os/arch/i686/idt.c

#include <kernel/tty.h>
#include <kernel/gdt.h>
#include <debugging.h>
#include "interrupt.h"
#include <stdint.h>
#include <stddef.h>

#define NUM_ENTRIES 0x100

struct InterruptDescriptor {
    uint32_t offset;
    uint32_t selector;
    uint8_t gate_type;
    uint8_t ring;
    bool present;
};

struct IDT {
    uint16_t size;
    uint64_t* base;
} __attribute__((packed));

static uint64_t entries[NUM_ENTRIES];
struct IDT idt;

enum EntryTypes {
    DivisionByZero = 0x00,
    _Reserved0     = 0x01,
    NMIInterrupt   = 0x02,
    Breakpoint     = 0x03,
    Overflow       = 0x04,
    BoundsExceeded = 0x05,
    InvalidOpcode  = 0x06,
    DeviceNotAvail = 0x07,
    DoubleFault    = 0x08,
    CoprocSegORun  = 0x09,
    InvalidTSS     = 0x0A,
    SegNotPresent  = 0x0B,
    StackSegFault  = 0x0C,
    GenerProtFault = 0x0D,
    PageFault      = 0x0E,
    _Reserved1     = 0x0F,
    x87FPUError    = 0x10,
    AlignmentCheck = 0x11,
    MachineCheck   = 0x12,
    SIMDFloatExcep = 0x13,
};

static void encodeIdtEntry(uint8_t *target, struct InterruptDescriptor source)
{
    // Encode the present bit
    if (source.present) {
        target[5] |= 0x80;
    } else {
        // Not present, return
        return;
    }

    // Encode the offset 
    target[0] = (source.offset >> 0x00) & 0xFF;
    target[1] = (source.offset >> 0x08) & 0xFF;
    target[6] = (source.offset >> 0x10) & 0xFF;
    target[7] = (source.offset >> 0x18) & 0xFF;
    
    // Encode the base
    target[2] = (source.selector >> 0x00) & 0xFF;
    target[3] = (source.selector >> 0x08) & 0xFF;
    
    // Encode the gate type
    target[5] |= source.gate_type;
    
    // Encode the DPL
    target[5] |= (source.ring << 5);
}

static void add_entry(struct InterruptDescriptor entry, uint8_t index)
{
    uint64_t descriptor = 0;
    encodeIdtEntry((uint8_t*) &descriptor, entry);

    entries[index] = descriptor;
}

static void load_idt() 
{
    idt.size = NUM_ENTRIES * 8 - 1;
    idt.base = entries;

    asm ( "lidt %0" : : "m"(idt) );
    asm ( "sti" );
}

struct InterruptDescriptor generate_entry(bool present, void isr(struct interrupt_frame*), uint8_t gate_type, uint8_t ring, uint8_t selector)
{
    struct InterruptDescriptor entry;
    entry.present = present;
    entry.offset = (uint32_t)isr;
    entry.gate_type = gate_type;
    entry.ring = ring;
    entry.selector = selector;
    return entry;
}

static inline void outb(uint16_t port, uint8_t val)
{
    __asm__ volatile ( "outb %b0, %w1" : : "a"(val), "Nd"(port) : "memory");
    /* There's an outb %al, $imm8 encoding, for compile-time constant port numbers that fit in 8b. (N constraint).
     * Wider immediate constants would be truncated at assemble-time (e.g. "i" constraint).
     * The  outb  %al, %dx  encoding is the only option for all other cases.
     * %1 expands to %dx because  port  is a uint16_t.  %w1 could be used if we had the port number a wider C type */
}

void idt_init()
{
    outb(0x21,0xfd);
    outb(0xa1,0xff);

    struct InterruptDescriptor entry;

    entry = generate_entry(true, exception, 0xF, 0, 0x8);
    for (int i = 0; i < 0x20; i++)
    {
        add_entry(entry, i);
    }

    entry = generate_entry(true, irq, 0xF, 0, 0x8);
    for (int i = 0x20; i < 0x28; i++)
    {
        add_entry(entry, i);
    }

    entry = generate_entry(true, divide_by_zero, 0xF, 0, 0x8);
    add_entry(entry, DivisionByZero);

    entry = generate_entry(true, general_protection_fault, 0xF, 0, 0x8);
    add_entry(entry, GenerProtFault);

    entry = generate_entry(true, double_fault, 0xF, 0, 0x8);
    add_entry(entry, DoubleFault);

    struct InterruptDescriptor print_entry;
    print_entry.present = true;
    print_entry.offset = (uint32_t)print;
    print_entry.gate_type = 0xE;
    print_entry.ring = 3;
    print_entry.selector = 0x8;

    add_entry(print_entry, 0x80);

    struct InterruptDescriptor input_entry;
    input_entry.present = true;
    input_entry.offset = (uint32_t)input;
    input_entry.gate_type = 0xE;
    input_entry.ring = 0;
    input_entry.selector = 0x8;

    add_entry(input_entry, 0x81);

    load_idt();

    tss.ss0 = 0x10;
    asm("mov %%esp, %0" : "=r" (tss.esp0));
    asm("mov $0x28, %ax");
    asm("ltr %ax");
}