Mark Smotherman
Last updated: July 2015

Summary: Interrupts are a vital part of sequencing a modern computer. They were developed for exception handling and were later applied to I/O events. Today, they also also play a role in protection and memory management. However, interrupts can be disruptive to both program design and to high performance.

.. under construction ..


Machines discussed:
Univac-I (1951) - overflow trap
NBS DYSEAC (1954) - I/O interrupt
Univac 1103/1103A (1955 innovation) - wind tunnel interrupt
IBM 704 (1955) - transfer trap (for debugging)
IBM Stretch (1957 paper) - vectored interrupt system
Lincoln Labs TX-2 (1957 paper) - multiple-sequence system
Electrologica X-1 (1957-1958 innovation) - vectored interrupt system
Manchester Atlas (1961) - page fault

Comments on interrupts

Design tree

Performance techniques

Deferred interrupt handler structure


In my 1989 paper, "A Sequencing-Based Taxonomy of I/O Systems and Review of Historical Machines," Computer Architecture News, vol. 17, no. 5, pp. 5-15, September 1989 (and reprinted as Paper 1 of Chapter 7, in M. Hill, N. Jouppi, and G. Sohi (eds.), Readings in Computer Architecture, Morgan Kaufmann, San Francisco, 2000),

Short Review of the History of Interrupts

However, the rest of the story includes

Strachey's comments on interrupts

Christopher Strachey, "Timesharing in Large Fast Computers," International Conference on Information Processing, June 1959, Paris, pp. 336-341 [IFIP conference; the proceedings were published by UNESCO in 1960].

... to be added ...

Denning's comments on interrupts

Peter J. Denning, "Principles for Reliable Operating Systems," January 28, 2003

Dijkstra's comments on interrupts

Hardy's comments on interrupts

Norm Hardy's history of interrupts

Design Tree

(adapted from Blaauw and Brooks, section 7.4, on interrupts)

  1. terminology and connotations (there are no well-defined, widely-accepted distinctions among the various terms)
    1. asynchronous, external
      1. interrupt - external device (I/O, clock, etc.)
      2. machine check - hardware error or failure
    2. synchronous, internal
      1. general terms
        1. alarm
        2. exception - special or undefined case (e.g., divide by zero)
        3. internal interrupt
        4. program check
        5. trap (sprung like a "mousetrap")
      2. terms associated with no continued execution
        1. abort - exception in the middle of an instruction
      3. terms associated with continued execution (either restart or resume)
        1. fault - missing information (e.g., page fault)
      4. terms associated with user request to OS
        1. mme (master mode entry)
        2. software interrupt (e.g., INT opcode in x86)
        3. svc (supervisor call)
        4. syscall
        5. trap (trap always on SPARC)

  2. entry point (i.e., where CPU execution diverted)
    1. fixed memory address (e.g., PDP-8)
      1. poll to find cause (e.g., PDP-8)
      2. code indicating cause placed in register
    2. one of many memory locations (interrupt vector)
      1. number of vectors
        1. single, globally shared (e.g., Electrologica X-1, S/360, x86)
        2. multiple, one per process (e.g., IBM Stretch)
      2. vector contents
        1. interrupt vector entry holds new PC (and maybe new PSW) (e.g., S/360, x86)
        2. interrupt vector holds blocks of instructions (e.g., SPARC)
    3. different PC (e.g., DYSEAC, TX-2)

  3. return linkage
    1. fixed memory location(s) (e.g., PDP-8, S/360)
    2. memory stack (e.g., x86, M68K)
    3. register (e.g., SPARC)

  4. permission
    1. single interrupt enable bit (e.g., PDP-8, 8086 w/o PIC)
      • enable/disable interrupt instructions
    2. interrupt enable mask in PSW (e.g., S/360)
    3. interrupt priority code in PSW (e.g., SPARC)
    4. nonmaskable (cannot be ignored)
    5. breakpoint bit in each instruction (e.g., DYSEAC)
      • device priority along with break and dismiss bit in each instructions (e.g., TX-2)

  5. techniques to accelerate interrupt handling
    1. cause determination (see cause register and interrupt vector)
    2. additional register sets
    3. interrupt poll to coalesce multiple interrupts (e.g., ITI on B5500 in 1960s, test pending interrupt on S/370 XA)

Performance techniques

on another note, Rob Warnock writing in comp.arch on 16 Jul 2001:

