8259 - Programmable Interrupt Controller (PIC)

Bit	Initialization Command Word 1
0	1=ICW4 is needed 0=no ICW4 needed
1	1=single 8259 0=cascading 8259's
2	1=4 byte interrupt vector 0=8 byte int vectors
3	1=level triggered mode 0=edge triggered mode
4	must be 1 for ICW1 (port must also be 20h or A0h)
5-7	must be zero for PC systems

Bit	Initialization Command Word 2
0-2	000= on 80×86 systems
3-7	A7-A3 of 80×86 interrupt vector

bit	Initialization Command Word 3 for Master Device
0	1=interrupt request 0 has slave 0=no slave
1	1=interrupt request 1 has slave 0=no slave
2	1=interrupt request 2 has slave 0=no slave
3	1=interrupt request 3 has slave 0=no slave
4	1=interrupt request 4 has slave 0=no slave
5	1=interrupt request 5 has slave 0=no slave
6	1=interrupt request 6 has slave 0=no slave
7	1=interrupt request 7 has slave 0=no slave

bit	Initialization Command Word 3 for Master Device
0-2	master interrupt request slave is attached to
3-7	must be zero

Bit	Initialization Command Word 4
0	1 for 80×86 mode 0 = MCS 80/85 mode
1	1 = auto EOI 0=normal EOI
2-3	slave/master buffered mode (see below)
4	1 = special fully nested mode (SFNM) 0=sequential
5-7	unused (set to zero)

bit	Operation Control Word 1 - IMR Interrupt Mask Register
0	0 = service IRQ0 1 = mask off
1	0 = service IRQ1 1 = mask off
2	0 = service IRQ2 1 = mask off
3	0 = service IRQ3 1 = mask off
4	0 = service IRQ4 1 = mask off
5	0 = service IRQ5 1 = mask off
6	0 = service IRQ6 1 = mask off
7	0 = service IRQ7 1 = mask off

bit	Operation Control Word 2 - ICR Interrupt Command Register
0-2	interrupt request level to act upon
3	must be 0 for OCW2
4	must be 0 for OCW2
5-7	EOI type (see table)

Bits			EOI - End Of Interrupt code (PC specific)
7	6	5
0	0	1	non-specific EOI command
0	1	0	NOP
0	1	1	specific EOI command
1	0	0	rotate in automatic EOI mode
1	0	1	rotate on non-specific EOI command
1	1	0	set priority command (uses bits 2-0)
1	1	1	rotate on specific EOI command

bit	Operation Control Word 3
0	1=read IRR on next read 0=read ISR on next read
1	1=act on value of bit 0 0=no action if bit 0 set
2	1=poll command issued 0=no poll command issued
3	must be 1 for OCW3
4	must be 0 for OCW3
5	1=set special mask 0=reset special mask
6	1=act on value of bit 5 0=no action if bit 5 set
7	not used (zero)

IRR - Interrupt Request Register, maintains a bit vector indicating which IRQ hardware events are awaiting service. Highest level interrupt is reset when the CPU acknowledges interrupt.
ISR - In Service Register, tracks IRQ line currently being serviced.
Updated by EOI command.

Hardware Interrupt Sequence of Events

8259 <A HREF=“int_table.html”>IRQ</A> signal is raised high by hardware setting the corresponding IRR bits true.
PIC evaluates the interrupt requests and signals the CPU where appropriate.
CPU acknowledges the INT by pulsing INTA (inverted)
INTA signal from CPU is received by the PIC, which then sets the highest priority ISR bit, and clears the corresponding IRR bit
CPU sends a second INTA pulse which causes the PIC to send the interrupt ID byte onto the bus. CPU begins interrupt processing.
Interrupts of lower and equal priority are blocked until a Non-Specific EOI (20h) is sent to the command port.

Operation Command Word (OCW), commands that set the 8259 in various interrupt modes. These can be written to the 8259 anytime after initialization.
The 8259 differentiates between the OCW1, OCW2 and OCW3 by the port address and the value of the data bits D4 and D3. ICW2 through ICW4 are order related and must follow ICW1. ICW1 is identified by an even port number and data bit D4 set to 1.
PCs operate in fully nested mode, where a Non-Specific EOI resets the interrupt identified by the highest bit value in the ISR
8259s can be chained together where the INT pin (output) of a slave 8259 can be used as the input to an IRQ line allowing up to 64 priority vectored interrupts. AT level machines use two 8259's for a total of 16 hardware interrupt levels
the first 8259 ports are located at 20h and 21h
the second 8259 ports are located at A0h and A1h
PC and AT interrupts are Edge Triggered while PS/2's are Level Triggered
some ASIC chips designed for Tandy 1000 Systems malfunction if specific and non-specific EOIs are mixed
for a more in-depth discussion of the 8259, see Intel's “Micro-processor and Peripherals Handbook, Volume I”