768



A device handler is a routine used by the OS to control the transfer of
data in that particular device for the task allotted (such as read, write,
save, etc.). The resident D: handler does not conform entirely with the
other handler--SIO calling routines. Instead, you use the DCB to
communicate directly with the disk handler. The device handler for R:
is loaded in from the 850 interface module. See De Re Atari, the 850
Interface Manual, and the OS Listings pages 64 - 65.

Locations 768 to 779 ($300 to $30B) are the resident Device Control
Block (DCB) addresses, used for I/O operations that require the serial
bus; also used as the disk DCB. DUP.SYS uses this block to interface
the FMS with the disk handler. The Atari disk drive uses a serial access
at 19,200 baud (about 20 times slower than the Apple!). It has its own
microprocessor, a 6507, plus 128 bytes of RAM, a 2316 2K masked
ROM chip (like a 2716), a 2332 RAM-I/O timer chip with another 128
bytes of RAM (like the PIA chip) and a WD 1771 FD controller chip.
See the "Outpost Atari" column, Creative Computing, May 1982, for
an example of using the disk DCB.

All of the parameters passed to SIO are contained in the DCB. SIO
uses the DCB information and returns the status in the DCB for
subsequent use by the device handler.

768            300            DDEVIC
     Device serial bus ID (serial device type) set up by the handler,
     not user-alterable. Values are:

     Disk drives  Dl - D4   49-52   ($31-$34)
     Printer      P1           64       ($40)
     Printer      P2           79       ($4F)
     RS232 ports  R1-R4     80-83   ($50-$53)

769            301            DUNIT
     Disk or device unit number: one to four, set up by the user.

770            302            DCOMND
     The number of the disk or device operation (command) to be
     performed, set by the user or by the device handler prior to
     calling SIO. Serial bus commands are:

     Read             82    ($52)
     Write (verily)   87    ($57)
     Status           83    ($53)
     Put (no verify)  80    (0)
     Format           33    ($21)
     Download         32    ($20)
     Read address     84    ($54)
     Read spin        81    ($51)
     Motor on         85    ($55)
     Verify sector    86    ($56)