5121



     store information about files currently being processed. The
     eight FCB's are 16-byte blocks that correspond in a one-on-one
     manner with the IOCB's. Each FCB consist of:

     Label    Bytes   Purpose
     FCBFNO   1       File number of the current file being
     processed.

     FCBOTC   1       Which mode the file has been OPENed for:
     append is one, directory read is two, input is four, output is
     eight, update is twelve.

     SPARE    1       Not used.

     FCBSLT   1       Flag for the sector length type; 128 or 256
     bytes

     FCBFLG   1       Working flag. If equal to 128 ($80), then the
     file has been OPENed for output or append and may acquire new
     data sectors. If the value is 64, then sector is in the memory buffer
     awaiting writing to disk.

     FCBMLN   1       Maximum sector data length; 125 or 253 bytes
     depending on drive type (single or double density). The last
     three sector bytes are reserved for sector link and byte count
     data.

     FCBDLN   1       Current byte to be read or modified in the
     operation in a data sector.

     FCBBUF   1       Tell FMS which buffer has been allocated
     to the file being processed.

     FCBCSN   2       Sector number of the sector currently in the
     buffer.

     FCBLSN   2       Number of the next sector in data chain.

     FCBSSN   2       Starting sectors for appended data if the file
     has been OPENed for append.

     FCBCNT   2       Sector count for the current file.

     DUP doesn't use these FCB's; it writes to the IOCB's directly.
     CIO transfers the control to FMS as the operation demands, then
     on to SIO.

5121           1401           FILDIR
     File directory, a 256 ($100) byte sequential buffer for entries to
     the disk directory.

5377           1501           ENDFMS
     Disk directory (VTOC -- Volume Table Of Contents) buffer. 64
     ($40) bytes are reserved, one byte for each possible file. It also
     marks the end of FMS. The VTOC (sector 360; $168) is a
     sequential bit map of each of the 720 sectors on the disk. It starts
     at byte ten and continues through to byte 99. When a bit is set