Category : Alternate Operating Systems - Quarterdeck DesqView, CP/M, etc
Archive   : QWHITE13.ZIP
Filename : EMS40.TEC

 
Output of file : EMS40.TEC contained in archive : QWHITE13.ZIP

ID:E4 EMS 4.0 Boards and DESQview
Quarterdeck Technical Note #136 Filename: EMS40.TEC
by Dan Sallitt CompuServe: EMS40.TEC
Last revised: 2/13/92 Category: HW

Subject: A description of the requirements for DESQview to multitask in
expanded memory. A good note to consult when shopping for an
expanded memory board. Answers the question "Why won't DESQview
multitask with my EMS hardware and software?"


Q: Why won't my EMS 4.0 board give me a Largest Available Expanded Memory
size greater than 0K?

The Largest Available Expanded Memory value indicates the largest chunk of
expanded memory that can be used to run a program in. When this value is 0,
no program can run in expanded memory; multitasking can only occur in
conventional memory.

Three things are necessary before any expanded memory board can be used for
multitasking with DESQview.

1) At least some memory below 640K must be able to take an expanded memory
map; it must have expanded memory management hardware associated with it.
For purchasers of expanded memory boards for the PC, XT, and 286 AT, this
means disabling motherboard memory below 640K and filling out main memory
with the expanded memory board. The more memory disabled and backfilled,
the larger the block of expanded memory that can be grabbed at one time to
run a program in.

2) The board must be capable of mapping pages of expanded memory below 640K.
If the board can't map pages of expanded memory below 640K, there's no point
in disabling main memory. If the board can't map pages of expanded memory
below, say, 256K, there's no point in disabling main memory below 256K.

3) The driver that manages the expanded memory on the board must be capable
of mapping pages of expanded memory below 640K. Here again, any limitation
on the driver's power is a limitation on the whole process.

If these three conditions are met, the Largest Available Expanded Memory
value will be equal to the amount of expanded memory that has been mapped
below 640K. For purchasers of expanded memory boards, this means that the
Largest Available Expanded Memory value will be equal to the quantity of
memory that has been disabled on the motherboard and "backfilled" from the
expanded memory board.

(This exact equivalence is always the case for users with EGA and VGA boards.
Due to DESQview's ability to map additional memory into available video areas
above 640K, users of monochrome and Hercules video cards may see a Largest
Available figure that is 64K greater than the size of the backfill; CGA users
may see a figure 96K greater.)

The catch: properties #2 and #3 are OPTIONAL for boards that meet the
Expanded Memory Specification 4.0 (EMS 4.0). Let the buyer beware.
DESQview (and QRAM as well) would also like expanded memory hardware and
software to permit the mapping of pages of expanded memory anywhere between
640K and 1024K. This property is very helpful in increasing the size of
Largest Available Conventional Memory in DESQview, and the Largest Available
Conventional Memory at the start of DESQview is an upper limit on Largest
Available Expanded Memory. Again, this property is optional for boards that
meet the EMS 4.0 specification.

It is worth repeating that even with a Largest Available Expanded Memory value
of 0 you can still multitask as many programs as will fit in conventional
memory; and you can still swap multiple programs in and out of expanded memory
very quickly. The one thing you cannot do with a Largest Available Expanded
Memory value of 0 is to multitask programs in expanded memory. Programs that
are swapped to expanded memory under these circumstances are frozen until you
swap them back into conventional memory.

************************************************************************
*This technical note may be copied and distributed freely as long as it*
*is distributed in its entirety and it is not distributed for profit. *
* Copyright (C) 1990-2 by Quarterdeck Office Systems *
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