7. IRQ and PIC ENGI 3655 Lab Sessions

Richard Khoury2 Textbook Readings  Interrupts ◦ Section

Richard Khoury3 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 interrupts

Richard Khoury4 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 instruction

Richard Khoury5 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 slave

Richard Khoury6 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 Retrace  IRQ 10 is reserved  IRQ 11 is reserved  IRQ 12: PS/2 Mouse controller  IRQ 13: Math coprocessor  IRQ 14: Hard disk controller 1  IRQ 15: Hard disk controller 2

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 functions  Three steps ◦ Remap the PIC’s interrupt numbers ◦ Install interrupt descriptors for the IRQ in the IDT ◦ Initialize the values of variables needed for the default IRQ functions

Richard Khoury8 Remapping Interrupts  The first thing we need to do is map IRQ to interrupts in the IDT ◦ By default, IRQ 0-7 are mapped to interrupts 8-15, and IRQ 8-15 are mapped to interrupts ◦ Not good; interrupts 8-15 are already used, and this will lead to conflicts  Recall from last week: 0-18 are already used, are reserved by Intel ◦ So we will remap IRQ 0-15 to interrupt 32-47

Richard Khoury9 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/.c

Richard Khoury10 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 slave

Richard Khoury11 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 PIC

Richard Khoury12 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  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!

Richard Khoury13 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  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 well

Richard Khoury14 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 function

Richard Khoury15 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 }

Richard Khoury16 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 functions

Richard Khoury17 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; }

Richard Khoury18 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!

Richard Khoury19 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 number

Richard Khoury20 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 0x20

Richard Khoury21 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 in the IDT ◦ Sets the IRQ handler pointers to 0  You’ll also need all the functions described ◦ Install/Uninstall IRQ handler pointers ◦ Top Part of the IRQ handler function ◦ Array of pointers to IRQ handler functions  Add to GlobalCFunctions.inc ◦ 16 Low parts and 1 Common part of the IRQ handler functions

Richard Khoury22 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 crashed ◦ Now, our OS immediately detects an IRQ0 ◦ We’ll deal with that next week

Richard Khoury23 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