Tandy 1000 Programmer's Reference Manual page 175

Chapter 5
DEVICE DRIVERS
A device driver is a binary file containing code to manipulate
hardware devices and provide consistent interfacing to MS-DOS.
The driver has a special header that identifies the device, defines
the strategy and interrupt entry points, and describes various
attributes of the device.
The .COM file must not use 100H as the driver ORG. Because it
does not use the Program Segment Prefix, the device driver file
must have an origin of zero (ORG 0 or no ORG statement).
Types of Devices
There are two kinds of devices: character devices and block
devices.
Character devices are designed to perform serial character I/O.
They have names such as CON, AUX, and CLOCK. You open
channels (handles or FCBs) for I/O operations t o them.
Block devices are system disk drives. They perform random 110
in pieces called blocks (usually the physical sector size). These
devices are not named as the character devices are, and therefore
cannot be opened directly. They are identified instead by the
drive letters (A, B, C, etc.).
Block devices can consist of one or more units with a single
driver responsible for one or more disk drives. For example, block
device driver ALPHA may be responsible for Drives A, B, C, and
D (four units (0-3) are defined and are identified by four drive
letters). The position of the driver in the list of all drivers deter-
mines which units correspond to which drive letters. If driver
ALPHA is the first first block driver in the device list, and it de-
fines
4
units (0-3), then they will be A, B, C, and
D.
If BETA is
the second block driver and defines three units (0-2), then they
will be E,
F, and G, and so on. The theoretical limit is 63 block
devices, but after 26 the drive letters are unconventional (such
as
I,
/,
and
7.
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