Q116256: Himem.sys Cannot Address More Than 64 MB of Memory

Article: Q116256
Product(s): Microsoft Disk Operating System
Version(s): MS-DOS:4.x,5.x,6.0,6.2,6.21,6.22; WINDOWS:3.1,3.11,95
Operating System(s): 
Keyword(s): kbhw kbtool win31 msdos win95 kbHardware
Last Modified: 24-JUL-2001

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The information in this article applies to:

- Microsoft MS-DOS operating system versions 4.0, 4.01, 5.0, 5.0a, 6.0, 6.2, 6.21, 6.22 
- Microsoft Windows versions 3.1, 3.11 
- Microsoft Windows 95 
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SYMPTOMS
========

Himem.sys does not report or use more than 64 megabytes (MB) of installed memory
on computers with an Industry Standard Architecture (ISA) or Micro Channel
Architecture (MCA) bus.

CAUSE
=====

ISA Buses
---------

ISA buses lack a standard BIOS interrupt routine for reporting more than 64 MB of
memory. Himem.sys depends on the INT 15 function 88 service to determine
installed memory, and this function is limited to 65,535 kilobytes (64 MB).

MCA Buses
---------

MCA-based computers add the INT 15 function C7 service to report installed
memory. Although this function can report more than 64 MB of memory, Himem.sys
has no code to support it. As a result, Himem.sys uses the standard INT 15
function 88 call, which is limited to 64 MB of memory.

MORE INFORMATION
================

This limitation does not apply to computers with an Extended Industry Standard
Architecture (EISA) bus. On EISA-based computers, Himem.sys scans each slot for
device information, which means that it can report up to 4 gigabytes (GB).

NOTE: You may need to use the /EISA switch on the Himem.sys line in your
Config.sys file.

Note that although Himem.sys addresses only the first 64 MB of memory, the
Windows 95 virtual memory manager (Vmm32.vxd) takes over memory management tasks
from Himem.sys. Vmm32.vxd can address more than 64 MB of memory.

Additional query words: 4.x 5.x 6.00 6.20 6.21 6.22 3.10 3.11 busses mem xms ram

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Keywords          : kbhw kbtool win31 msdos win95 kbHardware 
Technology        : kbWin3xSearch kbWin95search kbZNotKeyword3 kbWin310 kbWin311 kbMSDOSSearch kbMSDOS400 kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS500 kbMSDOS401 kbMSDOS500a
Version           : MS-DOS:4.x,5.x,6.0,6.2,6.21,6.22; WINDOWS:3.1,3.11,95

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