Overview - ZiLOG Z80-RIO Operating System User's Manual

CHAPTER 3
I/O STRUCTURE
3.1 OVERVIEW
The I/O architecture of RIO is designed to a) facilitate
user construction and implementation of device drivers to
service the I/O requests of system or user programs; and b)
simplify and standardize interface to all I/O drivers.
To this end, all I/O requests are made to RIO with
reference to a logical unit. RIO determines the proper
routing for the referenced unit and passes control for
servicing of the I/O request to the appropriate device
driver.
The internal structures supporting this facility include
the Active Device Table (ADT) and Logical File Table (LFT).
The Active Device Table has one entry for every device
known to the system at a given time and includes the device
name and entry point. Devices are made 'known' to the
system via the ACTIVATE command (see section 5.1), or they
may be removed from the ADT by the DEACTIVATE command (see
section 5.11). The current ADT contents can be reviewed with
the LADT command (List Activate Device Table - see section
5.31).
Devices which are known to the system may be used to
qualify a file name, thereby linking a logical unit to the
named device. Unqualified file names (those without a
device name prefix) are given a default routing to the
master device (see MASTER, section 5.32).
The link between a logical unit and a specific device
exists in the Logical File Table, each entry of which
contains the address of the device that the corresponding
unit is linked to. Before I/O requests may be processed via
a logical unit, the unit must be defined. This unit
definition may occur via the 'Assign' I/O request (see
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