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CWSPARAM provided by Charles W Sandmann (cwsdpmi@earthlink.net)
1206 Braelinn, Sugar Land, TX 77479
CWSPARAM OVERVIEW
There are certain parameters built into the CWSDPMI.EXE, CWSDPR0.EXE, and
CWSDSTUB.EXE images which might need to be modified for distributing some
programs for better performance. The modified image could be renamed, and
then the DPMI server name in a DJGPP V2 image may be stubedited to use the
new name. CWSPARAM can also be run directly on images created by appending
COFF images to CWSDSTUB.EXE. The parameters which can be modified are
discussed below:
Full name of paging file ("" to disable) ? [c:\cwsdpmi.swp]
This is where the file which provides virtual memory should be located. It
may be desirable to change the default to a faster disk or one with more
free space. To disable paging, enter two double quotes. The file name
MUST contain the disk, full path, and file name. Anything less may cause
unpredictable behavior. The maximum length of the name is 48 characters,
and it must be a DOS 8.3 style name. Press return to keep the current value.
Number of page tables to initially allocate (0=auto) ? [0]
Page tables are stored in the DOS memory area and consume this precious
resource. The default value of 0 chooses automatic allocation of enough
page tables to hold all of the physical memory without paging (with the
minimum enough to hold 20Mb without paging and a maximum to hold 128Mb).
If your program is a fixed size (and does not spawn any other DPMI images)
you can specify one page table for each 4Mb of your program's size. If
your program is small, this can save a small amount of DOS memory, which
might be needed for other purposes. If your program is large (typically
larger than the physical memory) specifying a number will consume more DOS
memory but improve performance since the page tables themselves will not
page. CWSDPMI will attempt to dynamically allocate additional page tables
at run time to prevent paging, but if all of the DOS memory is free,
performance would be degraded. Remember to add an extra page table if
mapping physical devices. As of r5, you must also allocate additional
space for the bitmaps for physical and virtual memory (one bit for each
4K page). Add one page for each 128Mb of total address space
(physical+virtual). Be generous, since someone may run the image on a
512Mb machine (or larger) and require 4 pages for the physical bitmap
even if you disable virtual memory.
Minimum application memory desired before 640K paging ? [512Kb]
Typically, CWSDPMI only uses extended memory to return as DPMI memory, and
saves the DOS memory area for DOS requests. On small memory machines with
applications not needing much DOS memory (or poorly tuned memory management)
there may be very little extended memory left available, with up to 600K of
DOS memory unused. This parameter determines the threshold which CWSDPMI
should start using DOS memory as DPMI memory. If an application is known to
not need much DOS memory, this value could be set to the size of the memory
required for the image, which provides roughly 0.5Mb additional memory before
paging happens. This parameter is one I would expect might be adjusted when
distributing an application for maximum performance on a wide variety of
machines.
Paragraphs of DOS memory to reserve when 640K paging ? [3840]
If the image needs to page in the DOS memory area (triggered by the parameter
value above), this parameter tells how much DOS memory to save for DOS
allocation requests. The value is in paragraphs (16 byte increments), so the
default is 60Kbytes. You will actually end up with a bit more probably, since
the page tables are allocated on 4Kbyte rounded boundaries. Some amount of
DOS memory needs to be saved for potentially expanding the file table, the
sbrk() algorithm, DMA buffers, etc. This amount can be tuned on an image
by image basis, and may be as little as 0.
Paragraphs of memory for extra CWSDPMI internal heap ? [256]
CWSDPMI is built with an internal 4K heap. Each nested task consumes around
300 bytes, each memory zone takes 12 bytes, and each HW interrupt (nested)
takes around 850 bytes. If extra DOS memory is available, the additional
paragraphs specified here are added to the heap at execution time by DOS.
The 8K default should be good for up to 40Mb in small requests. If you
compile very large C++ programs or allocate lots of memory in small pieces,
you might need to bump this parameter. If you are very tight on memory,
you could decrease it to zero and pick up another 4K of DOS memory. Each
paragraph allows a little more than one more memory zone (or in the worse
case around 85Kb of DPMI memory).
Maximum size of swap file ? [128Mb]
CWSDPMI dynamically allocates memory at run time for up to 2044Mb of swap file
space. The bitmap for the swap usage requires 32 bytes of memory per 1Mb of
swap usage. The default will provide up to 128Mb of swap space with a run
time memory requirement of 4Kb DOS memory. The dynamic allocation is limited
by the free space on the drive, so if you don't have much space on your hard
drive, you don't need to worry about this. If you run small programs and want
to save memory (maybe to stuff the allocation in a UMB) you can decrease this
value. If you want to run monster programs needing 2Gb of address space you
can increase this value (but this would increase the DOS memory usage). This
value is ignored by CWSDPR0.
Value of run option flags ? [0]
There are a few run-time options in CWSDPMI which should not be needed by most
users which can be activated by this field of bit flags:
Bit 0 (1): Disable allocation of run time page tables in UMBs (low only)
Bit 1 (2): Pre-allocate page table memory (in DPMI memory request)
Bit 2 (4): Disable DPMI 1.0 extensions (Null page protection, mapping)
Other values should be zero for future compatibility but are currently ignored.
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