ID:E4 EMS 4.0 boards and DESQview
Quarterdeck Technical Note #136
by Dan Sallitt

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 a chunk of expanded memory 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.

        Copyright (C) 1991 by Quarterdeck Office Systems
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