banan-os/kernel/arch/i386/GDT.cpp

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#include <kernel/GDT.h>
#include <stdint.h>
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namespace GDT
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{
union SegmentDesriptor
{
struct
{
uint16_t limit_lo;
uint16_t base_lo;
uint8_t base_hi1;
uint8_t type : 4;
uint8_t system : 1;
uint8_t DPL : 2;
uint8_t present : 1;
uint8_t limit_hi : 4;
uint8_t flags : 4;
uint8_t base_hi2;
} __attribute__((packed));
struct
{
uint32_t low;
uint32_t high;
} __attribute__((packed));
SegmentDesriptor() : low(0), high(0) {}
SegmentDesriptor(uint32_t base, uint32_t limit, uint8_t access, uint8_t _flags)
: low(0), high(0)
{
set_base(base);
set_limit(limit);
high |= ((uint16_t)access) << 8;
flags = _flags;
}
void set_base(uint32_t base)
{
base_lo = base & 0xFFFF;
base_hi1 = (base >> 16) & 0x00FF;
base_hi2 = (base >> 24) & 0x00FF;
}
void set_limit(uint32_t limit)
{
limit_lo = limit & 0xFFFF;
limit_hi = (limit >> 16) & 0x00FF;
}
} __attribute__((packed));
struct GDTR
{
uint16_t size;
void* address;
} __attribute__((packed));
static GDTR s_gdtr;
static SegmentDesriptor s_gdt[5];
extern "C" void load_gdt(void* gdt_ptr);
asm(
".global load_gdt;"
"load_gdt:"
"movl 4(%esp),%eax;"
"lgdt (%eax);"
"movw $0x10, %ax;"
"movw %ax, %ds;"
"movw %ax, %es;"
"movw %ax, %fs;"
"movw %ax, %gs;"
"movw %ax, %ss;"
"jmp $0x08,$flush;"
"flush:"
"ret;"
);
void write_entry_raw(uint8_t segment, uint32_t low, uint32_t high)
{
uint8_t index = segment >> 3;
s_gdt[index].low = low;
s_gdt[index].high = high;
}
void write_entry(uint8_t segment, SegmentDesriptor descriptor)
{
write_entry_raw(segment, descriptor.low, descriptor.high);
}
void initialize()
{
s_gdtr.address = s_gdt;
s_gdtr.size = sizeof(s_gdt) - 1;
write_entry(0x00, { 0, 0x00000, 0x00, 0x0 }); // null
write_entry(0x08, { 0, 0xFFFFF, 0x9A, 0xC }); // kernel code
write_entry(0x10, { 0, 0xFFFFF, 0x92, 0xC }); // kernel data
write_entry(0x18, { 0, 0xFFFFF, 0xFA, 0xC }); // user code
write_entry(0x20, { 0, 0xFFFFF, 0xF2, 0xC }); // user data
load_gdt(&s_gdtr);
}
}