HIMEM.SYS Reports Loading A20 Handler X,Y (78813)

The information in this article applies to:
    Microsoft MS-DOS operating system 5.0

This article was previously published under Q78813


If two numbers appear on the A20 handler message when HIMEM.SYS starts, the second number refers to a delay that pertains to some IBM AT compatible computers. This number is determined by HIMEM.SYS, and you cannot change it.


For example, if the following message is displayed when HIMEM.SYS starts

Installed A20 handler number 7,2.
the first number, 7, refers to the machine number. You can change this number by including the /MACHINE switch on the DEVICE command for HIMEM.SYS in your CONFIG.SYS file.

The second number, 2, refers to a delay number that is relevant for certain IBM AT compatible machines. You cannot specify the delay number; it is determined automatically by HIMEM.SYS.

In the example above, the Toshiba A20 handler (machine number = 7) is installed using delay number 2.

There are only three valid delay numbers: 0, 1, 2. Also, there are only specific valid combinations of machine and delay numbers. A delay number of 0 is never displayed; the absence of a delay number implies a delay number of 0. (Therefore, "1" is equivalent to "1,0".) The following list details the valid combinations of machine and delay numbers under MS-DOS 5.0. All machine numbers not listed here are not associated with delay numbers.
Machine Code    Machine Number, Delay Number

at              1
at              1,1
at              1,2
toshiba         7,2
at1             11
at2             12,1
css             12,1
at3             13,2
philips         13,2
The following combinations identify identical A20 handlers:
11   = 1
12,1 = 1,1
13,2 = 1,2
If you specify /MACHINE:1 as a HIMEM.SYS parameter, HIMEM will automatically attempt to determine which of the three delays is appropriate. However, if you specify /MACHINE number 11, 12, or 13, HIMEM.SYS will not attempt to determine the correct delay value. Instead, it will use the delay value as shown in the previous table. For example, if you specify /MACHINE:11, HIMEM.SYS will use machine number 1 ("at") with delay number 0.

These alternative values for "at" are useful only if you cannot load HIMEM.SYS on an IBM AT compatible computer.

Modification Type: Major Last Reviewed: 11/21/1999
Keywords: KB78813