HIMEM.SYS

Detailed Command Description

HIMEM.SYS

HIMEM is an extended-memory manager--a program that coordinates the use of

your computer's extended memory, including the high memory area (HMA), so

that no two applications or device drivers use the same memory at the same

time.

You install HIMEM by adding a DEVICE command for HIMEM.SYS to your

CONFIG.SYS file. The HIMEM.SYS command line must come before any commands

that start applications or device drivers that use extended memory; for

example, the HIMEM.SYS command line must come before the EMM386.EXE command

line.

Syntax

DEVICE=[drive:][path]HIMEM.SYS [/A20CONTROL:ON|OFF] [/CPUCLOCK:ON|OFF]

[/EISA] [/HMAMIN=m] [/INT15=xxxx] [/NUMHANDLES=n] [/MACHINE:xxxx]

[/SHADOWRAM:ON|OFF] [/TESTMEM:ON|OFF] [/VERBOSE]

In most cases, you won't need to specify command-line options. The default

values for HIMEM.SYS are designed to work with most hardware.

Parameter

[drive:][path]

Specifies the location of the HIMEM.SYS file. HIMEM.SYS should always be

located on the same drive that contains your MS-DOS files. If the

HIMEM.SYS file is in the root directory of your startup drive, you don't

need to include a path. However, you must always include the complete

filename (HIMEM.SYS).

Switches

/A20CONTROL:ON|OFF

Specifies whether HIMEM is to take control of the A20 line even if A20

was on when HIMEM was loaded. The A20 handler gives your computer access

to the HMA. If you specify /A20CONTROL:OFF, HIMEM takes control of the

A20 line only if A20 was off when HIMEM was loaded. The default setting

is /A20CONTROL:ON.

/CPUCLOCK:ON|OFF

Specifies whether HIMEM is to affect the clock speed of your computer.

If your computer's clock speed changes when you install HIMEM,

specifying /CPUCLOCK:ON may correct the problem; however, enabling this

option slows down HIMEM. The default setting is /CPUCLOCK:OFF.

/EISA

Specifies that HIMEM should allocate all available extended memory. This

switch is necessary only on an EISA (Extended Industry Standard

Architecture) computer with more than 16 MB of memory; on other

computers, HIMEM automatically allocates all available extended memory.

/HMAMIN=m

Specifies how many kilobytes of memory an application must require for

HIMEM to give that application use of the HMA. Only one application can

use the HMA at a time; HIMEM allocates the HMA to the first application

that meets the memory-use requirements set by this option. You can

specify a value from 0 to 63.

Set /HMAMIN to the amount of memory required by the application that

uses the most HMA memory.

The /HMAMIN option is not required; the default value is zero. Omitting

this option (or setting it to zero) specifies that HIMEM allocate the

HMA to the first application that requests it, regardless of how much of

the HMA the application is going to use.

The /HMAMIN option has no effect when Windows is running in 386 enhanced

mode.

/INT15=xxxx

Allocates the amount of extended memory (in kilobytes) to be reserved

for the Interrupt 15h interface. Some older applications use the

Interrupt 15h interface to allocate extended memory rather than using

the XMS (eXtended-Memory Specification) method provided by HIMEM. If you

use these applications, you can ensure enough memory is available to

them by setting xxxx to 64 KB larger than the amount required by the

application.

You can specify a value from 64 to 65535; however, you cannot specify

more memory than your system has available. If you specify a value less

than 64, the value becomes 0. The default value is 0.

/NUMHANDLES=n

Specifies the maximum number of extended-memory block (EMB) handles that

can be used simultaneously. You can specify a value from 1 to 128; the

default value is 32. Each additional handle requires an additional 6

bytes of memory.

The /NUMHANDLES option has no effect when Windows is running in 386

enhanced mode.

/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

/SHADOWRAM:ON|OFF

Specifies whether to disable shadow RAM (SHADOWRAM:OFF) or to leave the

ROM code running from RAM (SHADOWRAM:ON).

Some computers make ROM code run faster by "shadowing" it in RAM--that

is, by copying the ROM code into faster RAM memory at startup, which

uses some extended memory. On computers that use shadow RAM and have

less than 2 MB of RAM, HIMEM usually attempts to disable shadow RAM to

recover additional extended memory for Windows to use. (HIMEM can

disable shadow RAM only on certain types of systems.) When HIMEM

disables shadow RAM, the ROM code runs in the slower ROM instead of RAM;

therefore, your computer might run slightly slower than it did before.

/TESTMEM:ON|OFF

Determines whether HIMEM performs a memory test when your computer

starts. By default, HIMEM tests the reliability of your computer's

extended memory each time your computer starts. This test can identify

memory that is no longer reliable; unreliable memory can cause system

instability or loss of data. HIMEM's memory test is more thorough than

the standard power-up memory test performed by most computers. To

prevent HIMEM from performing the memory test, specify /TESTMEM:OFF.

Disabling the memory test will shorten the startup process. (The default

setting is /TESTMEM:ON.)

/VERBOSE

Directs HIMEM to display status and error messages while loading. By

default, HIMEM does not display any messages unless it encounters an

error. You can abbreviate /VERBOSE as /V. (To display status messages

without adding the /VERBOSE switch, press and hold the ALT key while

HIMEM starts and loads.)

HIMEM.SYS--Notes

HIMEM tests the reliability of your extended memory

The MS-DOS 6.2 version of HIMEM.SYS includes a new safety feature: it tests

your computer's extended memory each time your computer starts. HIMEM tests

the memory by writing and reading data to each memory address and checking

for differences in the data. If the data HIMEM reads from an address differs

from the data it just wrote to that address, then the memory at that address

is unreliable. Unreliable memory can cause system instability or loss of

data. HIMEM's memory test is more thorough than the standard power-up memory

test performed by most computers.

If HIMEM detects unreliable memory, it displays the following message and

does not load:

ERROR: HIMEM.SYS has detected unreliable extended memory at address

XXXXXXXXh.

If you receive this message, you should have your computer's memory checked

by a qualified computer hardware technician. Although you can disable the

memory test and load HIMEM by using the /TESTMEM:OFF switch, it's not

advisable to do so until you have had the memory checked by a technician.

Default memory allocation

Only one program at a time can use the high memory area (HMA). If you omit

the /HMAMIN=m switch (or set it to 0), HIMEM reserves the HMA for the first

program that requests it and that meets the memory requirements set by the

/HMAMIN=m switch. To ensure the most efficient use of your system's high

memory area, set /HMAMIN=m to the amount of memory required by the program

that uses the most HMA memory.

Loading MS-DOS into the high memory area

HIMEM or another XMS driver must be loaded in order to load MS-DOS into the

high memory area (HMA). You load MS-DOS into the HMA by using the DOS=HIGH

command in your CONFIG.SYS file. (The DOS=HIGH command can appear anywhere

in the CONFIG.SYS file.)

HIMEM.SYS--Examples

To run HIMEM using the default values, add the following command line to

your CONFIG.SYS file:

device=himem.sys

The command line shown above does not specify a path for the HIMEM.SYS file;

therefore, MS-DOS searches for the HIMEM.SYS file only in the root directory

of your startup drive. Because this command line does not include additional

options, HIMEM uses the default values for those options. HIMEM allocates

the HMA to the first program that requests it and allows the simultaneous

use of up to 32 extended-memory handles.

Increasing the number of memory handles that HIMEM provides

To specify that a maximum of 128 extended-memory handles can be used

simultaneously, add the following command line to your CONFIG.SYS file:

device=c:\dos\himem.sys /numhandles=128

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