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Microsoft KB Archive/35679

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HIMEM.SYS Level of Support of the XMS Version 2.00 Definition PSS ID Number: Q35679 Article last modified on 01-05-1989

2.00 2.01 2.02 2.03 2.04



In HIMEM.SYS Version 2.x, the Microsoft implementation of a XMS (eXtended Memory Specification) Version 2.00 device driver, all XMS API functions are implemented except for the last three, as follows:

Number Function

0FH Reallocate Extended-Memory Block 10H Request Upper-Memory Block 11H Release Upper-Memory Block

Functions 10H and 11H require the ability to access the upper-memory area. This is the area located between 640K and 1 megabyte, which, on some systems, has memory which is addressable by an application. Normally, this area is used by other hardware and software, such as video-adapter memory, EMS memory-manager page frames, and network drive-buffer areas, among others. To properly implement support of these two functions would take a 80386 specific version of HIMEM.SYS. These features are under review and will be considered for inclusion in a future release.

Function 0FH is also not implemented, which would give the ability to reallocate an extended memory block. Currently it is possible to allocate, free, lock, and unlock an extended memory block. It is NOT possible to reallocate the size of such a block. This feature is also under review and will be considered for inclusion in a future release.

There is another commercially available XMS device driver that does support the manipulation of the upper-memory area. It is “386 To The Max,” by Qualitas. As implied by the name, this driver will only work on 80386 systems with extended memory. The phone number for Qualitas is (301) 469-8848. Qualitas was involved in the design of the XMS Version 2.00 specification. Please note that this reference to this product does not constitute an endorsement.


Copyright Microsoft Corporation 1989.