CLS Does Not Clear Screen on AT&T 6300 (75095)

The information in this article applies to:
    Microsoft MS-DOS operating system 5.0
    Microsoft MS-DOS operating system 5.0a
    Microsoft MS-DOS operating system 6.0
    Microsoft MS-DOS operating system 6.2
    Microsoft MS-DOS operating system 6.21
    Microsoft MS-DOS operating system 6.22

This article was previously published under Q75095


After starting an AT&T 6300, MS-DOS uses only one line of the screen to display commands such as DIR, TREE, and so on; however, the startup information remains on the screen even after issuing the CLS command. In the other cases, the system will hang at the DOS prompt after pressing ENTER a few times.


To work around this problem, add the following line to the CONFIG.SYS file, or use the debug script listed below.
   device=ansi.sys /s
The same behavior occurs on AT&T 6310 machines.


The ANSI.SYS /S switch is an undocumented switch that will force the video to display in 25-line mode. MS-DOS 5.0 or 6.0 relies on byte 40h:84h of the BIOS to work in 43/30 line mode. The earlier version of MS-DOS did not use this byte. The following is a short program to patch the BIOS area into having the 25 (actually 24, because it is is 0-based) value required for a CGA/MDA adapter.

At the DOS prompt, type debug.

You will then see the Debug prompt (a hyphen). Type the following, pressing ENTER after each line:
Debug              User
Response           Input
--------           -----

C:\>               debug
-                  n
-                  a
xxxx:xxxx          mov ax,40
xxxx:xxxx          mov es,ax
xxxx:xxxx          es:
xxxx:xxxx          mov byte ptr [84],18
xxxx:xxxx          int 20
xxxx:xxxx          <press ENTER>
                   <press ENTER>
-                  r cx
:                  d
-                  w
-                  q
This will create PATCH.COM, a file you can execute from the command prompt. If the problem is resolved, put an entry to execute the PATCH.COM file in the AUTOEXEC.BAT file.

Modification Type: Major Last Reviewed: 5/12/2003
Keywords: KB75095