = bad	checksum (header read but is inconsistent)
;			 -70 = bad	bit slip marks (header read but may be bad)
;		  D1.W (output) -- <side><track> (low-order 12 bits)
;		  D2.W (output) -- <sector>
;		  D3.W (output) -- <volume>
;		  A3.L (output) -- ptr to <DNib-$96> (denibblizing table)
;		  A4.L (output) -- DiskQ6L pointer
;		  D4,D5,A0,A2 are trashed
;		  D6-D7 and A1,A5-A6 are preserved
; Function: This routine reads the next address mark that spins by.  On exit,
;		the attributes of the mark are passed back as:
;
;			D1 = <side> <track>
;			D2 = <sector>
;			D3 = <volume>
;
;		The disk should be up to speed with the correct head
;		selected, and interrupts should be disabled.
;
;		'Nibble must find' count may have to be tuned.
;_______________________________________________________________________

			BLANKS	ON
			STRING	ASIS

AdrMks		DC.B	$D5,$AA,$96,$DE,$AA,$FF


; code-saving proc called by RdAddrSetUp and FigTrkSpeed

SetUpPoll	MOVE.L	JSetUpPoll,-(SP)
			RTS

BogusSCC	DCB.B	6,0					; this is the bogus SCC register set	<SM4>

jtSetUpPoll								;										<SM4>
			TST.L	PollProc			; check to see if there is a pollproc	<SM4>
			BEQ.S	@1					; -> no, point A6 to a bogus place		<SM4>
			MOVEA.L SCCRd,A6			; get SCC read base address				<SM4>
			BRA.S	@2					;										<SM4>
@1			LEA		BogusSCC,A6			; fake SCC base address into A6			<SM4>
@2										;										<SM4>
			ADDQ.L 	#AData,A6			; SCC A-reg data						<1.7>
			MOVEA.L	VIA,A5				; get VIA base address					<1.7>
			ADDA.W 	#VBufD,A5			; 6522 disk-reg has head sel, wait/req	<1.7>
			ORI 	#HiIntMask,SR		; no ints from now on!				<29Jul85>

			MOVE.L	SP,PollStack		; init PollStack at current stack level
			ADD.L	D0,PollStack		;  compensate for 1 or 2 levels
			RTS

RdAddrSetup MOVEQ	#8,D0				; init PollStack 8 bytes down
			BSR.S	SetUpPoll

			MOVEQ	#RdDtaAdr,D0		; assume side 0
			BTST	#3,SideTrack(A1)	; side 1?
			BEQ.S	@1					; br if not
			MOVEQ	#RdDta1Adr,D0		; set for side 1
@1			BSR 	AdrDisk

RdAddr

			BSR		GetDrv1				;Get the drive offset				<C437/21nov86>
			TST.B	mfmDisk(A1,D1)		;Are we in MFM mode?				<C437/21nov86>
			BMI		mRdAddr				;-> yes, do the MFM version			<C437/21nov86>

			LEA 	AdrMks,A2			; point to our mark table
			MOVE.L	JRdAddr,-(SP)
			RTS

jtRdAddr

;	When reading immediatly after a write, the drive may suppress read data for
;	up to 620µsec, so we will wait at least that long for the first byte to be valid.
;	We look for 3 bytes @16µsec per byte brings the total wait to 668µsec.
;	There is an access to the VIA in the loop, which must synchronize with the 783khz
;	clock (1.2765µsec per access), so we compute the timeout for the fastest processor
;	which would be 668/1.2765 = 523 iterations, and add in another 10% for good measure
;	to get 575.  It doesn't hurt for it to be too big on slower processors, it would just
;	take longer to get noNybErr which should never occur unless there is a hardware error.

			MOVEQ	#3,D3				; try for 3 bytes
			MOVE.W	#575,D2				; setup loop timeout count			<2.0>
			MOVEQ	#0,D5				; D5 is used for offsets, so clear it
			movea.l	IWM,A4				;									<A302/30oct86><2.0>
			adda.w	#Q6L,A4 			; A4 points to shiftreg for speed	<A302/30oct86><2.0>

			MOVE.L	(SP)+,DskRtnAdr 	; return address

@1			_PollSCC					; poll the SCC modem port			<H4><SM4>

@2			MOVE.B	(A4),D5 			; read a nibble
			DBRA	D2,@3				; if no timeout yet, see if it was valid

