ID Number: Q49732
3.00
WINDOWS
Summary:
The following information describes why WIN /3 and WIN /R run slower
than WIN /2.
WIN /3 is slower than WIN /2 because it runs in 80386 protected mode
and virtualizes all devices and I/O operations. WIN /3 also provides
demand-paged virtual memory using the hard disk to swap paged-out RAM;
this feature involves extra validity checking and disk swapping when
page faults occur, which takes some extra time.
WIN /R is slower because it has limited memory space (the 640K maximum
provided by MS-DOS), and therefore, must perform a lot of moving and
discarding of code and data segments when the memory space is
"overcommitted" by the system and running applications.
WIN /2 is fastest because although it runs in the protected mode of
the 80286 and is somewhat slowed down by that, it has MUCH more memory
available for applications to run in; therefore, less moving and
discarding of code and data segments is necessary. The speed gains of
WIN /2 compared to WIN /R depend on how much extended memory is on the
machine and how much is needed by the system and all running
applications. For example, where WIN /R has to discard and move some
segments to make room for a new application to run, WIN /2 simply
allocates more of the global heap, which consists of all available
extended memory.
Some of these factors apply to Windows versions 2.x (2.03, 2.1, 2.11)
as well as Windows version 3.0. For example, Windows/386 versions 2.x
run in protected mode and virtualize devices and I/O, which slows
things down. However, Windows/286 versions 2.x run in real mode and
gain speed from that. On the other hand, Windows/386 versions 2.x
provide Lotus/Intel/Microsoft Expanded Memory Specification (LIM EMS)
version 4.0 to applications, which allows more applications to fit in
memory at the same time. However, Windows/286 does not provide its own
EMS memory for applications, instead relying on an already-installed
EMS board or "limulator" to provide it. If the system does not have
any EMS memory, Windows/286 is forced to discard and move memory more
often.