Build Your Own OS #05

What is an Interrupt?

An interrupt is a signal from a device that attached to our computer or from a program within the computer that requires the operating system to stop the current process and figure out what to do next. As you can see there are basically two types of interrupts.

Interrupts Handlers

Interrupts are handled via the Interrupt Descriptor Table (IDT). The IDT describes a handler for each interrupt. The interrupts are numbered (0–255) and the handler for interrupt i is defined at the ith position in the table. There are three different kinds of handlers for interrupts:

  • Task handler
  • Interrupt handler
  • Trap handler

Creating an Entry in the IDT

An entry in the IDT for an interrupt handler consists of 64 bits. The highest 32 bits are shown in the figure below:

Bit:     | 31              16 | 15 | 14 13 | 12 | 11 | 10 9 8 | 7 6 5 | 4 3 2 1 0 |
Content: | offset high | P | DPL | 0 | D | 1 1 0 | 0 0 0 | reserved |
Bit:     | 31              16 | 15              0 |
Content: | segment selector | offset low |
0xDEAD8E00
0x0008BEEF
idt[0] = 0xDEAD8E00
idt[1] = 0x0008BEEF

Handling an Interrupt

When an interrupt occurs the CPU will push some information about the interrupt onto the stack, then look up the appropriate interrupt hander in the IDT and jump to it. The stack at the time of the interrupt will look like the following:

[esp + 12] eflags
[esp + 8] cs
[esp + 4] eip
[esp] error code?
struct cpu_state {
unsigned int eax;
unsigned int ebx;
unsigned int ecx;
.
.
.
unsigned int esp;
} __attribute__((packed));
struct stack_state {
unsigned int error_code;
unsigned int eip;
unsigned int cs;
unsigned int eflags;
} __attribute__((packed));
void interrupt_handler(struct cpu_state cpu, struct stack_state stack, unsigned int interrupt);

Creating a Generic Interrupt Handler

Since the CPU does not push the interrupt number on the stack it is a little tricky to write a generic interrupt handler. This section will use macros to show how it can be done. Writing one version for each interrupt is tedious — it is better to use the macro functionality of NASM [34]. And since not all interrupts produce an error code the value 0 will be added as the “error code” for interrupts without an error code. The following code shows an example of how this can be done:

%macro no_error_code_interrupt_handler %1
global interrupt_handler_%1
interrupt_handler_%1:
push dword 0 ; push 0 as error code
push dword %1 ; push the interrupt number
jmp common_interrupt_handler ; jump to the common handler
%endmacro
%macro error_code_interrupt_handler %1
global interrupt_handler_%1
interrupt_handler_%1:
push dword %1 ; push the interrupt number
jmp common_interrupt_handler ; jump to the common handler
%endmacro
common_interrupt_handler: ; the common parts of the generic interrupt handler
; save the registers
push eax
push ebx
.
.
.
push ebp
; call the C function
call interrupt_handler
; restore the registers
pop ebp
.
.
.
pop ebx
pop eax
; restore the esp
add esp, 8
; return to the code that got interrupted
iret
no_error_code_interrupt_handler 0 ; create handler for interrupt 0
no_error_code_interrupt_handler 1 ; create handler for interrupt 1
.
.
.
error_code_handler 7 ; create handler for interrupt 7
.
.
.
  • Push the registers on the stack.
  • Call the C function interrupt_handler.
  • Pop the registers from the stack.
  • Add 8 to esp (because of the error code and the interrupt number pushed earlier).
  • Execute iret to return to the interrupted code.

Loading the IDT

The IDT is loaded with the lidt assembly code instruction which takes the address of the first element in the table. It is easiest to wrap this instruction and use it from C:

global  load_idt    ; load_idt - Loads the interrupt descriptor table (IDT).
; stack: [esp + 4] the address of the first entry in the IDT
; [esp ] the return address
load_idt:
mov eax, [esp+4] ; load the address of the IDT into register eax
lidt eax ; load the IDT
ret ; return to the calling function

6.6 Programmable Interrupt Controller (PIC)

To start using hardware interrupts you must first configure the Programmable Interrupt Controller (PIC). The PIC makes it possible to map signals from the hardware to interrupts. The reasons for configuring the PIC are:

  • Remap the interrupts. The PIC uses interrupts 0–15 for hardware interrupts by default, which conflicts with the CPU interrupts. Therefore the PIC interrupts must be remapped to another interval.
  • Select which interrupts to receive. You probably don’t want to receive interrupts from all devices since you don’t have code that handles these interrupts anyway.
  • Set up the correct mode for the PIC.
#include "io.h"    #define PIC1_PORT_A 0x20
#define PIC2_PORT_A 0xA0
/* The PIC interrupts have been remapped */
#define PIC1_START_INTERRUPT 0x20
#define PIC2_START_INTERRUPT 0x28
#define PIC2_END_INTERRUPT PIC2_START_INTERRUPT + 7
#define PIC_ACK 0x20 /** pic_acknowledge:
* Acknowledges an interrupt from either PIC 1 or PIC 2.
*
* @param num The number of the interrupt
*/
void pic_acknowledge(unsigned integer interrupt)
{
if (interrupt < PIC1_START_INTERRUPT || interrupt > PIC2_END_INTERRUPT) {
return;
}
if (interrupt < PIC2_START_INTERRUPT) {
outb(PIC1_PORT_A, PIC_ACK);
} else {
outb(PIC2_PORT_A, PIC_ACK);
}
}

Reading Input from the Keyboard

The keyboard does not generate ASCII characters, it generates scan codes. A scan code represents a button — both presses and releases. The scan code representing the just pressed button can be read from the keyboard’s data I/O port which has address 0x60. How this can be done is shown in the following example:

#include "io.h"    #define KBD_DATA_PORT   0x60    /** read_scan_code:
* Reads a scan code from the keyboard
*
* @return The scan code (NOT an ASCII character!)
*/
unsigned char read_scan_code(void)
{
return inb(KBD_DATA_PORT);
}

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