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#include <stdint.h>
#define SEG_DESCTYPE(x) ((x) << 0x04) // Descriptor type (0 for system, 1 for code/data)
#define SEG_PRES(x) ((x) << 0x07) // Present
#define SEG_SAVL(x) ((x) << 0x0C) // Available for system use
#define SEG_LONG(x) ((x) << 0x0D) // Long mode
#define SEG_SIZE(x) ((x) << 0x0E) // Size (0 for 16-bit, 1 for 32)
#define SEG_GRAN(x) ((x) << 0x0F) // Granularity (0 for 1B - 1MB, 1 for 4KB - 4GB)
#define SEG_PRIV(x) (((x) & 0x03) << 0x05) // Set privilege level (0 - 3)
#define SEG_DATA_RD 0x00 // Read-Only
#define SEG_DATA_RDA 0x01 // Read-Only, accessed
#define SEG_DATA_RDWR 0x02 // Read/Write
#define SEG_DATA_RDWRA 0x03 // Read/Write, accessed
#define SEG_DATA_RDEXPD 0x04 // Read-Only, expand-down
#define SEG_DATA_RDEXPDA 0x05 // Read-Only, expand-down, accessed
#define SEG_DATA_RDWREXPD 0x06 // Read/Write, expand-down
#define SEG_DATA_RDWREXPDA 0x07 // Read/Write, expand-down, accessed
#define SEG_CODE_EX 0x08 // Execute-Only
#define SEG_CODE_EXA 0x09 // Execute-Only, accessed
#define SEG_CODE_EXRD 0x0A // Execute/Read
#define SEG_CODE_EXRDA 0x0B // Execute/Read, accessed
#define SEG_CODE_EXC 0x0C // Execute-Only, conforming
#define SEG_CODE_EXCA 0x0D // Execute-Only, conforming, accessed
#define SEG_CODE_EXRDC 0x0E // Execute/Read, conforming
#define SEG_CODE_EXRDCA 0x0F // Execute/Read, conforming, accessed
#define GDT_CODE_PL0 (SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
SEG_PRIV(0) | SEG_CODE_EXRD)
#define GDT_DATA_PL0 (SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
SEG_PRIV(0) | SEG_DATA_RDWR)
#define GDT_CODE_PL3 (SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
SEG_PRIV(3) | SEG_CODE_EXRD)
#define GDT_DATA_PL3 (SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
SEG_PRIV(3) | SEG_DATA_RDWR)
struct gpt_ptr {
uint16_t limit;
uint32_t base;
}__attribute__((packed));
uint64_t gdt_desc[5];
struct gpt_ptr gp;
extern void flush_gdt(struct gpt_ptr *ptr);
uint64_t create_gdt_entry(uint32_t base, uint32_t limit, uint16_t flag) {
uint64_t ret;
ret = limit & 0x000F0000;
ret |= (flag << 8) & 0x00F0FF00;
ret |= (base >> 16) & 0x000000FF;
ret |= base & 0xFF000000;
ret <<= 32;
ret |= base << 16;
ret |= limit & 0x0000FFFF;
return ret;
}
void gdt_install(void) {
gp.limit = 64 * 5 - 1;
gp.base = (uint32_t)&gdt_desc;
gdt_desc[0] = create_gdt_entry(0, 0, 0);
gdt_desc[1] = create_gdt_entry(0, 0x000FFFFF, GDT_CODE_PL0);
gdt_desc[2] = create_gdt_entry(0, 0x000FFFFF, GDT_DATA_PL0);
gdt_desc[3] = create_gdt_entry(0, 0x000FFFFF, GDT_DATA_PL3);
gdt_desc[4] = create_gdt_entry(0, 0x000FFFFF, GDT_DATA_PL3);
flush_gdt(&gp);
}
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