===========================================================================
 BBS: The Abacus * HST/DS * Potterville MI
Date: 05-23-93 (17:46)             Number: 22
From: AARON LAPIKAS                Refer#: NONE
  To: ALL                           Recvd: NO  
Subj: Ymodem                         Conf: (35) Quick Basi
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Can someone tell me the difference between Xmodem 1K and Ymodem?  I know that th
ey are very similar (if not the same?), but the code for an Xmodem 1K transfer w
ill not work for a Ymodem transfer.

Also, I am looking to speed up my Xmodem 1K protocol.  Can anyone offer any advi
ce in accomplishing this?  Does anyone know a faster way to calculate a 16 bit C
RC than:

--------------------------------------------------------------------------- FUNC
TION CalcCRC& (B$, CRCHigh%, CRCLow%)      'Calculates CRC for Each Block

DIM CRC AS LONG
CRC = 0                                        'Reset for Each Text Block

FOR I = 1 TO LEN(B$)                           'Calculate for Length of Block
   ByteVal = ASC(MID$(B$, I, 1))

   TestBit = ((CRC AND 32768) = 32768) XOR ((ByteVal AND 128) = 128)
   CRC = ((CRC AND 32767&) * 2&)
   IF TestBit THEN CRC = CRC XOR &H1021&

   TestBit = ((CRC AND 32768) = 32768) XOR ((ByteVal AND 64) = 64)
   CRC = ((CRC AND 32767&) * 2&)
   IF TestBit THEN CRC = CRC XOR &H1021&

   TestBit = ((CRC AND 32768) = 32768) XOR ((ByteVal AND 32) = 32)
   CRC = ((CRC AND 32767&) * 2&)
   IF TestBit THEN CRC = CRC XOR &H1021&

   TestBit = ((CRC AND 32768) = 32768) XOR ((ByteVal AND 16) = 16)
   CRC = ((CRC AND 32767&) * 2&)
   IF TestBit THEN CRC = CRC XOR &H1021&

   TestBit = ((CRC AND 32768) = 32768) XOR ((ByteVal AND 8) = 8)
   CRC = ((CRC AND 32767&) * 2&)
   IF TestBit THEN CRC = CRC XOR &H1021&

   TestBit = ((CRC AND 32768) = 32768) XOR ((ByteVal AND 4) = 4)
   CRC = ((CRC AND 32767&) * 2&)
   IF TestBit THEN CRC = CRC XOR &H1021&

   TestBit = ((CRC AND 32768) = 32768) XOR ((ByteVal AND 2) = 2)
   CRC = ((CRC AND 32767&) * 2&)
   IF TestBit THEN CRC = CRC XOR &H1021&

   TestBit = ((CRC AND 32768) = 32768) XOR ((ByteVal AND 1) = 1)
   CRC = ((CRC AND 32767&) * 2&)
   IF TestBit THEN CRC = CRC XOR &H1021&
NEXT I

CRCHigh% = (CRC \ 256)                          'Break Word Down into Bytes
CRCLow% = (CRC MOD 256)                         ' for Comparison Later
ComputeCRC& = CRC                               'Return the Word Value

END FUNCTION
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I used to have the three lines:

   IF TestBit THEN CRC = CRC XOR &H1021&
   CRC = ((CRC AND 32767&) * 2&)
   TestBit = ((CRC AND 32768) = 32768) XOR ((ByteVal AND 4) = 4)

in a FOR/NEXT loop, but it is slightly quicker without the loop.  Any other idea
s would be GREATLY appreciated.

Aaron

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