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CHAPTER 3 CPU AND CONTROL UNITS
3.8
EIT (Exception/Interrupt/Trap)
EIT indicates that a program being executed is suspended by an event for the purpose
of executing another program. EIT is a generic term for exception, interrupt, and trap.
■ EIT (Exception, Interrupt, and Trap)
An exception is an event that occurs in relation to the current context. The CPU resumes execution of the
suspended program from the instruction that caused the exception.
An interrupt is an event that occurs independently of the current context. The event is generated by
hardware (event source).
A trap is also an event that occurs in relation to the current context. Some traps are programmed as system
calls. The CPU resumes execution of the suspended program from the instruction immediately following
the instruction that caused the trap.
■ Features
•
Multiple-interrupt support
•
Interrupt level masking function (15 levels available to the user)
•
Trap instruction (INT)
•
Emulator trigger EIT (hardware or software)
■ EIT Sources
EIT sources are as follows:
•
Reset
•
User interrupt (internal resource or external interrupt)
•
NMI (nonmaskable interrupt)
•
Delayed interrupt
•
Undefined instruction exception
•
Trap instruction (INT)
•
Trap instruction (INTE)
•
Step trace trap
•
Coprocessor absence trap
•
Coprocessor error trap
■ Return from EIT
RETI instruction is executed to return from EIT.
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