CAS file format

Files ending in .CAS are cassette tape images, derived from Dwight Elvey’s .IMG files. The .CAS files have all the bytes needed to be read by the Poly ROM’s tape loader, and probably match what was on the original Poly tape. Here is the format:

    31 bytes of E6 hex (the “sync” bytes before/between tape blocks)
    One byte of 01 hex (the SOH character preceding each record header)
    The record header, which includes:
            8 bytes of file name (space filled if needed)
            Record number (2 bytes, LS first, starts at 0000)
            Record length (1 bytes, 00 implies 256 bytes)
            Load address (2 bytes, LS first)
            Record type (1 byte, see below)
            Record checksum (such that all of the above, starting with file name, add to zero modulo 256)

The record header is immediately followed by data bytes, if any (up to 256) and a checksum which covers only those bytes. The record types are:

    00 – absolute (typical data for executable files)
    01 – comment
    02 – end
    03 – start address for executables
    04 – data (used by Assembler?)
    05 – BASIC program file
    06 – end (used by Assembler?)