ID:EP ExPANDED Memory. Which card is best
Quarterdeck Technical Note #149
by Dan Sweeney


Q: Which ExPANDed Memory card should I buy?

     The type of ExPANDed memory card recommended for use with 
DESQview depends on the motherboard configuration of your 
computer.  The ability to disable motherboard memory, and the 
amount of memory that can be disabled, are determining factors. 

     This discussion assumes that the motherboard memory can be 
disabled to a minimum of 256K.  If your computer's motherboard 
memory can't be disabled below 512K, then skip to the Other 
Choices section. 
     

Can the board MAP MEMORY BELOW 1024K?

     In all cases the memory card should be able to map memory 
below 1024K.  Many exPANDed memory boards can map 64K (four 16K 
pages - usually known as the page frame) below 1024K but not below 
640K.  These boards' hardware supports the EMS 3.2 memory 
specification.  (Adding an EMS 4.0 driver doesn't allow them to map
memory below 640K. This requires hardware as well as the software 
driver). 


Can the board MAP MEMORY BELOW 640K?

     The most desirable feature of an exPANDed memory board on a 
DESQview system is the ability to map memory below 640K.  Some 
boards can map memory down to 0K.... provided your motherboard 
memory can be totally disabled.  This property should not be 
confused with backfilling motherboard memory.  Backfilling 
motherboard memory simply fills your DOS program memory area back 
up to 640K. The ability to map memory to the area between 0 and 
640K is MANDATORY for multitasking of several large programs.


Does the board have ALTERNATE PAGE MAPPING REGISTERS?

     ExPANDed memory is allocated in 16K blocks called pages. When 
DESQview opens applications it instructs the memory manager to 
allocate enough of these pages (which may not be contiguous) and 
make them appear contiguous and map them below 640K. When you 
switch among applications the memory manager has to sort through 
all these 16K pages, find the correct ones, and map them into the 
area below 640K.  This can take a long time to do for a large 
memory area.  The delay may not be apparent to you unless you're 
running communications in the background.  If the memory card 
supports alternate page maps, DESQview can instruct the memory 
driver to save the necessary information to an alternate page map 
register (again this is hardware, managed by a software driver).  
When you switch windows, DESQview instructs the memory driver to
call the page map rather than sort through all the expanded 
memory.  This is very important if you need to run high speed 
communications in background, or have several large applications 
running simultaneously.  Large numbers of alternate page mapping 
registers on the board result in better multitasking performance 
when running several programs at once.  


Where can I find out about ExPANDed memory cards?

      Obtain a copy of InfoWorld's December 12, 1988 issue or PC 
Magazine Volume 8, Issue 21 (December 12, 1989) issue (page 169). 
Both articles give a very detailed comparison of ExPANDed memory 
boards.  Most exPANDed memory card manufacturers and their boards 
are listed.  Call InfoWorld at (415) 328-4602 or PC Magazine at 
(212) 503-5255 for information on obtaining reprints.


Other Choices

     If you have an 80286 based computer but cannot disable 
motherboard memory, there are two options available.  The first is 
a memory management unit like the All Computers CHARGECARD or the 
Sota POP card.  While these devices contain no memory, they in 
fact have the ability to convert ALL the conventional and exTENDed 
memory on an 80286 based machine into ExPANDed memory. 

     The second option is to convert the computer to an 80386. 


Is a 386 better for multitasking?

     YES!!!!!!  An 80386 is not only faster than the 8088 and 
80286 based computers (speed is only one of its many virtues). It 
is incredibly more intelligent!  It has the capability to run in 
virtual 8086 mode.  Used in conjunction with QEMM-386, a 386 
requires no special memory cards, and no motherboard memory needs 
to be disabled. The 80386 can allow QEMM to remap all exTENDed 
memory into exPANDed memory.  DESQview-386 can run bit-mapped 
graphics programs like Ventura Publisher, AutoCAD, GEM, etc. in 
small windows in the background.  In addition, DOS Extended 
programs like Lotus 1-2-3 Release 3 and AutoCAD 386 that use the 
VCPI (Virtual Control Program Interface) can co-exist with other 
programs that use exPANDed memory.  Memory protection is available 
in DESQview-386 for ill-behaved programs.  These capabilities are 
not available on 8088 or 80286 computers.  If you're considering 
this option and have the means (money), don't hesitate for a 
second.

        Copyright (C) 1991 by Quarterdeck Office Systems
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