Threads in Windows NT/Windows 95
1.0 Introduction
Windows is a preemptive, multi-tasking, multithreaded operating system. This is much
more flexible than the old MS-DOS "operating system" (there is a question as to whether
MS-DOS was truly an operating system), which allowed only a single task at a time. It is
also somewhat more flexible, in regard to tasking, than traditional UNIX, which is a
preemptive multi-tasking system, but which is not a multi-threaded operating system. A
"thread" is different from a "process." A multi-tasking operating system employs
processes, but does not employ threads. A multi-threaded operating system must employ
both processes (it is inherently multi-tasking as well as multi-threaded) and threads.
A "process" in a multi-tasking system is given a time slice of CPU time by the
operating system. When that time slice expires (the time slice is measured by a quantum
timer), then the operating system preempts the process, and allows the next process to
run. Normally, each process in a multi-tasking operating system is given it's own data
storage space, which is normally organized as part of the system stack.
A process can be broken down itself into threads. The breaking of a process into
threads is normally performed under application programmer control (the application
programmer is the programmer who wrote the process). A thread is not just a smaller
process. The primary differences between a process and a thread are:
threads share the memory space of a single process (although each thread is
assigned its own section of the memory space).
the processor overhead of switching the CPU from one thread to another
thread is typically lower than for switching the CPU from one process to
another process
a thread will typically receive a smaller time slice of CPU time than a process,
since a thread normally receives part of the time slice assigned to its parent
process
Figure 1 describes the differences between processes versus threads.
Quantum Quantum
Processes
Threads
Process 2
Process 1
Figure 1. Processes versus Threads
