On Fri, 12 Feb 1999 Michael Chan <[log in to unmask]> wrote:
>On Wed, 10 Feb 1999 16:24:30 -0800, you wrote:
>
>Since the A20 line is controlled by the chipset of the keyboard, if
>that chipset is malfunctional, it is possible to have such error
>message. Replaced another keyboard and see if this help.
>
>>I'm teaching a computer hardware course at a community college and I
>>have a bunch of old 486 VESA system boards that my students are learning
>>on.  I have the same problem with a number of these boards, and so far
>>have not been able to determine the cause.
>>The problem is as follows:  When we boot up DOS 6.22 and the Himem.sys
>>driver is loaded in Config.sys we get the message: "ERROR: Unable to
>>control A20 line   XMS driver not loaded".
>>Anyone familiar with this problem?  Thanks in advance for any advice.
>>-Rick VanHeyst
>
>Regards,
>Michael
>
By your quoting, you appear to put your words into someone else's mouth.

The keyboard controller in question for address line A20 control
is the keyboard serial interface controller on the motherboard,
not the key scan decoder on the keyboard. Changing the keyboard
has no relevance to the problem.

Controlling address line decoding from the keyboard controller
is a strangeness inherited from early IBM PC, PC/AT & i386 designs:
address line A20 didn't exist on the original PC, so to emulate
the 8088/8086 address wrap-around at 1MB it has to be locked off
then it must be re-enabled to allow access to the 65520 byte
High Memory Area or to Extended memory - and there just happened
to be a spare I/O port bit on the controller chip.

(There's an explanation of i386 Real Mode and HMA in H-P Messmer
"The Indispensable PC Hardware Book" 2nd edn, p79)

Motherboard chipset designers have found more logical and faster ways
to control A20, but unfortunately HIMEM cannot recognise some of them
automatically, so you might have to point it in the right direction
with the /MACHINE (or /M) switch.

The following table is from the HIMEM.EXE entry in DOS 6.2 HELP.COM

/MACHINE:xxxx
    Specifies what type of computer you are using. Usually, HIMEM can detect
    your computer type successfully; however, there are a few computers that
    HIMEM cannot detect. On such systems, HIMEM uses the default system type
    (IBM AT or compatible). You might need to include the /MACHINE option if
    your computer is a type that HIMEM cannot detect and if HIMEM does not
    work properly on your system by using the default system type.

    Currently, systems that require this option include Acer 1100, Wyse, and
    IBM 7552.

    The value for xxxx can be any of the codes or their equivalent numbers
    listed in the following table.

Code         Number  Computer type
-------------------------------------------------------------------------
at           1       IBM AT or 100% compatible
ps2          2       IBM PS/2
ptlcascade   3       Phoenix Cascade BIOS
hpvectra     4       HP Vectra (A & A+)
att6300plus  5       AT&T 6300 Plus
acer1100     6       Acer 1100
toshiba      7       Toshiba 1600 & 1200XE
wyse         8       Wyse 12.5 Mhz 286
tulip        9       Tulip SX
zenith       10      Zenith ZBIOS
at1          11      IBM PC/AT (alternative delay)
at2          12      IBM PC/AT (alternative delay)
css          12      CSS Labs
at3          13      IBM PC/AT (alternative delay)
philips      13      Philips
fasthp       14      HP Vectra
ibm7552      15      IBM 7552 Industrial Computer
bullmicral   16      Bull Micral 60
dell         17      Dell XBIOS



Len Warner <[log in to unmask]> http://easyweb.easynet.co.uk/~len/ ICQ:10120933

               PCBUILD mailing list is brought to you by:
                            The NOSPIN Group
                            http://nospin.com