Atari 400 Technical Reference Manual page 123

Handlers that do not have short timeouts associated with the
reading of data (such as the Keyboard and Cassette Handlers),
must monitor the [BREAK] key flag BRKKEY [0011] and return with a
status of $80 when a [BREAK] condition occurs. See Appendix L,
E5i and Section 12 for a discussion of [BREAK] key monitoring.
CIO checks for reads from device/files that have not been opened
or have been opened for output only; the handler will not be called in
those cases.
PUTBYTE
This entry is called in response to a PUT CHARACTERS
or
PUT
RECORD command to CIO. The handler is expected to accept a single
byte in the A register
or
return an
error
status in the Y
register.
At handler entry, the following parameters can be of interest:
x
=
index to originating IOCB.
V
=
$92
(status
=
function not implemented by handler).
A = data byte.
ICDNOZ [0021]
=
device number (1-4, for multiple device
handlers).
ICAX1Z/ICAX2Z [002A/002B]
=
device-specific information.
The handler sends the data byte directly to the device, or to a
handler-maintained buffer, and returns to CIO with the operation
status in the Y register. If a handler-maintained buffer fills,
the handler will send the buffered data to the device before
returning to CIO.
CIO checks for WRITEs to device/files that have not been opened,
or have been opened for input only. The handler will not be called in
those cases.
Now
that the normal operation of PUTBYTE has been defined, a
special case must be added. Any handler that will operate within
the environment of the ATARI 8K BASIC language interpreter has a
different set of rules. Because BASIC can call the handler
PUTBVTE entry directly. without going through CIO, the zero-page
IOCB (ZIOCB) can or may not have a relation to the PUTBYTE call.
Therefore, the handler must use the outer level IOCB to obtain
any information that would normally be obtained from ZIOeB. Note
also that the OPEN protection normally provided by CIO is
bypassed (i. e. PUTBYTE to a non-OPEN device/file and PUTBVTE to a
read-only OPEN).
OPERATING SYSTEM C016555 -- Section 9
138