ZiLOG Z80-RIO Operating System User's Manual page 136

The parameter is set up as follows:
(IY)
(IY+l)
(IY+2)
(IY+4)
(IY+6)
(IY+8)
Logical unit number (1 byte) -
identifies the particular dataset
being accessed. Used by
as
to
route the call to the correct driver.
Request code (1 byte) - identifies
the action to be taken. Two request
codes are given for each operation
in the following list. One is even,
the other odd. If the first is
used, the return will be on completion
of the operation. If the second is
used, then return will be immediate,
with completion occurring under
interrupt control.
Data transfer area (2 bytes) - gives
the address at which data transfer
is to begin. If no data transfer is
expected for a given request, this
field should be zero.
Length (2 bytes) - gives the length
of the operation. For most operations,
this is the number of bytes to transfer,
but refer to specific operation
descriptions. If no data transfer is
expected, the length should be zero.
On return, this gives the length
actually completed.
Completion return address (2 bytes) -
specifies address to branch to when
the operation completes if the request
code specified immediate return. If
return on completion is specified,
this element is ignored.
Error return address (2 bytes) - if
non-zero, specifies the address to
branch to if an error occurs. The
error will still be indicated in the
completion return code. If the
address is zero, return is as though
there were no error.
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