ZiLOG Z80-RIO Operating System User's Manual page 20

'I/O unit') which enables I/O activity independent of a
particular device. Units are 'defined' by linking or
mapping a unit to a given device. I/O requests may not be
made on undefined units, although some requests inherently
result in unit definition.
Three units are predefined by RIO to handle console input
(unit 1), console output (unit 2) and high volume printed
output (unit 3). Unit 0 is used by system functions and is
not available to the user. Units 4-20 (in the standard
system) are available for user programming. Units 1, 2,
and 3 have the mnemonics CONIN, CONOUT and SYSLST,
respectively which can be used interchangeably with the
literal unit designations, where applicable.
I/O requests are made with a standard vector format,
containing information such as unit, data transfer address,
data length, completion codes, and an optional supplemental
parameter vector address. I/O requests are made by
providing a pointer to the request vector (see below) and
making a system call.
Note that programs which use the RIO I/O structure can
remain unchanged so long as compatible I/O devices
are provided. For instance, a BASIC system could
immediately utilize a line printer by redefining SYSLST.
No other software changes would be required.
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