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!
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 Thank you again! -AJ On Sun, Nov 15, 2015 at 4:49 AM, Lawrence Wilkinson <ljw-cct...@ljw.me.uk> wrote: > On 15/11/15 11:15, Microtech Dart wrote: > > (snip) >> "The cyclical redundancy check (CRC) shall consist of two bytes, >> calculated >> over the 512 bytes of interchange data, and the 4-byte block address, >> *starting >> with all ONEs, CRC initial value*, and using the CRC Generating >> polynomial: >> x16 + x12 + x5 + 1 >> >> *"starting with all ONEs, CRC initial value"*? What does THAT mean? Do I >> need to do some kind of register shift? xorin or xorout? >> > It's the initial value of each bit, corresponding to the "Init before > calculation" lines in the code at the bottom of the page. There doesn't > seem to be any provision for entering this value in the online > computation (and all-zeros is *not* a good initialisation vector!) > > One way around this would be to come up with a byte sequence that > gave a CRC of all ones (i.e. 84 CF) and pre-pend that to the main > sequence, and then it works. But I assume you're using some sort of > code, so just make sure it's initialised appropriately. > > >> I've even played with http://reveng.sourceforge.net/, but I'm having >> trouble even understanding the meaning of the input values and parameters >> with this. >> >> I appreciate any feedback that anyone can give, here. >> >> Best of luck! CRCs are fun! > > -- > Lawrence Wilkinson lawrence at ljw.me.uk > The IBM 360/30 page http://www.ljw.me.uk/ibm360 > > -- Thanks, -AJ http://MicrotechM1.blogspot.com