			MOVEQ	#NoNybErr,D0		; set CCR
			BRA 	RdAddrXit

@3			BPL.S	@1					; try again if no nibble
			SUBQ.W	#1,D3				; leave D3=0
			BNE.S	@1					; if we can find 3, should be ok
			
			MOVE.W	#MustFindCt,D0		; D0 holds nibble must find count
			TST.B	NewIntf(A1,D1)		; new interface drive?				<02Nov85>
			BEQ.S	@4
			MOVE.W	#MustFindCt-32,D0

@4			LEA 	(DNib-$96),A3		; A3 points to denibblizing table

RdAddr1 	MOVE.L	A2,A0				; point to our mark table
			MOVEQ	#3,D1				;

@1			MOVE.B	(A4),D5 			;
			BPL.S	@1					;
			_PollSCC					; poll the SCC modem port			<H4><SM4>
@2			DBRA	D0,@3				;
			MOVEQ	#NoAdrMkErr,D0		; after 1500 tries report error
			BRA.W	RdAddrXit

@3			CMP.B	(A0)+,D5			;
			BNE.S	RdAddr1 			;
			SUBQ.W	#1,D1				;
			BNE.S	@1					; leave D1=0  (returns <side><track>)

; We found an address mark header so retrieve the information from the mark.

			_PollSCC					; poll the SCC modem port			<H4><SM4>

RdAddrMk	MOVE.B	(A4),D5 			; read track number byte
			BPL.S	RdAddrMk			;
			MOVE.B	0(A3,D5.W),D1		; denibblize it
			MOVE.B	D1,D4				; init the checksum
			ROR.W	#6,D1				; make room for 6 bits of side . . .

			_PollSCC					; poll the SCC modem port			<H4><SM4>

@2			MOVE.B	(A4),D5 			; read sector number byte
			BPL.S	@2					;
			MOVEQ	#0,D2				; zero extend the sector number				<2.0>
			MOVE.B	0(A3,D5.W),D2		; denibblize it
			EOR.B	D2,D4				; calculate the checksum

			_PollSCC					; poll the SCC modem port			<H4><SM4>

@3			MOVE.B	(A4),D5 			; read side number byte
			BPL.S	@3					;
			MOVE.B	0(A3,D5.W),D1		; denibblize it
			EOR.B	D1,D4				; calculate the checksum
			ROL.W	#6,D1				; D1,bits 0-11 = <side><track>

			_PollSCC					; poll the SCC modem port			<H4><SM4>

@4			MOVE.B	(A4),D5 			; read volume number byte
			BPL.S	@4					;
			MOVE.B	0(A3,D5.W),D3		; denibblize it
			EOR.B	D3,D4				; calculate the checksum

			_PollSCC					; poll the SCC modem port			<H4><SM4>

@5			MOVE.B	(A4),D5 			; read checksum nibble
			BPL.S	@5					;
			MOVE.B	0(A3,D5.W),D5		; denibblize it
			EOR.B	D5,D4
			BNE.S	BadCkSum			; report bad checksum

			MOVEQ	#1,D4				; 2 bit slip marks
RdAddrEnd
			_PollSCC					; poll the SCC modem port			<H4><SM4>

@1			MOVE.B	(A4),D5 			; get first trail nibble			<2.1>
			BPL.S	@1					;									<2.1>
			CMP.B	(A0)+,D5			; trail mark ok?
			BNE.S	NoSlip				; br if not
			DBRA	D4,RdAddrEnd

