3.2.6) What is the Percom configuration block?

The following is largely verbatim from Bill Wilkinson's "Insight: Atari"
column, Compute! magazine #65, October 1985, page 110-111.  See:

As defined by the Percom standard (Percom was the first manufacturer of disk
drives for the Atari other than Atari), a config block is a set of 12 bytes
within the memory of the disk control microprocessor--which is inside your
disk drive(s).  You read a drive's config block by passing "N" to it as an SIO
command.  You can write a new config block to a drive via an "O" command.  The
"N" and "O" commands closely parallel the "R" and "W" sector input/output
commands, except the data length is always 12 bytes and, no sector number is
needed.  The 12 bytes in the block are shown in the table:

   Byte #  # of   Description
     0       1    Number of Tracks
     1       1    Step Rate (values have no universal meaning)
    2-3      2    Sectors per Track (byte 2 is high byte; byte 3 is low byte)
     4       1    Number of Sides or Heads (0=one head, 1=two heads)
     5       1    Density (0=FM/Single, 4=MFM/Double)
    6-7      2    Bytes per Sector (byte 6 is high byte; byte 7 is low byte)
     8       1    Drive Selected?
     9       1    Serial Rate Control (values have no universal meaning)
   10-11     2    Miscellaneous (reserved)

The successful establishment of the Percom standard allowed for Atari disk
drives capable of either single or double density operations, and thus for
Atari users with Percom compatible drives to use single or double density
disks interchangeably.  However, the extent of support for the full standard
varies widely among particular disk drives.  Atari's own 810 and 1050 drives
do not support the Percom config block at all, but the Atari XF551 and most
3rd party disk drives designed for the Atari support at least portions of the
Percom standard.  For example, the Indus GT supports Percom config block bytes
5-7 (Density and Bytes/Sector) only.
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