42



     decreased by one for each byte transferred.

42             2A             ICAX1Z
     Auxiliary information first byte used in OPEN to specify the type
     of file access needed.

43             2B             ICAX2Z
     CIO working variables, also used by some serial port functions. 
     Auxiliary information second byte.

44,45          2C,2D          ICAX3Z/4Z
     Used by BASIC NOTE and POINT commands for the transfer of
     disk sector numbers. These next four bytes to location 47 are also
     labelled as: ICSPRZ and are defined as spare bytes for local CIO
     use.

46             2E             ICAX5Z
     The byte being accessed within the sector noted in locations 44
     and 45. It is also used for the IOCB Number multiplied by 16.
     Each IOCB block is 16 bytes long. Other sources indicate that the
     6502 X register also contains this information.

47             2F             ICAX6Z
     Spare byte. Also labelled CIOCHR, it is the temporary storage
     for the character byte in the current PUT operation.

-------------------------------------------------------------------

48             30             STATUS
     Internal status storage. The SIO routines in ROM use this byte to
     store the status of the current SIO operation. See page 166 of the
     OS User's Manual for status values. STATUS uses location 793
     ($319) as temporary storage. STATUS is also used as a storage
     register for the timeout, BREAK abort and error values during
     SIO routines.

49             31             CHKSUM
     Data frame checksum used by SIO: single byte sum with carry to
     the least significant bit. Checksum is the value of the number of
     bytes transmitted (255; $FF). When the number of transmitted
     bytes equals the checksum, a checksum sent flag is set at location
     59 ($3B). Uses locations 53773 ($D20D) and 56 ($38) for com-
     parison of values (bytes transmitted).

50,51          32,33          BUFRLO/HI
     Pointer to the data buffer, the contents of which are transmitted
     during an I/O operation, used by SIO and the Device Control
     Block (DCB); points to the byte to send or receive. Bytes are
     transferred to the eight-bit parallel serial output holding register
     or from the input holding register at 53773 ($D20D). This register