RdAddrXit	BRA 	DskRtn				; return with D0 positive, BPL for ok

BadCkSum	MOVEQ	#BadCkSmErr,D0
			BRA.S	RdAddrXit

NoSlip		MOVEQ	#BadBtSlpErr,D0
			BRA.S	RdAddrXit
			eject								;							<A351/05nov86>
;_______________________________________________________________________
;
; Routine:	RdData
; Arguments:	  A0.L (input)	-- ptr to 512-byte data buffer
;		  A3.L (input)	-- ptr to <DNib-$96> (denibblizing table)
;		  A4.L (input)	-- DiskQ6L pointer
;		  A5.L (input)	-- ptr to 6522 A-reg (has head sel, wait/req)
;		  A6.L (input)	-- ptr to  SCC channel A data register
;
;		  D0.W (output) --
;			0  = no error
;			  -71 = no	data mark found
;			  -72 = bad checksum
;			  -73 = bad bit slip marks
;		  A3-A6 are preserved
;		  all other registers are trashed
; Function: This routine reads the next data mark that spins by. The 524
;		bytes of data are split as follows: the first 12 bytes are put
;		into TagData, the remaining 512 bytes into the buffer at (A0).
;		This routine denibblizes and computes the checksum on the fly,
;		so it may be used for one-to-one reading.
;
;		The disk should be up to speed with the correct head
;		selected, and interrupts should be disabled.  This routine
;		should be called immediately after RdAddr for disk reads.
;
; To do:
;
; Currently (on the SCC version Macintosh), there is no time to check each
; nibble when denibblizing data nibbles for $FF mappings; this should be added
; when this routine is put in ROM or run on a faster processor.
;
; An optimization could be added for missed address marks: if the calling
; routine is after a particular sector number and already knows which track
; it is on, this routine could be modified to check for that sector in the
; byte just before the encoded data, and return immediately on no match . . .
;_______________________________________________________________________

DtaMks		DC.B	$D5,$AA,$AD,$DE,$AA,$FF

RdData

			BSR		GetDrv1				;Get the drive offset				<C437/21nov86>
			TST.B	mfmDisk(A1,D1)		;Are we in MFM mode?				<C437/21nov86>
			BMI		mRdData				;-> yes, do the MFM version			<C437/21nov86>

			LEA 	DtaMks,A1
			MOVE.L	JRdData,-(SP)
			RTS

jtRdData	MOVE.L	(SP)+,DskRtnAdr 	; save return address here
			MOVEQ	#48,D2				; D2 holds nibble must find count
			MOVEQ	#0,D3				; clear D3 for use as an index
			MOVEQ	#-64,D0 			; $C0 used to mask off bits 0-5
			MOVE.L	#$1FE000A,D4		; byte read counts for 2 buffers
			MOVEQ	#0,D5				; init CkSumA
			MOVEQ	#0,D6				; init CkSumB
			MOVEQ	#0,D7				; init CkSumC

; try to find a valid header until we've timed out

RdData1 	MOVE.L	A1,A2				; address of address marks
			MOVEQ	#3,D1

@1			MOVE.B	(A4),D3
			BPL.S	@1
			_PollSCC					; poll the SCC modem port			<H4><SM4>
@2			DBRA	D2,@3
			MOVEQ	#NoDtaMkErr,D0		; after 48 tries report error
			BRA.S	RdAddrXit

@3			CMP.B	(A2)+,D3
			BNE.S	RdData1
			SUBQ.W	#1,D1
			BNE.S	@1					; leave D1=0

; We found a data mark header so retrieve the information from the mark.


@4			MOVE.B	(A4),D3 			; read the encoded sector number
			BPL.S	@4					;
			LEA 	TagData+1,A1		; A1 points to buffer for extra bytes
			MOVE.B	0(A3,D3.W),(A1)+	;

;_______________________________________________________________________
;
; We found the header so the actual info is about to spin by.  There are 699
; nibbles followed by 3 checksum nibbles.  We use a four nibble loop for the
; data nibbles . . . the first 12 bytes go into a separate buffer, then 512
; bytes are placed in the user's buffer.
;
;	D7 = CkSumC 		A7 = stack (where poll data is pushed)
;	D6 = CkSumB 		A6 = ptr to SCC chan A data reg
;	D5 = CkSumA 		A5 = ptr to 6522 A-reg
;	D4 = loop counters		A4 = ptr to Q6L (IWM)
;	D3 = buffer 		A3 = ptr to denibblizing table
;	D2 = buffer 		A2 = ptr to trail mark table
;	D1 = buffer for odd bits	A1 = ptr to data buffer
;	D0 = $C0 mask			A0 = ptr to user buffer
;_______________________________________________________________________

RdData2 	_PollSCC					; poll the SCC modem port			<H4><SM4>

@0			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	RdData2 			;
			MOVE.B	0(A3,D3.W),D1		; D1 = [00][00][A7][A6][B7][B6][C7][C6]
			ROL.B	#2,D1				; D1 = [A7][A6][B7][B6][C7][C6][00][00]
			MOVE.B	D1,D2				;
			AND.B	D0,D2				; D2 = [A7][A6][00][00][00][00][00][00]

@1			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	@1					;
			OR.B	0(A3,D3.W),D2		; D2 = ByteA' (no time for bad nibs)

