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#include <gdt.h>
#include <string.h>
#define GDT_ENTRIES 6 // The contents of each entry is defined in gdt_init.
/* One entry of the table */
struct gdt_entry {
uint16_t limit_low;
uint16_t base_low;
uint8_t base_middle;
uint8_t access;
uint8_t granularity;
uint8_t base_high;
} __attribute__((packed));
typedef struct gdt_entry gdt_entry_t;
/* Structure defining the whole table : address and size (in bytes). */
struct gdt_ptr {
uint16_t limit;
uint32_t base;
} __attribute__((packed));
typedef struct gdt_ptr gdt_ptr_t;
/* The TSS is used for hardware multitasking.
We don't use that, but we still need a TSS so that user mode process exceptions
can be handled correctly by the kernel. */
struct tss_entry {
uint32_t prev_tss; // The previous TSS - if we used hardware task switching this would form a linked list.
uint32_t esp0; // The stack pointer to load when we change to kernel mode.
uint32_t ss0; // The stack segment to load when we change to kernel mode.
uint32_t esp1; // Unused...
uint32_t ss1;
uint32_t esp2;
uint32_t ss2;
uint32_t cr3;
uint32_t eip;
uint32_t eflags;
uint32_t eax;
uint32_t ecx;
uint32_t edx;
uint32_t ebx;
uint32_t esp;
uint32_t ebp;
uint32_t esi;
uint32_t edi;
uint32_t es; // The value to load into ES when we change to kernel mode.
uint32_t cs; // The value to load into CS when we change to kernel mode.
uint32_t ss; // The value to load into SS when we change to kernel mode.
uint32_t ds; // The value to load into DS when we change to kernel mode.
uint32_t fs; // The value to load into FS when we change to kernel mode.
uint32_t gs; // The value to load into GS when we change to kernel mode.
uint32_t ldt; // Unused...
uint16_t trap;
uint16_t iomap_base;
} __attribute__((packed));
typedef struct tss_entry tss_entry_t;
// ========================= //
// Actual definitions
static gdt_entry_t gdt_entries[GDT_ENTRIES];
static gdt_ptr_t gdt_ptr;
/* For internal use only. Writes one entry of the GDT with given parameters. */
static void gdt_set_gate(int num, uint32_t base, uint32_t limit, uint8_t access, uint8_t gran) {
gdt_entries[num].base_low = (base & 0xFFFF);
gdt_entries[num].base_middle = (base >> 16) & 0xFF;
gdt_entries[num].base_high = (base >> 24) & 0xFF;
gdt_entries[num].limit_low = (limit & 0xFFFF);
gdt_entries[num].granularity = (limit >> 16) & 0x0F;
gdt_entries[num].granularity |= gran & 0xF0;
gdt_entries[num].access = access;
}
/* Write data to the GDT and enable it. */
void gdt_init() {
gdt_ptr.limit = (sizeof(gdt_entry_t) * GDT_ENTRIES) - 1;
gdt_ptr.base = (uint32_t)&gdt_entries;
gdt_set_gate(0, 0, 0, 0, 0); //Null segment
gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF); //Kernel code segment 0x08
gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF); //Kernel data segment 0x10
gdt_set_gate(3, 0, 0xFFFFFFFF, 0xFA, 0xCF); //User code segment 0x18
gdt_set_gate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF); //User data segment 0x20
asm volatile ("lgdt %0"::"m"(gdt_ptr):"memory");
}
/* vim: set ts=4 sw=4 tw=0 noet :*/
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