1. 8259A PROGRAMMABLE
INTERRUPT CONTROLLER

2. Interrupt handling by
CPU vs. 8259

3. Why 8259A ??
• 8085 Processor has only 5 hardware interrupts to handle
a maximum of 5 I/O devices.
(RST 7.5, RST6.5, RST 5.5, INTR, TRAP)
• If the I/O devices are more than 5, more number of
interrupt pins are required.
• In such case of multiple interrupt systems 8259 – PIC is
designed to care of the interrupts.
• Every 8259 IC can handle a maximum of 8 interrupts
along with priorities.

4. • When an interrupts occurs ,it Vectors an interrupt request to the
vector table.
• However all the 8 interrupts are spaced at an interval of four to
eight locations.
• Resolve 8 levels of interrupt priorities in variety of modes &
Mask each interrupt request individually.
• Read the status of pending interrupts, in-service interrupts and
masked interrupts.
• Accepts either the level triggered or edge triggered interrupt
request.
• Expandable to 64 priority levels by cascading additional 8259As &
compatible with 8-bit as well as 16-bit processors.

5. 8259A PIC- BLOCK DIAGRAM
It includes 8 blocks.
• Control logic
• Read/Write logic
• Data bus buffer
• Three registers (IRR,ISR and IMR)
• Priority resolver
• Cascade Buffer

6. 8259A PIC- PIN DIGRAM

7. 8259A PIC- BLOCK DIAGRAM

8. 8259A PIC- BLOCK DIAGRAM

9. 8259A PIC- INTERRUPTS AND CONTROL
LOGIC SECTION
This section consists of
• IRR (Interrupt Request
Register)
• ISR (In-Service Register)
• Priority Resolver
• IMR (Interrupt Mask
Register)
• Control logic block
IRR
• 8 interrupt inputs set
corresponding bits of IRR
• Used to store the
information about the
interrupt inputs
requesting service.

10. 8259A PIC- INTERRUPTS AND CONTROL
LOGIC SECTION
ISR
• Used to store information
about the interrupts
currently being serviced.
PRIORITY RESOLVER
• Examines three registers and
determines whether INT
should be sent to MPU.
• Determines the priorities of the
bits set in IRR as dictated by
priority mode set by OCWs.
• The bit corresponding to
highest priority input is set in
ISR during input.

11. 8259A PIC- INTERRUPTS AND CONTROL
LOGIC SECTION
IMR
• This register can be programmed by an OCW to store
the bits which mask specific interrupts.
• An interrupt which is masked by software (By
programming the IMR) will not be recognized and
serviced even if it sets corresponding bits in the IRR.

12. 8259A PIC- INTERRUPTS AND CONTROL
LOGIC SECTION
CONTROL LOGIC
• Has two pins:
INT (Interrupt) Output
( Interrupt Acknowledge) Input
• INT Connected to Interrupt pin of MPU.
 When interrupt occurs this pin goes high.

13. 8259A PIC- BLOCK DIAGRAM
DATA BUS BUFFER
• 8 bit
• Bidirectional
• Tri-state Buffer used to Interface the 8259 to the
system data bus.
• Control words, Status words and vectoring data
are all passed through the data bus buffer.

14. 8259A PIC- READ/WRITE CONTROL LOGIC
SECTION
• Contains ICW and OCW registers programmed by the
CPU to set up the 8259 to operate in various modes.
• Also accepts read command from CPU to permit the
CPU to read status words.
•  Chip Select  Active Low input
 Used to select the Device.
•  Read  Active Low input
 Used by CPU to read the status of
ISR,IRR,IMR or the Interrupt level.
•  Write  Active Low input
 Used to write OCW and ICW onto the 8259.
*ICW Initialization Control Word
OCW Operation Control Word

15. 8259A PIC- CASCADE BUFFER/
COMPARATOR
• Generates control signals for cascade operation.
• 8259 cascaded with other 8259s
 Interrupt handling capacity to 64 levels
 Former is called master and latter is slave.
• 8259 can be set up as master or slave by

16. 8259A PIC- CASCADE BUFFER/
COMPARATOR
CAS 0-2
• For master 8259 these pins are outputs and for slaves these are
inputs.
• When 8259 is a master the CALL op-code is generated by master in
response to the first Interrupt acknowledge.
• The vectoring address must be released by slave 8259.
• The master puts out the identification code to select one of the slave
from 8 slaves through these pins.
• The slave thus selected puts out the address of ISR during second
and third interrupt acknowledge pulses from the CPU.

17. Interrupt Sequence
Following events occur:
1) One or more interrupt request lines(IR0-IR7) are raised high.
2) Priority resolver checks for the 3 register and sets INT high.
3) In response to INTR signal 8085 mp sends an INTA pulse.
4) Upon receiving INTA from 8085, corresponding IRR
line(IR0-IR7) is reset.
4) 8259 places the opcode for call instruction.
5) Call instruction initiates two more interrupt acknowledge
cycles.
6) During the two interrupt acknowledge cycles, 8259 places
the lower byte of ISR and higher byte of ISR address.
7) When interrupt cycle is completed an EOI command is
issued by the microprocessor.

18. 8259A- OPERATING MODES
FULLY NESTED MODE:
• General purpose mode / default mode.
• IR0 to IR7 are arranged from highest to lowest.
• IR0 Highest IR7Lowest
AUTOMATIC ROTATION MODE:
• In this mode, a device after being serviced, receives the
lowest priority.
SPECIFIC ROTATION MODE:
• Similar to automatic rotation mode, except that the user
can select any IR for the lowest priority, thus fixing all
other priorities.

19. 8259-INTERRUPT PROCESS IN FULLY NESTED MODE

20. 8259A- OPERATING MODES
END OF INTERRUPT (EOI):
• After the completion of an interrupt service, the
corresponding ISR bits needs to be reset to update the
information in the ISR. This is called EOI command.
It can be issued in three formats:
NON SPECIFIC EOI COMMAND:(manually)
• When this command is sent to 8259A, it resets the
highest priority ISR bit.
SPECIFIC EOI COMMAND:(manually)
• This command specifies which ISR bit is to reset.

21. 8259A- OPERATING MODES
AUTOMATIC EOI:
• In this mode, no command is necessary.
• During the end of interrupt acknowledge cycle, the ISR
bit is reset.
• Used for master only

22. Command words (ICW & OCW)
The command words of 8259A are classified in two groups,
1) Initialization command words (ICWs)
(Total of 4 –ICWs)
2) Operation command words (OCWs)
(Total of 3 - OCWs)
8259 must be initialized by writing two to four command
words into their respective command word registers.
If A0=0,D4=1: The control word is ICW1
If A0=1: The control word is ICW2

23. ICW1 and ICW2 are compulsory
command words for initialization sequence
of 8259A while ICW3 and ICW4 are
optional.

25. ADI=1 for 8086 based system
For 8086 Don’t Care

26. For 8085 system they are filled by A15-A11 of the interrupt vector address and
Least significant 3 bits are same as the respective bits of the vector address.
For 8086 system they are filled by most significant 5 bits of interrupt type and
the least significant 3 bits are 0, pointing to IR0.