April
1999, Issue 105
Win32
and Real Time
by
Peter Petersen and Tom Schotland
Tempted
to use Windows NT or 95 as an RTOS? Peter and Tom
have an alternative. They create a Win32-compatible
environment with dedicated embedded-system development
tools—less overhead, no real-time deficiencies.
Software
development tools are available and have been tested
by countless developers. And, CPU vendors offer a range
of 32-bit Intel i80386-compatible microcontrollers like
Intel’s ’386EX, AMD’s Élan series, and
National Semiconductor’s NS486SXF.
But,
there are problems, too. Windows NT and 95 need a lot
of computer resources. Windows 95 needs at least 16
MB to run smoothly, and Windows NT needs 32 MB to perform
well (not counting the application’s needs).
Both
systems require 100–150 MB of hard disk space.
Even Microsoft’s Windows NT Embedded variant requires
16 MB of RAM and 20 MB of ROM or other permanent storage.
Another
important issue is real time. Windows was designed for
home and office applications, which (except for games
or multimedia applications) have weak or no real-time
requirements.
The
ability to assign real-time priorities to a process
may mislead developers to believe that Windows 95 and
NT support real-time processing. But, Windows dynamically
changes priorities at runtime to achieve a more equal
or fair CPU time distribution. Sharing the CPU equally
is not the goal of a real-time system.
Win32’s
priority scheme implementation is also subject to priority
inversion for application threads and deferred procedure
calls of device-driver interrupt-service routines.
And,
nonpreemptable system calls can use a nondeterministic
amount of time. The internal timer resolution is fixed
at 10 ms—too coarse for many systems. Besides,
Windows does not guarantee that timer callbacks can
be serviced within a maximum delay.
Application
speed is another issue. Demon processes to implement
various OS services such as virtual memory management
can steal CPU time from a time-critical application.
Multitasking
with Windows NT and 95 is not as efficient as in dedicated
real-time systems. For example, running the code in
Listing 1 under Windows
NT, Windows 95, and an RTOS yields the results graphed
in Figure 1.
|
Figure 1—The benchmark performs 400,000
semaphore operations with 200,000 task switches.
The times shown here are for a 120-MHz Pentium.
|
Last
but not least, licensing costs are a crucial factor
for embedded systems development.