			MOVE.B	D7,D3				;
			ADD.B	D7,D3				; ex <- CkSumC[7]
			ROL.B	#1,D7				; CkSumC' <- ROL (CkSumC)

			EOR.B	D7,D2				; ByteA <- ByteA' XOR CkSumC'
			MOVE.B	D2,(A1)+			; store first byte
			ADDX.B	D2,D5				; CkSumA',ex <- CkSumA + ByteA + ex
			ROL.B	#2,D1				; D1 = [B7][B6][C7][C6][00][00][A7][A6]
			MOVE.B	D1,D2				;
			AND.B	D0,D2				; D2 = [B7][B6][00][00][00][00][00][00]

@2			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	@2					;
			OR.B	0(A3,D3.W),D2		; D2 = ByteB' (no time for bad nibs)
			EOR.B	D5,D2				; D2 = ByteB
			MOVE.B	D2,(A1)+			; store second byte
			ADDX.B	D2,D6				; CkSumB',ex <- CkSumB + ByteB + ex

			ROL.B	#2,D1				; D1 = [C7][C6][0][0][A7][A6][B7][B6]
			AND.B	D0,D1				; D1 = [C7][C6][0][0][0][0][0][0]

			_PollSCC					; poll the SCC modem port			<H4><SM4>

@3			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	@3					;
			OR.B	0(A3,D3.W),D1		; D1 = ByteC' (no time for bad nibs)
			EOR.B	D6,D1				; D1 = ByteC
			MOVE.B	D1,(A1)+			; store third byte
			ADDX.B	D1,D7				; CkSumC'' <- CkSumC' + ByteC + ex
			SUBQ.W	#3,D4				; decrement counter by 3 bytes
			BPL.S	RdData2 			; loop	(careful->subq changes ex bit)

			SWAP	D4					; first time thru switch counters, buffers
			TST.B	DskVerify			; doing a read verify?
			BEQ.S	RdData3A			; br if not
			BRA.S	RdVer1				; br if so (only verify, don't read)

RdData3 	_PollSCC					; poll the SCC modem port			<H4><SM4>

RdData3A	MOVE.B	(A4),D3 			; read a nibble
			BPL.S	RdData3 			;
			MOVE.B	0(A3,D3.W),D1		; D1 = [00][00][A7][A6][B7][B6][C7][C6]
			ROL.B	#2,D1				; D1 = [A7][A6][B7][B6][C7][C6][00][00]
			MOVE.B	D1,D2				;
			AND.B	D0,D2				; D2 = [A7][A6][00][00][00][00][00][00]

@1			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	@1					;
			OR.B	0(A3,D3.W),D2		; D2 = ByteA' (no time for bad nibs)

			MOVE.B	D7,D3				;
			ADD.B	D7,D3				; ex <- CkSumC[7]
			ROL.B	#1,D7				; CkSumC' <- ROL (CkSumC)

			EOR.B	D7,D2				; ByteA <- ByteA' XOR CkSumC'
			MOVE.B	D2,(A0)+			; store first byte
			ADDX.B	D2,D5				; CkSumA',ex <- CkSumA + ByteA + ex
			ROL.B	#2,D1				; D1 = [B7][B6][C7][C6][00][00][A7][A6]
			MOVE.B	D1,D2				;
			AND.B	D0,D2				; D2 = [B7][B6][00][00][00][00][00][00]

@2			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	@2					;
			OR.B	0(A3,D3.W),D2		; D2 = ByteB' (no time for bad nibs)
			EOR.B	D5,D2				; D2 = ByteB
			MOVE.B	D2,(A0)+			; store second byte
			ADDX.B	D2,D6				; CkSumB',ex <- CkSumB + ByteB + ex
			TST.W	D4					; check counter
			BEQ.W	RdCkSum 			; we are done when counter is 0

			ROL.B	#2,D1				; D1 = [C7][C6][0][0][A7][A6][B7][B6]
			AND.B	D0,D1				; D1 = [C7][C6][0][0][0][0][0][0]

			_PollSCC					; poll the SCC modem port			<H4><SM4>

@3			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	@3					;
			OR.B	0(A3,D3.W),D1		; D1 = ByteC' (no time for bad nibs)
			EOR.B	D6,D1				; D1 = ByteC
			MOVE.B	D1,(A0)+			; store third byte
			ADDX.B	D1,D7				; CkSumC'' <- CkSumC' + ByteC + ex
			SUBQ.W	#3,D4				; decrement counter by 3 bytes
			BRA.S	RdData3 			; loop	(careful->subq changes ex bit)

