uncommit.txt

		How to Use Uncommitted Memory

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 might be, but it 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 marks the 
faulting address as committed, 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.