IO Interrupts

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I/O Interrupts

• Thomas Roth

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What is I/O inpterrupt

• I/O interrupts are the way the computer is forced
  branched to the required subroutine
• Subroutines are require to
• 1.Transfer a data word between an I/O device and
  main storage (read/write operations)
• 2. Checks conditions related to the I/O device
• 3. Handle the ending of the I/O device operation

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How I/O interrupts are used in routine
instructions

• The diagram above is an example of a job program
  routine
• The branch from job routine to I/O routine occurs
  at point A

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I/O interrupts in routine instruction (continue)

• Branching occurs because the program is at a
  point that the I/O operation is required.
• E.G. the reading of a card in the card reader
• When the I/O routine ends, another branch is made
  back to the job routine .
• I/O routine and job routine dont overlap due to
  the CPU executing only one instruction at a time.
• Also , when the I/O device operation is started,
  it continues as the program-instruction
  execution takes place in the CPU.

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Interrupted Action

• When the I/O device operation starts, and then
  moved far enough to be read, then it give a
  signal to the CPU ( which is a interrupt
  request).
• But also the interrupt doesnt occur until after
  the current instruction has been completed.
• Thus a forced branch occurs to an
  interrupt-handling subroutine (more on interrupt
  subroutine in later slide/s).

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A example of a interrupted action
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Interrupted action (continue)

• In order to return back to the program routine in
  progress (before the interrupt happened), an
  program-controlled branch is created.
• This type of branch is NOT forced by the CPU.

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Program-controlled branch

• In a program-controlled branch to an I/O routine,
  a program has address information for the
  branch-to location and (if needed) the return
  address.
• A CPU-forced branch must properly branch to the
  proper subroutine and return, through another
  branch, to the interrupted program at the end of
  the subroutine.
• At the time of the actual interrupt, the next
  instructions address is in the
  instruction-address register.

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Interrupt Subroutine

• The first word of the interrupt-subroutine
  location stores the next instruction at the time
  of the interrupt that a CPU-enforced branch is
  executed.
• This branch is also a branch-and-store-IAR
  instruction

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format of a branch-and-store-IAR instruction
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Interrupt Subroutine (continue)

• The contents of the instruction are kept in the
  first word location of the interrupt subroutine
  until necessary to branch back to the interrupted
  program, which is when the interrupt is handled.
• Also, when the program is originally loaded into
  the system, the subroutine addresses are stored
  into the interrupt vectors.

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Saving Data used by The Interrupted Program

• Before machine registers and certain index are
  used for data manipulation within the subroutine,
  the interrupt subroutine must save their
  contents, but only the ones being used by the
  interrupt subroutine.
• This must be maintained because the interrupted
  program may use the same registers.
• At the end, the affected registers contents are
  loaded back into the core storages registers.

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Causes of interrupts

• To determine what I/O device is causing a
  interrupt, examine the interrupt-level staus
  word, if necessary.
• After the device that is causing the interrupt is
  found, the device-status word is examined the
  condition in the device that cause the interrupt
  to happened.

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Causes of interrupts ( continue)

• The program must determine which operation
  occurs.
• When the subroutine determines whether the
  interrupt is for a read or punch (by addressing a
  sense-device I/O control command) either one of
  two actions happen
• 1. For read response, a execute I/O instruction
  addresses a read I/O control command that moves
  the data word from the 1442 to core storage.
• 2. For punch response, a execute I/O instruction
  addresses a write I/O control command that moves
  the data word from the core storage to the 1442
  punch.

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Causes of interrupts part 3

• After the end of the level-0 interrupt
  subroutine, saved contents of registers are
  loaded from core storage back into registers, and
  a branch is made back to the interrupted program.
• The interrupted-level stats words defined for
  interrupt levels 1, 2, 3, 4, and 5.
• After the words are loaded into the accumulator,
  the device causing the interrupt can be determine
  by the contents of the word.
• After the device causing the interrupt is
  determined, to determine the condition within the
  device that caused the interrupt, a
  sense-device-status command can be executed.

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Sources

• Diagrams and facts are used from the following
  places
• http//www.ibm1130.net/functional/IOInterrupts.htm
  l