RVerify		_PollSCC					; poll the SCC modem port			<H4><SM4>

RdVer1		MOVE.B	(A4),D3 			; read a nibble
			BPL.S	RVerify				;										<1.1/12feb88>
			MOVE.B	0(A3,D3.W),D1		; D1 = [00][00][A7][A6][B7][B6][C7][C6]
			ROL.B	#2,D1				; D1 = [A7][A6][B7][B6][C7][C6][00][00]
			MOVE.B	D1,D2				;
			AND.B	D0,D2				; D2 = [A7][A6][00][00][00][00][00][00]

@1			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	@1					;
			OR.B	0(A3,D3.W),D2		; D2 = ByteA' (no time for bad nibs)

			MOVE.B	D7,D3				;
			ADD.B	D7,D3				; ex <- CkSumC[7]
			ROL.B	#1,D7				; CkSumC' <- ROL (CkSumC)

			EOR.B	D7,D2				; ByteA <- ByteA' XOR CkSumC'
			CMP.B	(A0)+,D2			; check first byte
			BNE.S	@4					; (to RdVerErr) exit on errors
			ADDX.B	D2,D5				; CkSumA',ex <- CkSumA + ByteA + ex
			ROL.B	#2,D1				; D1 = [B7][B6][C7][C6][00][00][A7][A6]
			MOVE.B	D1,D2				;
			AND.B	D0,D2				; D2 = [B7][B6][00][00][00][00][00][00]

@2			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	@2					;
			OR.B	0(A3,D3.W),D2		; D2 = ByteB' (no time for bad nibs)
			EOR.B	D5,D2				; D2 = ByteB
			CMP.B	(A0)+,D2			; compare second byte
			BNE.W	RdVerErr			; exit on errors
			ADDX.B	D2,D6				; CkSumB',ex <- CkSumB + ByteB + ex
			TST.W	D4					;
			BEQ.S	RdCkSum 			; we are done when counter is 0

			ROL.B	#2,D1				; D1 = [C7][C6][0][0][A7][A6][B7][B6]
			AND.B	D0,D1				; D1 = [C7][C6][0][0][0][0][0][0]

			_PollSCC					; poll the SCC modem port			<H4><SM4>

@3			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	@3					;
			OR.B	0(A3,D3.W),D1		; D1 = ByteC' (no time for bad nibs)
			EOR.B	D6,D1				; D1 = ByteC
			CMP.B	(A0)+,D1			; compare third byte
@4			BNE.S	RdVerErr			; exit on errors
			ADDX.B	D1,D7				; CkSumC'' <- CkSumC' + ByteC + ex
			SUBQ.W	#3,D4				; decrement counter by 3 bytes
			BRA.S	RVerify				; loop	(careful->subq changes ex bit)	<1.1/12feb88>

; now read the four checksum nibbles and compare . . .

RdCkSum 	_PollSCC					; poll the SCC modem port			<H4><SM4>

@0			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	@0					;
			MOVE.B	0(A3,D3.W),D1		; D1 = [00][00][A7][A6][B7][B6][C7][C6]
			BMI.S	DCkSumBad			; branch if bad nibble
			ROL.B	#2,D1				; D1 = [A7][A6][B7][B6][C7][C6][00][00]
			MOVE.B	D1,D2				;
			AND.B	D0,D2				; D2 = [A7][A6][00][00][00][00][00][00]

@1			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	@1					;
			MOVE.B	0(A3,D3.W),D3		; D3 = [0][0][A5][A4][A3][A2][A1][A0]
			BMI.S	DCkSumBad			; br if bad nibble
			OR.B	D3,D2				; D2 = CkSumA
			CMP.B	D2,D5				; check against calculated value
			BNE.S	DCkSumBad			; br if bad
			ROL.B	#2,D1				; D1 = [B7][B6][C7][C6][00][00][A7][A6]
			MOVE.B	D1,D2				;
			AND.B	D0,D2				; D2 = [B7][B6][00][00][00][00][00][00]

