Q65935: Explanation of Why BASIC Programs Can Open Devices As Files

Article: Q65935
Product(s): See article
Version(s): 1.00 1.01 1.02 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S900817-47 B_BasicCom | mspl13_basic
Last Modified: 19-OCT-1990

This article explains how Microsoft BASIC compilers, QuickBASIC, and
GW-BASIC can open character device drivers as if they were files, as
in the following line of code:

   OPEN "\DEV\CON" FOR OUTPUT AS #1

BASIC can interact with character device drivers in this way because
this feature is built into MS-DOS.

This information applies to Microsoft QuickBASIC versions 1.00, 1.01,
1.02, 2.00, 2.01, 3.00, 4.00, 4.00b, and 4.50 for MS-DOS; to Microsoft
BASIC Compiler versions 6.00 and 6.00b for MS-DOS; to Microsoft BASIC
Professional Development System (PDS) 7.00 and 7.10 for MS-DOS; and to
all versions of GW-BASIC (3.23, 3.22, 3.20 and earlier) for MS-DOS.

DOS supports two types of device drivers: character device drivers and
block device drivers. Character device drivers drive those devices
that input and output data one character at a time. A typical
character device is the keyboard or the line printer.

DOS provides the ability to open the character device driver for I/O;
you can access the driver by name as if it were a file. The name can
be up to eight characters in length. This "name" is internal to the
device driver and is unrelated to the actual piece of hardware you are
interacting with. DOS keeps a list internally of the names of
character device drivers installed at boot time plus some default
devices that are always present.

DOS provides several character device drivers by default. Any program
can open CON (the console), AUX (the communications port), PRN (the
line printer), and LPT1 or LPT2. For example, when you install
ANSI.SYS, this device driver replaces the default DOS CON device
driver; therefore, anything printed to the CON device will actually be
filtered by ANSI.SYS.

For an example of sending escape codes to ANSI.SYS in BASIC through
the CON device, search for a separate article in this Knowledge Base
using the following words:

   ANSI and ESCAPE and CON and QUICKBASIC

Some character device drivers can interact with your program by
sending and receiving control data strings. BASIC provides the IOCTL$
function and the IOCTL statement to input and output data control
strings to character device drivers.

Note: Very few device drivers support IOCTL data control strings.
BASIC's IOCTL and IOCTL$ work correctly only with device drivers that
have this ability built in.

For more information on device drivers, consult a book on programming
DOS such as "Advanced MS-DOS Programming, Second Edition" by Ray
Duncan, published by Microsoft Press (1988).

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