Windows NT Only Recognizes up to 64 MB RAM on Some Computers

• Microsoft Windows NT operating system version 3.1
• Microsoft Windows NT Advanced Server version 3.1
• Microsoft Windows NT Server version 3.5
• Microsoft Windows NT Server version 3.51
• Microsoft Windows NT Server version 4.0
• Microsoft Windows NT Workstation version 3.5
• Microsoft Windows NT Workstation version 3.51
• Microsoft Windows NT Workstation version 4.0

SUMMARY

Windows NT detects up to 4 gigabytes (GB) of RAM, but if you are using a computer with more than 64 megabytes (MB) of RAM, Windows NT may detect only up to 64 MB. This behavior is based on how the BIOS returns the amount of memory to Windows NT. Even though the BIOS may correctly display memory above 64 MB, it may not pass this information correctly to Windows NT.

NOTE: The AST Manhattan may display this behavior and may require updating the EISA configuration by running the EISA configuration utility. AST Manhattan machines may also require disabling video memory range in BIOS in order for NT to report the actual amount of memory (RAM) installed.

MORE INFORMATION

Windows NT supports only memory the BIOS reports using INT 15, 88H, E801H, and E820H. Depending on how the BIOS reports memory is how Windows NT will recognize it. If the BIOS is using INT 88H, it can only support up to 64 MB RAM. If the BIOS is using INT E801H can support more, but it can only be in three ranges: 0 - 640K, 1 MB - 16 MB, 16 MB and up. For the BIOS to support INT E801H, INT 88H can only support up to 16 MB. This may be a problem, since it means that some existing applications or products only see 16 MB of memory on computers with more than 64 MB. Also, supporting E801 typically means the manufacturer of the computer ships a TSR that turns off INT E801 and allows INT 88H to return 64 MB.

If the BIOS is using INT E820, it does not have this problem, and it allows for other features such as reporting RAM that is in use by the BIOS. In addition to this just being a feature, it effectively reclaims 256 kilobytes (K) of memory which currently utilizes INT 88H or INT E801H. INT E820 can report addresses that cannot be used by devices and ensure that different drivers don't attempt to use the same address for their devices. INT E820 can return as many different 'runs' of real memory as it wants, so large amounts of memory work on Extended Industry Standard Architecture (EISA) and Industry Standard Architecture (ISA) with INT E801, but there are limitations.

Windows NT only supports the memory returned by the BIOS. For Windows NT to support computers with more than 64 MB RAM, the computer must return this memory through INT E820 or INT E801.