Appeal 2007-2127
Reexamination Control No. 90/006,621
l. Multithreaded versus sequential programs
A sequential or linear program has a single thread of execution that
executes instructions from the start to the finish with no simultaneity or
overlap among instructions (no concurrency); there is only one program
counter that traces the path of execution of instructions through the program.
By contrast, the defining characteristic of multithreading is that multiple
threads execute instructions concurrently, i.e., more than one thread is
executing at the same time, and each thread may be executing instructions in
a different part of the program, as indicated by its individual program
counter. If there was a processor available for each thread, all threads could
run simultaneously at exactly the same time, but, for a machine with one
processor, the threads must take turns running their tasks a little at a time.
As stated in LaFore, Peter Norton's Inside OS/2, page 134:
A computer program, when it is running, executes a sequence of
instructions which are stored in the computer's memory. This
activity—executing a sequence of stored instructions—is called a
thread, or sometimes a thread of execution. Think of an instruction
pointer that points to the next instruction to be executed. Generally
the instruction pointer moves forward through memory, pointing to
each instruction in sequence, but sometimes it jumps to a different
part of memory (to accommodate loops, function calls, and other
non-sequential activities). This instruction pointer tracks the activity
of a particular thread.
When a process is started it begins executing a single
thread. . . . However, . . . a thread can create a new thread. Both
threads can then run at the same time. What does this mean? Again,
imagine that each thread has its own instruction pointer, showing
where it is in the instruction sequence in memory. The scheduler lets
one instruction pointer move through memory for a while, then the
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