Introduction to CRC Routines
Initialization - CRCnCLR, CRC3INIT
CRC Updating   - CRCnUPD
CRC Completion - CRCnDONE
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                  CRC Routines

The CRC Routines may be used to check the validity of an
incoming serial byte stream or check for validity of data
copies.  These routines compute and check a true 16-bit
Cyclic  Redundancy Code (CRC).

    The use of these routines will guarantee detection of:
	1) all single- and double-bit errors
	2) all errors with an odd number of error bits
	3) all burst errors of length 16 or less

    Additionally, the following statistics:
	1) 99.9969% of all 17-bit error bursts
	2) 99.9984% of all possible longer error bursts

CRC and CRC3 routines use the X^16+X^12+X^5+1 polynomial
commonly used in Floppy disk controllers and Modem programs.
The CRC1 routine uses the X^16+X^15+X^2+1 polynomial also
used in Synchronous communications while CRC2 uses the
X^16+X^15+X^13+X^7+X^4+X^2+X+1 polynomial found in Public-
Domain File CRC checking programs.

All four families of routines are used in the same way, except
that a unique initialization routine, CRC3INIT, is required
with CRC3 before any processing is performed.  This is because
the CRC3 update routine is Table-driven compared to the others
which calculate the CRC "on the fly".  The added routine builds
the table of pre-computed remainders.  Typical use is as:

 CRCMAKE:		; Call CRC3INIT here if using CRC3
	CALL	CRCCLR		; Clear the CRC
	  <loop CALLing CRCUPD>	; ..acquire values
	CALL	CRCDONE		; Get the finished value
	LD	(CRCVAL),HL	; ..and save
 CRCCK:			; Routine to check incoming CRC
	CALL	CRCCLR		; Clear CRC
	  <loop CALLing CRCUPD>	; ..acquire new set of values
	CALL	CRCDONE		; Get the finished value in HL
	LD	DE,(CRCVAL)	; Get the first value in DE
	CALL	COMPHD		; Compare HL to DE
	JR	NZ,ERROR	; ..Error if CRC's Don't match

CRCCLR, CRC1CLR, CRC2CLR, CRC3CLR - Clear CRC Accumulator

  ENTER: None
  EXIT : None
  USES : None

 Usage:  These routines must be executed before beginning a new
  operation on a File, Block or Sector of data.  Do NOT call
  these routines before completing calculations on a block or
  the accumulated data will be incorrect.

 CRC3INIT - Initialize CRC3 table for use

  ENTER: HL = Pointer to 512-byte space for table
  EXIT : None
  USES : AF,BC,DE

 Usage:  This routine MUST be executed before using the CRC3
  Updating routine, or incorrect results will be obtained.  It
  initializes a table with pre-computed CRC remainders for use
  by the updating routine.

CRCUPD, CRC1UPD, CRC2UPD, CRC3UPD - Update CRC Accum. Count

  ENTER: A = Byte to be included in CRC
  EXIT : None
  USES : None

 Usage: This routine is called once for every byte to be
  included in the CRC calculation.

CRCDONE, CRC1DONE, CRC2DONE, CRC3DONE - Return CRC Value

  ENTER: None
  EXIT : HL = Calculated CRC Value
  USES : HL

 Usage: This routine is used to terminate CRC accumulation and
  return the calculated 16-bit CRC value.