Q96711: HIMEM.SYS Reports Error: Unable to Control A20 Line!
Article: Q96711
Product(s): Microsoft Disk Operating System
Version(s): MS-DOS:5.x,6.0,6.2,6.21,6.22; WINDOWS:95
Operating System(s):
Keyword(s): msdos win95
Last Modified: 20-FEB-2002
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The information in this article applies to:
- Microsoft MS-DOS operating system versions 5.0, 5.0a, 6.0, 6.2, 6.21, 6.22
- Microsoft Windows 95
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SYMPTOMS
========
If your computer displays the "ERROR: Unable to control A20 line!" message when
you start your computer, HIMEM.SYS did not load properly and therefore cannot
make extended memory or the high memory area (HMA) available. If HIMEM.SYS is
not loaded correctly, programs such as EMM386.EXE and Microsoft Windows 3.0 and
3.1 are unable to use extended memory, and Microsoft MS-DOS cannot load into the
HMA.
CAUSE
=====
This problem occurs if HIMEM.SYS incorrectly identifies your machine type, or if
the /CPUCLOCK and /MACHINE switches need to be added to the DEVICE command for
HIMEM.SYS.
WORKAROUND
==========
To correct this problem, add the /CPUCLOCK and /MACHINE switches to the DEVICE
command that starts HIMEM.SYS, as follows:
1. If you are using MS-DOS 5.0, create a startup disk. To do so, insert a
formatted floppy disk in drive A and type the following:
" sys c: a:" (without the quotation marks)
NOTES:
- Do not use drive B when you create the startup disk. Most computer BIOS
startup procedures do not look for a disk in drive B when starting the
computer, even though the light on drive B may turn on momentarily.
- If you are using MS-DOS 6.0, 6.2 or Windows 95, you can skip to step 3.
You do not need to create a startup floppy disk because you can use the
interactive startup key (F8) to bypass commands in your CONFIG.SYS file if
they cause your system to stop responding (hang). For more information on
using interactive startup, see your MS-DOS documentation.
2. Copy your CONFIG.SYS file to the startup disk by typing the following:
" copy c:\config.sys a:\ " (without the quotation marks)
3. Edit your CONFIG.SYS file.
a. To edit the file using MS-DOS Editor, type the following at the MS-DOS
command prompt:
" edit c:\config.sys" (without the quotation marks)
b. Make sure your CONFIG.SYS file contains a DEVICE command for HIMEM.SYS.
This command should appear before any other DEVICE commands. Specify the
command as follows:
" device=c:\dos\himem.sys" (without the quotation marks)
c. Add the /CPUCLOCK:ON switch to the DEVICE command for HIMEM.SYS. The
command should now appear as follows:
" device=c:\dos\himem.sys /cpuclock:on" (without the quotation marks)
d. If you are using MS-DOS 6.0 or later, you should also add the /V (verbose)
switch after the /CPUCLOCK:ON switch to force HIMEM.SYS to display
informational messages when it loads.
e. If you are using MS-DOS Editor, choose Exit from the File menu. When
MS-DOS Editor displays a dialog box prompting you to save your file,
choose the Yes button or press ENTER.
4. Restart your computer by pressing CTRL+ALT+DEL, or if you are using Windows
95, click Start, click Shut Down, click Restart (or Restart The Computer),
and then click OK.
Watch the messages that your computer displays during startup. If HIMEM.SYS is
working properly, it should display a message similar to the following:
Installed A20 handler number 1. 64K High Memory Area is available.
If the error message no longer appears, HIMEM.SYS starts correctly, and your
system is working properly, the problem has been corrected. If you still see
the "ERROR: Unable to control A20 line!" message, continue with the next
step.
5. Edit your CONFIG.SYS file again. Locate the DEVICE command for HIMEM.SYS and
add the /MACHINE switch to it. The /MACHINE switch specifies the type of
computer you have. The switch should be followed by a colon (:) and your
machine number (see the following list). For example, the following DEVICE
command specifies that your computer is a Toshiba(R) 5100 (machine number
7):
device=c:\dos\himem.sys /cpuclock:on /v /machine:7
The following table lists machine types in alphabetical order, and shows the
corresponding number to specify with the /machine switch:
Machine Type Number
------------------------------------------------------
Abacus 386 1
Acer 1100 6
AT&T(R) 6300 Plus 5
Bull Micral 60 16
Chaplet 1
Compaq Presario 1
CompuAdd(R) 386 systems 1 or 8
CSS Labs 12
Datamedia 386/486 2
Everex(TM) AT Plus 1800 1
Everex Notebook ELX 1
Excel Computer Systems 13
Hitachi(R) HL500C 8
Hewlett-Packard(R) (HP[R]) Vectra(R) 14
HP Vectra (A and A+) 4
IBM(R) PC/AT(R) 1, 11, 12, or 13
IBM PS/2(R) 2
Intel(R) 301z or 302 8
JDR 386/33 1
OPT 386-25 motherboard 1
Pak 386SX 1
PC Limited 4
PC 380/33C, PC 350/33C, or PC300/33C 2
BIOS revision 1.14
Philips 13
Phoenix Cascade BIOS 3, 1 or 8
Toshiba 1600 and 1200XE 7
Toshiba 5100 7
Tulip(R) SX 9
Unisys(R) PowerPort 2
WYSE(R) 12.5 MHz 286 8
Zenith(R) ZBIOS 10
If your computer is not on the list, you are not sure which number to use, or
the machine number for your computer doesn't correct the problem, try these
machine numbers in the following order: 1, 11, 12, 13, 8, 2-10, 14-16.
6. If you're using MS-DOS Editor, choose Exit from the File menu. When MS-DOS
Editor displays a dialog box prompting you to save your file, choose Yes or
press ENTER.
7. Restart your computer by pressing CTRL+ALT+DEL or if you are using Windows
95, click Start, click Shut Down, click Restart (or Restart The Computer),
and then click OK.
Watch the messages that your computer displays during startup. If HIMEM.SYS is
working properly, it should display a message similar to the following:
Installed A20 handler number 1. 64K High Memory Area is available.
If your computer fails when you restart it or you receive the "ERROR: Unable to
control A20 line!" message, the machine number you specified is incorrect for
your hardware. If the computer hangs and you are using MS-DOS 5.0, insert the
startup disk in drive A and restart your system. If you are using MS-DOS 6.0 or
later, use the F8 key to start the interactive startup procedure. When you are
prompted to load HIMEM.SYS, press the N key.
Next, edit your CONFIG.SYS file, specify a different machine number, save your
changes, and restart your system. You may need to try several machine numbers
before you identify the one that works for your computer.
REFERENCES
==========
If you are using MS-DOS 5.0, refer to the Microsoft MS-DOS "User's Guide and
Reference," pages 610-613, for more information.
If you are using MS-DOS 6.0, 6.2, or Windows 95, refer to the MS-DOS Help Command
Reference by typing the following at the MS-DOS command prompt:
" help himem.sys" (without the quotation marks)
Additional query words: 5.00 6.00 6.20 6.22 7.00 dos high memory tshoot errmsg
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Keywords : msdos win95
Technology : kbWin95search kbZNotKeyword3 kbMSDOSSearch kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS500 kbMSDOS500a
Version : MS-DOS:5.x,6.0,6.2,6.21,6.22; WINDOWS:95
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