Hardware Reference
In-Depth Information
Table 3.1
DOS I/O channels
Channel
Meaning
Function
Notes
COM1:, COM2:, COM3:,
Communications
Serial I/O
Via RS-232 ports
and COM4:
CON:
Console
Keyboard (input) and
This channel combines
screen (output)
the functions associated
with the keyboard and the
display (i.e. a 'terminal')
LPT1:, LPT2:, and LPT3:
Line printer
Parallel printer
This interface (output)
conforms to the
Centronics standard
PRN:
Printer
Serial or parallel
printer (output)
NUL:
Null device
Simulated I/O
Provides a means of
simulating a physical I/O
channel without data
transfer taking place
DOS commands
DOS responds to command lines typed at the console and terminated with
a
<RETURN>
or
<ENTER>
keystroke. A command line is thus composed of a
command keyword, an optional command tail, and
<ENTER>
. The command
keyword identifies the command (or program) to be executed. The command
tail can contain extra information relevant to the command, such as a filename
or other parameters. Each command line must be terminated using
<ENTER>
or
<ENTER>
(not shown in the examples which follow).
As an example, the following command can be used to display a directory
of all BASIC source code (i.e. those with a BAS extension) within a directory
named TEST in drive C:, indicating the size of each:
DIR C: \TEST\*.BAS
Note that, in this example and the examples that follow, we have omitted the
prompt generated by the system (indicating the current drive).
It should be noted that the command line can be entered in any combination
of upper- or lower-case characters. DOS converts all letters in the command
line to upper-case before interpreting them. Furthermore, whilst a command
line generally immediately follows the system prompt, DOS permits spaces
between the prompt (e.g.
C:
\
>
) and the command word.
As characters are typed at the keyboard, the cursor moves to the right in order
to indicate the position of the next character to be typed. Depending upon the
keyboard used, a
<BACKSPACE>
,or
<DELETE>
key, can be used to delete the
last entered character and move the cursor backwards one character position.
Alternatively, a combination of the
CONTROL
and
H
keys (i.e.
<CTRL-H>
)maybe
used instead. Various other control characters are significant in DOS and these
are shown in Table 3.2.
