Computer Systems

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Computer Systems

• Exceptions and Interrupts

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Exceptions

• The CPU operates in one of two states user or
  supervisor.
• The OS is running in the supervisor mode Users
  program is running in the user mode.
• Each state has different stack pointer SSP and
  USP.

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State Transition Diagram

• Exceptions are calls to the operating system.

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Exception Table

• There are different types of exception.
• Each type of exception has its exception
  handler.
• Exception handler is a program, the program is
  part of operating system.
• An exception vector table contains the address
  of all exception handlers.
• When there is an exception from the user
  program, the CPU is forced to operate in its
  supervisor mode.
• Based on the type of exception, the OS finds out
  the address of exception handler from the table.

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Interrupts and Exception

• Interrupt is a specific instance of the
  exception.
• When an interrupt occurs, the CPU decides
  whether to service it. The time between the CPU
  receiving an interrupt and time at which it
  responds is the interrupt latency.

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Instruction Cycle with Interrupts
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Simple InterruptProcessing
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Interrupts and Exception

• Nonmaskable interrupts This interrupt can not
  be deferred and must be served. (e.g. power off)
• Prioritized interrupts Each interrupt has a
  predefined priority, a new interrupt with lower
  priority can not interrupt the current interrupt
  handling.
• Vectored Interrupts After the I/O receives IACK
  from the CPU, it can identify itself by providing
  CPU with location of interrupt handling routine.

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Multiple Interrupts - Sequential
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Multiple Interrupts - Nested
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68000 Familys Exceptions

• Hardware (External)
• Interrupts
• Bus error
• Reset

• Software (Internal)
• Trace 
• Errors
• illegal instructions
• address error 
• privilege violation 
• divide by zero 
• Programmer initiated