CRC Error Sector:

The CRC, or cyclic redundancy checksum, is another data item affixed to each sector by the 1771. This is an error check code, so that bad data could be identified by the 810. What is done here is that the CRC is intentionally set to a bad value, such that good data would generate a CRC error. The application program expects the CRC error in certain sectors throughout the disk, and attempts to read these sectors. As with all non-standard sectors, the error must be present, or the program will not run. One of the neat things that is done is to combine CRC errors and deleted data marks in one sector. The unmodified 810 could never hope to duplicate these sectors, but will return to the program error status indicating the bad CRC or deleted data mark. How is this done?