7. IRQ and PIC - PowerPoint Presentation

Slide1

7. IRQ and PIC

ENGI 3655 Lab SessionsSlide2

Richard Khoury

2

Textbook Readings

Interrupts

Section 13.2.2Slide3

Richard Khoury

3

Interrupts

Last week, we learned about interrupts

Two kinds, software and hardware

Software signal errors

Hardware signal that a device needs attention

We coded routines to handle interrupts

256 interrupt descriptors in the IDT

Special routine for the first 32 interrupts, default handler for the others

This week, we move on to hardware interruptsSlide4

Richard Khoury

4

Hardware Interrupts

I/O Controller

Input or output ready, or error

Translate IRQ into Interrupt no.

Send IRQ

Signal CPU’s Interrupt Request line

Execute instruction

Check IR line

Execute instruction

Check IR line

Jump to location pointed in IDT

Save state information

Interrupt Handler Routine

Restore state information

Perform regular I/O functions

Programmable Interrupt Controller (PIC)

CPU

OS

Execute instructionSlide5

Richard Khoury

5

PIC

Programmable Interrupt Controller receives interrupts from system, asserts priority,

and notifies the CPU

You should guess that it is

system-specific

The Intel PIC is the 8259A chip

Can handle 8 IRQs – insufficient for a modern computer

Motherboard has two cascading 8259A chips, a master and a slaveSlide6

Richard Khoury

6

Default Hardware Interrupts

MASTER PIC

IRQ 0: Programmable Interval Timer (PIT)

IRQ 1: Keyboard controller

IRQ 2: Cascade from slave PIC

IRQ 3: Serial Port COM2

IRQ 4: Serial Port COM1

IRQ 5: Parallel Port LPT2

IRQ 6: Floppy disk controller

IRQ 7: Parallel Port LPT1

SLAVE PIC

IRQ 8: Real-Time Clock

IRQ 9: CGA RetraceIRQ 10 is reserved

IRQ 11 is reservedIRQ 12: PS/2 Mouse controllerIRQ 13: Math coprocessorIRQ 14: Hard disk controller 1IRQ 15: Hard disk controller 2Slide7

Handling IRQs

We will need a function to set up IRQ handling in our OS

Called by the HAL after the GDT and IDT initialization functionsThree stepsRemap the PIC’s interrupt numbersInstall interrupt descriptors for the IRQ in the IDTInitialize the values of variables needed for the default IRQ functionsSlide8

Richard Khoury

8

Remapping Interrupts

The first thing we need to do is map IRQ 0-15 to interrupts 0-255 in the IDT

By default, IRQ 0-7 are mapped to interrupts 8-15, and IRQ 8-15 are mapped to interrupts 112-120

Not good; interrupts 8-15 are already used, and this will lead to conflicts

Recall from last week: 0-18 are already used, 19-31 are reserved by Intel

So we will remap IRQ 0-15 to interrupt 32-47Slide9

Richard Khoury

9

Remapping Interrupts

We will need to reinitialize the two PICs

As with any chip, this will require us to write data into the ports

Each PIC has a command port and a data port

Master PIC command port: 0x20, data port 0x21

Slave PIC command port: 0xA0, data port 0xA1

Recall: we have a port-writing function in our HAL

port.h

/.cSlide10

Richard Khoury

10

Remapping Interrupts

Initialization of a PIC is done by sending four successive Initialization Control Words (ICW)

The details are in the specs (on the website)

ICW1 is the initialization command word (goes to the command port)

Hints: Call address interval does not apply for us, and we are using the PIC in edge triggered mode

ICW2 is the new IDT value of the first IRQ to each chip (data)

The next IRQ of the PIC will have the following seven interrupt values

We will use 32 for the master and 40 for the slaveSlide11

Richard Khoury

11

Remapping Interrupts

ICW3 is the IRQ pin connecting the master and the slave (IRQ2) (data)

Hint: remember that C does not handle binary numbers directly, unlike Assembly!

