8259A Interrupt Controller on the PC

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I received this from Richard Clayton (richard@locomotive.com). It is
written by one of his colleagues, Chris Hall (chris@locomotive.com).

This file won't save you from having to look at the data sheet as
well... it's just an explanation of what those guys at Intel are trying
to say in the specs, and how the 8259 is used in a PC. :)



             8259A Interrupt Controller on the PC

                                        GMCH  1 Dec 93 10:49

1.  Introduction & Scope

The 8259A is a wonderful device.  It has a number of modes of
operation, and some magic priority rotation features.  On the
PC, however, only the very simplest mode is used.

This discussion is limited to the way in which the 8259A is
used on the PC.

The function of the 8259A is to take up to eight interrupt
sources, and feed any active interrupts, one at a time, to the
CPU.  The processor can individually mask off interrupt
sources.  The 8259A has a priority mechanism, so that lower
priority interrupt sources do not interrupt the CPU while it is
servicing a higher priority interrupt -- but a higher priority
interrupt will be passed on to the processor.

2.  The Jargon

An interrupt source, fed into an 8259A, is known as an IRQ --
Interrupt Request.  An 8259A has 8 IRQ inputs.

The 8 IRQ inputs are fed into an 8 bit Interrupt Request
Register (IRR), via some "rising edge" detection logic (8 bit
Edge Sense Register -- ESR).

The 8259A can be told to mask off any of the IRQ.  The 8259A
has an 8 bit Interrupt Mask Register (IMR).  A one bit in the
IMR masks off the corresponding IRQ.

To perform its priority arbitration the 8259A has an 8 bit In
Service Register (ISR).  In the register a bit is set to 1 when
the corresponding interrupt has been passed to the CPU, and the
CPU has not yet signalled End of Interrupt (EOI)

The CPU interrupt input is known as INTR.  The 8259A interrupt
output is known as INT, which is connected to the CPU (wait for
it) INTR, or to another 8259A's IRQ.

The 8 IRR bits are ANDed with the NOT of the IMR, giving the
interrupt request input to the priority arbitration logic.
Reading between the lines, there is an INT latch, which is set
by the OR of the bits of (IRR AND NOT IMR) higher than the
highest priority bit in the ISR.

On an original PC there are 8 possible interrupt sources IRQ0
to IRQ7, fed into one 8259A (I/O address #020..#03F).

On AT's and beyond, there are 16 possible interrupt sources
IRQ0 to IRQ15, fed into two 8259A's.  One 8259A (known as #1,
I/O address #020..#03F) is the "Master" and the other is a
"Slave" (known as #2, I/O address #0A0..#0BF).  Only the
Master's INT is connected to the CPU's INTR.  The Slave's INT
is connected to the Master's IRQ2.

3.  The Mechanisms

The PC sets the 8259A into:

  * Edge Triggered Interrupts
  * Cascaded (on AT and later) ; Single (on earlier machines)
  * Not Special Fully Nested (to do with Slave 8259A, see below)
  * Not Buffered Normal EOI (Not Automatic EOI on INTA)

With this in mind, we will start with the simple cases, and
work up.

3.1  One 8259A, All IRQ Unmasked, No Interrupts In Service
     and None Active.

So we start from the simplest possible quiescent state.  The
sequence of actions is as follows:

   0  The ESR, ISR, IRR and IMR are all zero.
   1  IRQ3 becomes active (goes to 1)
   2  B3 of the ESR is set to 1
   3  B3 of the IRR is set to 1
   4  B3 of the IMR is 0, so the IRR B3 is passed to the
      priority arbitration logic.
   5  All bits of the ISR are 0 (no interrupts are in
      service), so the priority arbitration logic sets the
      INT latch -- so the INT output is set active.
   6  Eventually the CPU issues the first of two INTA
      cycles.  The contents of the IRR are frozen.  The
      8279A selects the highest priority IRR (B3) and sets
      the corresponding ISR (B3).
   7  Setting B3 of the ISR clears B3 of the ESR.
   8  The CPU issues the second of two INTA cycles.  The
      8279A issues the interrupt vector associated with the
      highest priority ISR (B3).  The contents of the IRR
      are unfrozen.
   9  The INT latch is cleared -- so the INT output is set
  10  B3 of the IRR is set to 0 (IRR is unfrozen and B3 of
      ESR is zero).
  11  At some time in the future, the CPU issues an EOI
      command, which clears B3 of the ISR.

