TITLE:              DPMS: What are the Tradeoffs?
DOCUMENT ID:        TID14947
DOCUMENT REVISION:  1
DATE:               05MAY94
ALERT STATUS:       Yellow
README FOR:         NA

NOVELL PRODUCT and VERSION:
Novell DOS 7

ABSTRACT:
NA

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ISSUE:

DPMS: What Are the Tradeoffs?

Novell, to provide the greatest flexibility possible, includes a wide
range of tools in Novell DOS 7 for configuring and using your system. 
It is important to note that each of these tools, while providing
very important and advanced features to your system, has a cost
associated with it.  For example, tools that can enhance the speed of
your system will typically cost memory, because these programs are
usually in the form of memory resident software, such as TSRs and
device drivers.  On the other hand, tools that increase the
availability of conventional memory can cost in system performance.

Novell introduced DOS Protected Mode Services (DPMS) with Novell DOS
7.  DPMS is a memory management technique in which memory resident
software can be written to load part of their code into the extended
memory area, above 1 MB.  This technique makes it possible for more
conventional and upper memory to be available to large applications
and other memory resident software that must be loaded into the first
megabyte of memory.

Any program written for DPMS is responsible to request the DPMS
"server" to provide the protected mode services it requires. 
DPMS.EXE is the DPMS server.  It handles the allocation of extended
memory and keeps track of programs that use the extended memory.  The
program that uses the DPMS informs the server which parts of itself
can be loaded into extended memory.  When the memory resident
software is loaded, it "interprets" that it was using DPMS, and
instructs DPMS.EXE to run the code that was put into the extended
memory area.  To do this, there must still be at least a small
portion of code in the conventional or upper memory area to perform
these requests to the DPMS server.  This conventional memory
"footprint" is controlled by the software.  That is, the memory
resident program decides which parts of itself can and cannot be
loaded into extended memory.

Sometimes there is a decrease in the performance of a particular
device driver or TSR that uses DPMS versus the "non-DPMS" version of
the software.  This are two reasons for this.  First, to access
extended memory, the processor must run in Protected Mode.  However,
DOS typically operates in Real Mode.  So the DPMS server must switch
the processor in and out of protected mode to run the memory resident
software that makes use of its services.  Second, the footprint of
the TSR left in conventional memory is quite often loaded into upper
memory, assuming there is a large enough block of upper memory
available for it.  It is important to note that access to the upper
memory area is somewhat slower than running in conventional memory.

Because of these two reasons, that the processor must be constantly
switched in and out of protected mode and that upper memory can be
slower than conventional memory, DPMS programs can operate slower
than non-DPMS programs would.  DPMS is one of the tools provided by
Novell DOS for the greatest configuration flexibility.  It allows the
user to choose what is of the most importance on their particular
system, either system performance or memory management.  It is
important to note the costs of using any of the tools provided with
any operating system and to use the tools most appropriate for your
needs.

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