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Microprocessor System Design Programmable Interrupt Controller

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Answering an Interrupt

Save status
FR, IP, CS
Service the interrupt
Interrupt service routing (ISR) or Interrupt handler
Based on Interrupt vector number
From Interrupt vector table
Four bytes for every interrupt: CS:IP
Return to original position by IRET

Difference with Call

CALL FAR can jump to any location (1M range)
Hardware interrupts can come at any time.
Interrupts are maskable.
After CALL only CS:IP is saved
End of routine: RETF vs. IRET.

Interrupt Categories

Hardware interrupts:
Only 3 pin, but how 256 interrupt?
INTR (in), NMI (in), and INTA (out)
INTR can be masked by CLI / STI
Active high.
80x86 finished instruction.
Push FR, CS, IP
NMI: INT 02.
Software interrupts
INT nn
Example: INT 21H (DOS functions)

Predefined Interrupts

INT 00 (divide error)
INT 01 (single step)
Set Trap flag (how?)
PUSHF, POP AX, …
Trace in debug
INT 02 (NMI)
INT 03 (breakpoint)
INT 04 (signed number overflow)
INT) instruction
Examine Interrupt vector table.
Examine INT 12H (size of conventional RAM in AX)

Edge Triggered and Interrupt Sharing

Level triggered mode: IRQ line should be brought down before EOI.
Edge triggered mode: noise on IRQ lines might cause false interrupts.
New computer and busses.
Level triggered.
Interrupt sharing.

The Intel 8259 is a Programmable Interrupt Controller (PIC) designed for Intel 8085 and Intel 8086 microprocessors The initial part was 8259, a later version of the suffix A was compatible and usable up with 8086 or 8088. The processor 8259 combines Multiple interrupt input sources in a single interrupt output to the host microprocessor, extending the interrupt levels available in a system beyond one or two levels found on the processor chip. The 8259A was the interrupt controller for the ISA bus on the original IBM PC and the IBM PC AT.

The 8259 was introduced as part of the family of MCS 85 Intel in 1976. The 8259A was included in the original PC introduced in 1981 and maintained by the PC / XT when it was introduced in 1983. A second 8259A was added with the introduction of PC / AT. The 8259 has coexisted with the Intel APIC architecture since its introduction in PC Symmetric Multi-Processor. Modern PCs have begun to eliminate 8259A in favor of the Intel APIC architecture. However, although it is no longer a separate chip, the 8259A interface is still provided by the Southbridge chipset on modern x86 motherboards.

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