IRQ3 can remain active beyond step 10, without generating any
further interrupts -- because B3 of IRR has been cleared.  To
produce another interrupt requires IRQ3 to go inactive (0), and
then active (1) again.

3.2  Meaning of "Edge Triggered Interrupt Mode"

The behaviour of the ESR, IRR and ISR described above is what
happens in the famous Edge Triggered Interrupt Mode.

The purpose is to allow for IRQ signals to be short down/up
pulses.  When the 8259A is reset the ESR is set to zero.  An
upward transition of the IRQ sets the corresponding ESR bit to
1, which allows the IRQ state to be copied to the IRR --
provoking the interrupt.  When the interrupt is acknowledged
the ISR bit is set, which resets the ESR bit, which forces the
IRR bit to zero -- irrespective of the IRQ.  So even if IRQ is
still 1 when the ISR bit is cleared, at End of Interrupt, no
further interrupts will be generated.

3.3  What Happens if IRQ Changes with the Interrupt is In Service

It is clear what happens if IRQ does not do any further down/up
transitions until after EOI.  It is OK for IRQ to go down
before EOI, but going up again is not explicitly described in
the manuals.

If a down/up IRQ transition cannot be prevented before EOI,
then it can be (reasonably safely) assumed that this will
generate a further interrupt after EOI -- provided the IRQ is
still up (active, 1) at EOI.  Multiple down/up transitions can
be assumed to have the same effect.

What happens if there are one or more down/up IRQ transitions
followed by a final down transition before EOI, is also
undocumented.  I guess that this has no effect.  The
corresponding IRR bit will follow the IRQ, but this may be
expected to have no effect on the (supposed) INT latch, because
the ISR bit prevents it.

Obviously, it would be safer to ensure that IRQ does not go
down and then up again before EOI (just down is OK).  If this
is not possible, then I believe the given assumptions to be
reasonable -- perhaps MEJ's boys could help us !

3.4  Master and Slave Handling

The PC does not use "Special Fully Nested Mode".  What this
means is that once one of the Slave's interrupts is In Service
it takes precedence over all other Slave interrupts.

Slave interrupts are generally indistinguishable from Master
interrupts.  The only tricky bit is that an EOI must be sent to
both Master and Slave.  EOI should be sent to the Slave first
-- to allow any lower priority interrupts on the slave to
assert themselves.  The EOI to the Master then allows any lower
or equal priority interrupts to assert themselves.

3.5  Fiddling with Interrupt Masking

Clearing masking or unmasking an interrupt when all is quiet
(no IRQ, IRR or ISR) is trivially OK, and produces no side

If an interrupt was masked and is unmasked, then any "pending"
IRQ will immediately take effect.

According to the 8259A diagram published by Intel, the IMR mask
gates the IRR bits into the interrupt priority resolution
logic.  Masking and unmasking an interrupt while its IRR is
active is equivalent, as far as the interrupt priority
resolution logic is concerned, to the IRQ coming and going.
The effect can be seen to be the same as in 3.3 above.

There is doubt and uncertainty about what happens if IRQ
bounces up and down while ISR is set.  To avoid difficulties it
would be reasonable (unless the Intel diagram I am working from
is entirely wrong) to mask off the interrupt until EOI, and
then unmask it again -- assuming that there is some other way
of detecting and dealing with the reasons for IRQ changing.

----------end of document-----------


Note the worry about IRQ boucing up and down... this all comes
back to the amusing behaviour of the serial chip whilst it is
being serviced.

Also, I have seen discussion (in the Crynwr packet driver source; on
a Simtel20 mirror near you) of broken Chips & Technology (?) 8259s which
do not handle non-specific EOIs properly.

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