Specifying A20 Handlers with HIMEM.SYS /M in MS-DOS (72302)

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

This article was previously published under Q72302


MS-DOS version 5.0 includes HIMEM.SYS version 2.77. MS-DOS version 6.0 includes HIMEM.SYS version 3.07. MS-DOS versions 6.2, 6.21, and 6.22 include HIMEM.SYS version 3.10. These versions include an A20 handler for the following computers that can be used with the /M: or /MACHINE switch:
   CODE            Number          A20 Handler
   ----            ------          -----------
   at              1               IBM PC/AT
   ps2             2               IBM PS/2
   pt1cascade      3               Phoenix Cascade BIOS
   hpvectra        4               HP Vectra (A and A+)
   att6300plus     5               AT&T 6300 Plus
   acer1100        6               Acer 1100
   toshiba         7               Toshiba 1600 and 1200XE
   wyse            8               Wyse 12.5 MHz 286
   tulip           9               Tulip SX
   zenith          10              Zenith ZBIOS
   at1             11              IBM PC/AT
   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
There are two values that are considered defaults. /M:1 is the standard default for IBM AT-class machines, and t/M:2 is the standard default for IBM PS/2 machines. For most 100-percent-compatible machines, the /M:1, /M:11, /M:12, and /M:13 A-20 handler switches should work. Although the other switches are hardware specific, one may be required for proper operation on certain machines.

NOTE: In the above list, the device driver should detect the correct handler to use. The following computers may need the corresponding switch added to the CONFIG.SYS file:
   System                    Switch
   ------                    ------
   Bull Micral 60            /machine:16
   COMPUADD 386 systems      /machine:1 or /machine:8
   Datamedia 386/486         /machine:2
   Hitachi HL500C            /machine:8
   Intel 301z or 302         /machine:8
   JDR 386/33                /machine:1
   Toshiba 5100              /machine:7
   UNISYS PowerPort          /machine:2


The /M switch specifies which hardware the computer is using. If the wrong switch is used, HIMEM.SYS may not load properly or the system may hang. For more information, query on the following words:

HIMEM and /M

Modification Type: Major Last Reviewed: 11/26/2003
Keywords: KB72302