ICW4 defines the PIC behaviour (data)

Hints:

we will use normal end-of-interrupt (EOI) operations, we will not use buffered mode, and Special Fully Nested Mode only applies if there are more than 2 PICSlide12

Richard Khoury

12

Remapping Interrupts

After we’re done reinitializing the PICs, we can clear the data port to zero

Or restore the previous state if we saved it before initialization

After that, the IRQs are mapped to interrupts 32-47

So we have to update the information in the IDT

Without modifying the code of IDT.c/.h

That’s something we learned last week!Slide13

Richard Khoury

13

Installing IRQs

Recall: function

i86_install_ir

Writes an interrupt descriptor in the IDT and maps it to a function, the interrupt handler routine (IHR)

We’ll use it to install the IRQ handler routines to interrupts 32-47

Recall: The IHR is a three-part function

Low Part is a global, interrupt-specific, Assembly function that pushes the interrupt number and error code on the stack and calls the Common Part

Common Part is a common Assembly function that pushes all the registers on the stack and calls the Top Part

Top Part is a C function that receives the registers as argument and takes interrupt-specific action

That’s what our IRQ handler routines will be as wellSlide14

Richard Khoury

14

IRQ Handler Functions

Structure of Low Part

Identical to ISR function except we need to differentiate the IRQ number and interrupt number

_irq#:

push byte 0

push byte ##

jmp irq_common_stub

Where ## = 32 + #

Structure of Common Part

Identical to ISR function except we call the irq_handler function instead of the fault_handler functionSlide15

Richard Khoury

15

IRQ Handler Functions

Structure of Top Part

void irq_handler(struct isrregs *r)

{

Execute hardware’s handler function, or default handler if none is available

Send end-of-interrupt signal to PIC

Do NOT run an infinite loop

}Slide16

Richard Khoury

16

IRQ Handler Functions

Each IRQ corresponds to a piece of hardware

When we install each of them we will write a handler for it

So for now, we need to make sure that our OS’s IRQ functions can allow us to install and uninstall custom IRQ handler functionsSlide17

Richard Khoury

17

IRQ Handler Functions

There are 16 IRQs, so we will create an array of 16 pointers to handler functions

void *irq_routines[16];

Installing and uninstalling a handler function for an IRQ is simply changing the corresponding pointer

void irq_install_handler(uint32_t irq, void (*handler)(struct isrregs *r))

{ irq_routines[irq] = handler; }

void irq_uninstall_handler(uint32_t irq)

{ irq_routines[irq] = 0; }Slide18

Richard Khoury

18

IRQ Handler Functions

Initially, the pointers will all be 0

We haven’t installed any hardware yet!

Don’t forget to initialize them at 0 in the end of the IRQ initialize function!Slide19

Richard Khoury

19

IRQ Handler Functions

First function of Top Part:

Checks the registers it received in argument and retrieves the IRQ handler pointer corresponding to the correct interrupt

If not zero, call that function (pass registers as argument, as the handler function will need them)

If zero, take default action

Create an array of messages and display the one corresponding to the interrupt numberSlide20

Richard Khoury

20

End Of Interrupt Signal

Second function of the Top Part

The EOI signals to the PIC that the interrupt has been dealt with

It’s a command word that has to be written in the PIC command port

Which PIC?

IRQ 0-7, Master PIC

IRQ 8-15, both PICs, because Slave PIC is connected to Master PIC

The command word is 0x20Slide21

Richard Khoury

21

Lab Assignment

Write IRQ.h & IRQ.c

You need an

i86_irq_initialize

function

Called by

cpu_initialize

after initializing the IDT

Reinitialize the PICs

Installs new descriptors for ints 32-47 in the IDT

Sets the IRQ handler pointers to 0

You’ll also need all the functions describedInstall/Uninstall IRQ handler pointersTop Part of the IRQ handler functionArray of pointers to IRQ handler functionsAdd to GlobalCFunctions.inc16 Low parts and 1 Common part of the IRQ handler functionsSlide22

Richard Khoury

22

Lab Assignment

Don’t forget to call the IRQ initialization function from the CPU initialization function

Otherwise it won’t do anything

Once you’re done (and if you did it correctly) it’s now safe to reinitialize interrupts

Recall: we had cleared them with the CLI command in the

bootloader

before going into protected mode

We can restore them by adding this line in

main()

, after initializing the HAL

__

asm__ __volatile__ ("sti");If we did this previously, our OS crashedNow, our OS immediately detects an IRQ0 We’ll deal with that next weekSlide23

Richard Khoury

23

Lab Assignment

So for this lab, you:

Write IRQ.c/IRQ.h

Add functions in GlobalCFunctions.inc

Add one line in i86.c

Add one line in main.c

You should not be changing any other files

Especially IDT.c/IDT.h