CS452 - Real-Time Programming - Spring 2008

Lecture 19 - Interrupt Service Routines

Questions & Comment

1. Friday
2. ASUS P4P800-VM
3. 82801EB southbridge
   • 8254 timers
   • 8259 interrupt controllers
4. W83627THF
   • probably two 16550As at ports at 0x2f8-0x2ff, irq 3 and 0x3f8-0x3ff, irq4

Interrupt Service Routines

Example

error = Delay( int interval );

Delay( ) is a stub that Sends a message to the clock server.

• It returns when the clock server Replys
• The message says: wake me up after interval ticks have elapsed.

Possible errors

• illegal value of interval
• no clock server

Clock server manages a queue of tasks to wake up

Hardware

8254 timer

• Four 16-bit registers
  • 8-bit status/control register
• You want mode 2
  • interrupt line sets for exactly one clock cycle
  • needs to be latched in interrupt contyroller

8259 interrupt controller

• where the interrupt is turned off
• three registers
  1. interrupt mask register: don't need an EOI
  2. interrupt request register: waiting for service
  3. in-service register: waiting for EOI

Remember

1. three places that interrupts can be turned off
   1. device
   2. interrupt controller
   3. CPU
2. CPU requires edge-triggered

Basic Service

1. Save more state
2. Locate interrupt source
   • by argument (vector)
   • by polling
3. Service the hardware
   • save volatile data
   • turn off interrupt, including EOI to PIC
4. return from interrupt

Where should each part of this be done?

Low-level primitive

int AwaitEvent( int EventType );

Implementation notes

1. Event may already have occurred
2. AwaitEvent( ) can service the device.
   • Then it must return the volatile data
3. Need to think of priorities when you partition the work
4. Kernel has interrupts turned off
   • Therefore need IdleTask( ). Why?

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