@2			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	@2					;
			MOVE.B	0(A3,D3.W),D3		; D3 = [0][0][B5][B4][B3][B2][B1][B0]
			BMI.S	DCkSumBad			; branch if bad nibble
			OR.B	D3,D2				; D2 = CkSumB
			CMP.B	D2,D6				; check against calculated value
			BNE.S	DCkSumBad			; br if bad
			ROL.B	#2,D1				; D1 = [C7][C6][00][00][A7][A6][B7][B6]
			AND.B	D0,D1				; D1 = [C7][C6][00][00][00][00][00][00]

			_PollSCC					; poll the SCC modem port			<H4><SM4>

@3			MOVE.B	(A4),D3 			; read a nibble
			BPL.S	@3					;
			MOVE.B	0(A3,D3.W),D3		; D3 = [00][00][C5][C4][C3][C2][C1][C0]
			BMI.S	DCkSumBad			; branch if bad nibble
			OR.B	D3,D1				; D1 = CkSumC
			CMP.B	D1,D7				; check against calculated value
			BEQ.S	RdSlip				;

DCkSumBad	MOVEQ	#BadDCkSum,D0		; bad checksum
			BRA.S	RdDataXit

RdVerErr	MOVEQ	#DataVerErr,D0
			BRA.S	RdDataXit

; finally we must read the bit slip marks before giving our stamp of approval

RdSlip		MOVEQ	#1,D4				; 2 bit slip marks
@1			MOVE.B	(A4),D3 			; get next trail nibble
			BPL.S	@1					;

			_PollSCC					; poll the SCC modem port			<H4><SM4>

@2			CMP.B	(A2)+,D3			; trail mark ok?
			BNE.S	NoDSlip 			; br if not
			DBRA	D4,@1

			MOVEQ	#0,D0				; report success!!!
RdDataXit	BRA 	DskRtn				; return with D0 positive, BPL for ok

NoDSlip 	MOVEQ	#BadDBtSlp,D0		; bad slip nibbles is error code 3 .	. .
			BRA.S	RdDataXit

; Denibblizing Table (nibbles $96 thru $FF converted to $00 thru $3F)
; $FF means illegal nibble: if we checked for illegal nibbles (currently we
; don't have time for it), this table could be extended down to $80 . . .

DNib

			DC.B	$00,$01,$FF,$FF,$02,$03 ; $96,$97,	 ,	 ,$9A,$9B
			DC.B	$FF,$04,$05,$06,$FF,$FF ;	 ,$9D,$9E,$9F,	 ,
			DC.B	$FF,$FF,$FF,$FF,$07,$08 ;	 ,	 ,	 ,	 ,$A6,$A7
			DC.B	$FF,$FF,$FF,$09,$0A,$0B ;	 ,	 ,	 ,$AB,$AC,$AD
			DC.B	$0C,$0D,$FF,$FF,$0E,$0F ; $AE,$AF,	 ,	 ,$B2,$B3
			DC.B	$10,$11,$12,$13,$FF,$14 ; $B4,$B5,$B6,$B7,	 ,$B9
			DC.B	$15,$16,$17,$18,$19,$1A ; $BA,$BB,$BC,$BD,$BE,$BF
			DC.B	$FF,$FF,$FF,$FF,$FF,$FF ;	 ,	 ,	 ,	 ,	 ,
			DC.B	$FF,$FF,$FF,$FF,$FF,$1B ;	 ,	 ,	 ,	 ,	 ,$CB
			DC.B	$FF,$1C,$1D,$1E,$FF,$FF ;	 ,$CD,$CE,$CF,
			DC.B	$FF,$1F,$FF,$FF,$20,$21 ;	 ,$D3,	 ,	 ,$D6,$D7
			DC.B	$FF,$22,$23,$24,$25,$26 ;	 ,$D9,$DA,$DB,$DC,$DD
			DC.B	$27,$28,$FF,$FF,$FF,$FF ; $DE,$DF,
			DC.B	$FF,$29,$2A,$2B,$FF,$2C ;	 ,$E5,$E6,$E7,	 ,$E9
			DC.B	$2D,$2E,$2F,$30,$31,$32 ; $EA,$EB,$EC,$ED,$EE,$EF
			DC.B	$FF,$FF,$33,$34,$35,$36 ;	 ,	 ,$F2,$F3,$F4,$F5
			DC.B	$37,$38,$FF,$39,$3A,$3B ; $F6,$F7,	 ,$F9,$FA,$FB
			DC.B	$3C,$3D,$3E,$3F 		; $FC,$FD,$FE,$FF