As far as the areas that I've been personally involved in -- mostly high-speed networking -- the best hardware/software tradeoffs have been mostly just judgment calls by very experienced folks. What I've noticed is that a *LOT* of "lessons learned" decades ago are still being re-learned by later generations, over and over again. We as an industry don't seem to have been doing a very good job of passing on the "secret sauce" to subsequent generations [in many cases, even *within* the same companies!].
For example, way back in the early 1970s at Digital Communications Assoc. (in Atlanta, made communications front-ends for DEC PDP-10s and IBM 360s using DEC PDP-8e's, successfully sold in *competition* with DEC's own PDP-11-based front-ends!], we made simple interrupt-per- character PIO-based terminal controllers that were -- in terms of host CPU cycles -- *more* efficient (and a lot cheaper!) than DEC's complex DMA-based controllers, partly because we used three simple techniques, two programming and one hardware: (1) interrupt "coalescing", (2) "dallying" before dismissing an interrupt, and (3) interrupt "holdoff". That is, (1) during a single interrupt, process *all* of the data that's available or that *becomes* available (looping to repeat the availability test as needed) during a single interrupt; (2) spin-wait/test a certain amount (which may even be context/data-dependent!) before dismissing an interrupt, so as to catch new data and process it without having to dismiss/interrupt again; and (3) once you finally *do* dismiss an interrupt, arrange the hardware so that you won't get another one until a certain (sometimes dynamically) chosen time interval has expired (also known as "interrupt rate-limiting", which, together with the other two, prevents "thrashing" in the interrupt service routine).
[There's another variant of #2 that's worth mentioning: when doing PIO writes to a FIFO and hitting a FIFO-full condition, "dallying" (spin-waiting) a certain amount first rather than enabling a watermark interrupt and going away. If you pick the dally amount appropriately, by "wasting" a few cycles you actually can save *lots* of total CPU time, since you (hopefully) avoid a later heavy-weight interrupt/context-switch/dismiss.]
These techniques were *NOT* unique to DCA!! In talking to other engineers at conferences & trade shows, we learned that such things were generally widely known among workers in the field (especially embedded systems, datacomm, and real-time folks), but were not often discussed openly, being considered somewhat "trade secrets". [That is, why tell your competitor why his performance sucks and yours doesn't?!?]
In the 30+ years since then, I've been astounded how many times the techniques of coalescing/dallying/holdoff have been rediscovered, but I've been *more* astounded how many times they haven't been used *at all* in many places where they were definitely needed, or how they were reinvented badly. [E.g., if you're implementing hardware assistance for interrupt holdoff, it is *extremely* important that the holdoff period be effective only *after* the dismissing of an interrupt, and not delay the response to or processing of a "new" event that arrives when the system has been idle for longer than the holdoff.] "Computer Science" courses don't seem to ever discuss such practicalities, either.

(E.g., see US 3,789,365 issued January 1974 and US 4,342,082 issued July 1982)

Deferred interrupt handler structure

In this section I want to trace the development of the idea of "bottom half" or "deferred" interrupt handlers (a.k.a. fork routines, interrupt queueing, interrupt stacking, as well as the term "Cutler kernel" structure).

The idea is that a "first half" interrupt handler dynamically creates a new thread or "fork routine" for later scheduling and execution, as opposed to an interrupt handler that runs to completion or a barebones interrupt handler that merely awakens a previously-created driver process. (For example, in RSX-11M, interrupt handlers fork a routine for later execution by adding a four-word fork block to a FIFO fork queue. The fork routines must be executed before any user processes are scheduled.)


  1. run the more-involved handling with interrupts re-enabled
  2. serialize handler access to kernel data structures in a uniprocessor (using a special queue of deferred handler tasks)
  3. allow the later invocation a non-resident system routine as part of the interrupt handling

The idea appears to date from at least the early 1960s with the soft-real-time monitor developed for the Project Mercury space program. [See the discussion of "Cutler kernel" in Bruce R. Montague, "JN: An Operating System for an Embedded Java Network Computer," UCSC-CRL-96-29, 1996.]

Fred Brooks said that having to stack interrupts influenced Seymour Cray to design the CDC 6600 with polling rather than interrupts.

CDC 3600

two systems with deferred handling are cited by Richard Watson in Timesharing System Design Concepts, 1970, pp. 126,144:

Montague, op cit., also mentions the IBM TSS (mid-1960s) as using deferred handling.

genealogical tree involving Dave Cutler

Linux deferred handlers

See Jonathan Corbet, Alessandro Rubini, and Greg Kroah-Hartman, Linux Device Drivers, Third Edition


Hardware support in x86

Corrections are welcome!

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