KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q60140: Location of Keyboard Buffer Area in MS-DOS; BASIC PEEK, POKE

Article: Q60140
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S900326-109 b_basiccom pss | mspl13_basic
Last Modified: 12-NOV-1990

The actual location of the keyboard buffer on the IBM PC or PS/2 is
variable, but by default, the buffer is a 32-byte area located at
segment 0, offset 1054 (41E hex). Because many applications assume
that this is the default location, please be careful if you change its
address or size.

The buffer is composed of 16 2-byte entries. It holds up to 16
keystrokes until they are read via the BIOS services through interrupt
22 (16 hex). Because this is a circular queue buffer, two pointers
indicate the head and tail of the queue. It is usually best to
manipulate the pointers rather than the actual data.

This information is taken from "The New Peter Norton Programmer's
Guide To The IBM PC and PS/2" on pages 56-60, published by Microsoft
Press (1988).

There are other adjacent memory locations that are used in conjunction
with the keyboard buffer. The most important of these are listed
below. All of these addresses are in segment 0.

Offset 1050 (41A hex)
---------------------

A 2-byte address that points to the current head of the BIOS keyboard
buffer at offset 1054.

Offset 1052 (41C hex)
---------------------

A 2-byte address that points to the current tail of the BIOS keyboard
buffer at offset 1054.

Note: One interesting way to clear the keyboard buffer is to set the
head of the queue equal to the tail. To do this in BASIC, simply PEEK
the two bytes at 1052 and POKE them into location 1050.

Offset 1152 (480 hex)
---------------------

A 2-byte word pointing to the start of the keyboard buffer area.

Offset 1154 (482 hex)
---------------------

A 2-byte word pointing to the end of the keyboard buffer area.

Note: Be careful if you choose to change the addresses at 1152 or 1154
because many applications may not check these memory locations to
determine the keyboard buffer area. These applications will assume the
default configuration.

References:

For more articles about reading from and writing to the keyboard
buffer, search in this Knowledge Base for the following words:

   interrupt AND keyboard AND buffer

Keyboard scan codes are documented in Appendix D of "Microsoft
QuickBASIC 4.5: Programming in BASIC"; in Appendix A of "Microsoft
QuickBASIC 4.0: Language Reference" for 4.00 and 4.00b; in Appendix A
of "Microsoft BASIC Compiler 6.0: Language Reference" for 6.00 and
6.00b; and in Appendix A of "Microsoft BASIC 7.0: Language Reference"
manual for BASIC PDS versions 7.00 and 7.10.

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.