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void init_gdt() {

	gdt_ptr.base = (u32)&gdt_entries;
	gdt_ptr.limit = sizeof(gdt_entry_t) * GDT_SIZE - 1;

	gdt_entries[0] = create_gdt_entry(0, 0, 0, 0); // 0x00

	gdt_entries[1] = create_gdt_entry(0, 0xffffffff, 0x98, 0xc0); // code segment 0x08
	gdt_entries[2] = create_gdt_entry(0, 0xffffffff, 0x92, 0xc0); // data segment 0x10
	gdt_entries[3] = create_gdt_entry(0x00, 0x00, 0x96, 0xc0);	// kernel stack 0x18

	gdt_entries[4] = create_gdt_entry(0x30000, 0x0, 0xf8, 0xc0); // user code segment 0x20 / 0x23
	gdt_entries[5] = create_gdt_entry(0x30000, 0x0, 0xf2, 0xc0); // user data segment 0x28 / 0x2B
	gdt_entries[6] = create_gdt_entry(0x00, 0x20, 0xf6, 0xc0); // user stack 0x30 / 0x33

	load_gdt((u32)&gdt_ptr);

	print_ok();
	print("New GDT loaded\n");

}

gdt_entry_t create_gdt_entry(const u32 base, const u32 limit, const u8 access, const u8 gran) {

	return (gdt_entry_t) {

		.base_low 		= (base & 0xffff),
		.base_middle 	= (base >> 16) & 0xff,
		.base_high 		= (base >> 24) & 0xff,

		.limit_low 		= limit & 0xffff,
		.granularity 	= (gran & 0xf0) | ((limit >> 16) & 0x0f),

		.access 		= access

	};

}

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[bits 32]
[GLOBAL load_gdt]

load_gdt:
	mov eax, [esp + 4] 	; get pointer passed by stack
	lgdt [eax]			; load the gdt pointer

	mov ax, 0x10		; offset in gdt for data segment
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax

	mov ax, 0x18		; ...
	mov ss, ax

	jmp 0x08:flush		; load code segment (0x08 is offset to code segment)

flush:
	ret

Code: Select all

void task() {

	char *vid = (char *)0x88000; // 0xb8000 - 0x30000
	*vid = 'f';

	while(1);

}