HIMEM.SYS Reports Error: Unable to Control A20 Line! (96711)


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
    • Microsoft Windows 95
This article was previously published under Q96711

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:
    • 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:

    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.

    • Copy your CONFIG.SYS file to the startup disk by typing the following:

    copy c:\config.sys a:\


    • Edit your CONFIG.SYS file.
      • To edit the file using MS-DOS Editor, type the following at the MS-DOS command prompt:

      edit c:\config.sys


      • 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


      • 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


      • 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.
      • 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.

    • 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.
    • 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.
    • 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.
    • 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

Modification Type: Major Last Reviewed: 5/12/2003
Keywords: 5.00 6.00 7.00 msdos win95 KB96711
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