ZiLOG Z80-RIO Operating System User's Manual page 30

Data Transfer Address
The memory address at which data movement will begin.
Data Length
Number of bytes of information in the transfer. This will
be reset by the device to reflect the actual number of
bytes transferred upon completion of the operation.
Completion Return Address
If bit 0 (least significant) of the request code is set
(=1), those devices which are interrupt-driven will return
control to the calling routine as soon as possible and
continue the operation under interrupt control. At the
time that the operation is completed, transfer will be made
to the completion address which should exercise the
responsibilities of an interrupt service routine (i.e., it
must preserve all registers). However, an RETI instruction
should not be executed, since lower-level interrupts are
enabled by the interrupt handler. (If immediate return is
desired, care must be taken not to change any words in the
parameter vector, or use or change the data, until the
operation is complete.) If return on completion is
indicated (i.e., bit 0 is reset=O) the completion return
address is ignored.
Error Return Address
If nonzero, the error return address will be used as the
return address in the event of an error condition. The
routine thus entered should execute an RET instruction
after processing the error condition. Since the error
condition is detected by the I/O driver, and the call to
the error return address is made there, the programmer
should not make assumptions about the elements on the
stack above the return address.
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