How to Use Uncommitted Memory
in a DPMI 1.0 Host

Overview

One of the really nice features of DPMI 1.0 is its support of Uncommitted Memory, that is the ability to allocate address space without allocating memory to back it up at the same time.

For example, say you need space for an object whose size you can't compute at the start, but has a reasonable upper limit of, say, 100 megabytes. Using uncommitted memory, you can allocate 100 MB of address space and then just start using it.

When you first write to anywhere in the address space, the DPMI 1.0 host signals a Visible Page Fault (VPF) which your program has hooked and is now given the opportunity to handle. Your program's response to the VPF is to mark the 4KB block's Page Attributes as Read/Write and Committed, and then simply re-execute the instruction which triggered the VPF. The DPMI host takes over from there, maps in a 4KB block of memory, and your program continues on as if nothing had happened.

Subsequent accesses to different 4KB blocks of the uncommitted memory trigger additional VPFs to which your program responds in the same way. This provides a very efficient mechanism with a small amount of overhead once per 4KB block of the address space and commits only the amount of memory actually needed.

I used this technique in my linker, QLINK. At the start of the program, it doesn't know how big each segment (as well as other structures) might be, but can make a reasonable guess that none will be bigger than a few megabytes. When data is written to each segment, QLINK's VPF handler changes the faulting page's attributes, and continues on.

This feature allows QLINK to pass through each object file only once instead of the usual two passes needed by other linkers.

Details

To use uncommitted memory, follow these steps:

1. Allocate address space via DPMI function 504h (Allocate Linear Memory Block) with EDX = 0 to create Uncommitted Pages.
2. Hook the Page Fault handler via DPMI function 210h (Get Extended Processor Exception Handler -- PM), interrupt 0Eh.
3. When the Page Fault handler hooked above is called, find the matching handle from step 1, and change the corresponding 4KB block's Page Attributes (via DPMI function 507h -- Set Page Attributes) to Read/Write and Committed.
4. Return to the caller to re-execute the instruction which triggered the VPF.

Author

This page was created by Bob Smith -- please any questions or comments about it to me.

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