QIC Tape help with CRC - Dwight?

Lawrence Wilkinson ljw-cctech at ljw.me.uk
Mon Nov 16 00:24:53 CST 2015

Ah, I thought you were writing a specific program, but if you're
just documenting things then specifying the CRC polynomial and
the init value as all-ones should be sufficient.

The corresponding init parameter of reveng is "-i ffff".

On 15/11/15 23:52, Microtech Dart wrote:
> Lawrence, Dwight & Chuck...thank you!
> The revelation that I can append 84CF to the 512-byte data block and the
> 4-byte block address, is golden, and this works!
The other thing to point out is that if you append the CRC to
your original block, the resulting CRC should be 0000.

e.g. 84 CF FF FF gives 0000

This means there's no need to do a specific 'CRC check', but
after your data block you keep feeding the CRC bytes in and
then check that the result is 0000.

> It would have taken a lot longer for me to figure out on my own that this
> was a work-around, and at least one working interpretation of the confusing
> text I was quoting from the manuals.
> I don't grasp all of the rest of what you shared fully, but now I at least
> have an example of something that works to play with, and from there, I
> hope to get more comfortable with this procedure, and write some programs
> that do this checking to validate each block I read with my Universal QIC
> reader system.
> I've updated my web page about QIC-24 decoding CRC section to include this
> knowledge:
> http://mightyframe.blogspot.com/2015/08/qic-24-tape-data-block-format-decoding.html#CRC
When specifying a CRC it's always good to provide a short
example that can be used to validate an implementation - it
doesn't need to be a whole block, but just 10 or so bytes
together with the resulting CRC.
> Thank you again!
> -AJ
Lawrence Wilkinson                          lawrence at ljw.me.uk
The IBM 360/30 page                   http://www.ljw.me.uk/ibm360

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