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Section Two is concerned with traps, hardware interrupts and software interrupts.
Traps and hardware interrupts introduce sudden switches into the CPU’s normal instruction execution sequence. This provides a mechanism for handling special conditions which occur outside the CPU’s immediate control.
Use is made of this facility as part of another mechanism called the “system call”, whereby a user program may execute a “trap” instruction to cause a trap deliberately and so obtain the operating system’s attention and assistance.
The software interrupt (or “signal”) is a mechanism for communication between processes, particularly when there is “bad news”.
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