Atari 400 Technical Reference Manual page 125

The handler will perform the indicated operation, if possible,
and return to CIa with the operation status in register Y.
The IaCB need not be opened nor closed in order for you
to re~uest CIa to perform a SPECIAL operation; the handler
must check where there are restrictions. See Section 5 for a
discussion of the CIa actions involved with a SPECIAL
operation using both open and closed IaCB's, and note the
impact of this on the use of the buffer address parameter.
Error Handling
Error handling has been simplified somewhat by having CIa handle
outer level errors and having SIO handle Serial bus errors,
leaving the handler to process the remaining errors. These
errors include:
out-of-range parameters.
[BREAK] key abort.
Invalid command.
Read after end of file.
The current handlers respond to errors using the following
guidelines:
They keep the recovery simple (and therefore predictable and
repeatable).
They Do not interact directly with you for recovery
instructions.
They lose as little data as possible.
They make all attempts to maintain the integrity of file
oriented device storage -- this involves the initial design
of the structural elements as well as error recovery
techniques.
Resource Allocation
Nonresident handlers needing code and/or data space in RAM should
use the techniques listed below, to assure nonconflict with other
parts of the OS, including other nonresident handlers.
OPERATING SYSTEM C016555 -- Section 9
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