QIC Tape help with CRC - Dwight?

I guess I should throw my own experience on this. First off, it's useful to think about a CRC as just another checksum. Instead of using a simple addition, however, a somewhat more complex (table-drive) operation is used (that's your polynomial). In other words, if S is the CRC sum and I is the current input, a CRC is nothing more than S = sum(S,I) for every I. In the case of an arithmetic checksum the function "sum" is a simple addition. In a CRC it's more involved. You can see that the initial value of S is important. In CRCs, this initial value can be anything, but usually, it's 0 or all binary ones. Note that CRC calculation doesn't use arithmetic per se, but rather boolean ops and shifts. The character of the operation is dictated by the CRC polynomial. I'll go out on a limb here and say that most often the polynomial used in storage devices, where a 16-bit CRC is calculated is the standard CCITT CRC-16 one. There are exceptions, but really, assuming that polynomial is an excellent starting point. Once you've selected a polynomial candidate, the second thing to do is to determine exactly what the input to the CRC computation is. You'd think, for example, that on most IBM-type floppy disks, that it's the data in a sector--and you'd be wrong. Most floppy CRC computation includes the data address mark (DAM) as well. So, in a typical 1.44M IBM floppy, that data address mark would be hex "FB", followed by sector data. The third thing to consider is what order that bits are presented to the CRC calculator--that is, high-order or low-order bit first. Floppy disks mostly go one way and USRTs (synchronous comm) go the other way. So you could have the same value treated two different ways and get two completely different CRCs, based on the same polynomial. This is a real gotcha when dealing with early disk formats, as many of those used an inexpensive USRT or shift register rather than an expensive-for-the-time LSI controller. I find it useful to have a pair of simple test programs that, given a CRC polynomial and initial value and an input file, produce a CRC for comparison. One routine calculates little-bit endian and the other does the same for big-endian calculations. CRC_reveng is a great routine, but much of the time, it's overkill. For whatever it's worth, Chuck