Microprocessor System Design Programmable Interrupt Controller.

Microprocessor System DesignProgrammable Interrupt Controller

Outline

• Interrupts in PC

• Interrupts and calls

• 8259 – PIC

• Programming 8259

• Interfacing 8259 in PC

What is Interrupt

8086/88 Interrupts

• 256 Interrupts.

• Types 00 ……. FF.

• Type is provided in a specified timing.

Interrupt Vector Table

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

1. CALL FAR can jump to any location (1M range)

2. Hardware interrupts can come at any time.3. Interrupts are maskable.4. After CALL only CS:IP is saved5. 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)

Interrupt and Flags

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)

8259

Control Word (initialization)

ICW1, ICW2

ICW3, ICW4

Example

Masking and Prioritization

• OCW (operation command word)

Programming OCWs

OCW3

8259 in PC XT

ICW1: 13HICW2: 08HICW3: 09H

Interrupt Sources in PC

Sources of NMI

8259s in AT

IRQ9 Instead of IRQ2

Interrupts (Summary)

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.