CS452 - Real-Time Programming - Spring 2009

Lecture 13 - Interrupts

Practical Detail

Interrupts

General

Interrupt input (hardware) asserted

First instruction of interrupt handler put into pipeline

Change processor mode to int
Put next instruction address of interrupted task into lr_irq
Put CPSR of interrupted task into spsr_irq
New CPSR has interrupts disabled, mode changed
Set pc to 0x18

Similar procedure for fast interrupt

Extra duplicated registers: r8-r14
Mode is fiq
pc is set to 0x1c
FIQ has higher priority

Many sources for interrupts

How do you find out which one has been asserted?
How do turn off the interrupt?
- at the CPU
- in the interrupt controller
- in the device
And in what order?

ARM Specific

Two controllers, nested

VICINTSOURCE[0-31] to primary
VICINTSOURCE[32-63] to secondary
You care about
- VICINTSOURCE4: timer counter 1
- VICINTSOURCE5: timer counter 2
- VICINTSOURCE[23,24]: UART1 [receive, transmit]
- VICINTSOURCE[25,26]: UART2 [receive, transmit]
- VICINTSOURCE36: watchdog timer
- VICINTSOURCE51: timer counter 3
- VICINTSOURCE52: UART1 general
- VICINTSOURCE54: UART2 general

Registers at

primary: 0x800b0000
secondary: 0x800c0000

47 registers per controller

11: chip control
16: vector addresses
16: vector control
4: chip id

Priority

FIQ-configured interrupts (not vectored)
IRQ-configured vecored interrupts
IRQ-configured non-vectored interrupts

Low-level primitive

This is what everything depends on

int AwaitEvent( int EventType );

What does it do?

Blocks on an interrupt of the type specified
When the interrupt occurs, may return one word of volatile data

Implementation notes

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

How is it used?

Example: Clock Server

Provides a sleep prinitive

int Delay( int interval )

block and return later
Send to a server which
- maintains an ordered list of sleepers
- receives clock ticks
- on each tick checks the list and replies to each sleeper who is ready to wake up

Server must receive clock ticks

provided by a Notifier

Notifier code

InitializeHardware( );
FOREVER {
  tick = AwaitEvent( REAL_TIME_CLOCK );
  // check for errors
  error = Send( parentTid, .... );
  //check for errors
}

What does the kernel do?

Software

gets volatile data from device
turns off source of interrupt
- device
- interrupt controller
finds task waiting on the interrupt
makes it ready

later

enables interrupts in CPU

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