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#include <kernel/tty.h>
#include <stdint.h>
extern void stack_top; // platform/boot.s
struct gdt_entry {
uint64_t limit_low : 16;
uint64_t base_low : 24;
uint64_t accessed : 1; // set by the processor
// CODE | DATA
uint64_t rw : 1; // readable? | writeable?
uint64_t conforming : 1; // conforming? | expands down?
uint64_t code : 1; // 1 | 0
uint64_t codeordata : 1; // 1 for everything other than TSS and LDT
uint64_t ring : 2;
uint64_t present : 1; // always 1
uint64_t limit_high : 4;
uint64_t available : 1; // ???
uint64_t long_mode : 1;
uint64_t x32 : 1;
uint64_t gran : 1; // 1 - 4kb, 0 - 1b
uint64_t base_high : 8;
} __attribute__((packed));
struct tss_entry {
uint32_t prev_tss; // unused
uint32_t esp0; // kernel mode stack pointer
uint32_t ss0; // kernel mode stack segment
// total size = 0x68 (?) - 3 * sizeof(uint32_t) = 5c
uint8_t _unused[0x5c];
} __attribute__((packed));
struct lgdt_arg {
uint16_t limit;
uint32_t base;
} __attribute__((packed));
enum {
SEG_null,
SEG_r0data,
SEG_r0code,
SEG_r3data,
SEG_r3code,
SEG_TSS,
SEG_end
};
static struct gdt_entry GDT[6];
static struct tss_entry TSS;
static struct lgdt_arg lgdt_arg; // probably doesn't need to be global
static void gdt_prepare();
static void gdt_load();
static void gdt_prepare() {
// null segment
GDT[0].present = 0;
// ring0 data
GDT[1].limit_low = 0xFFFF;
GDT[1].limit_high = 0xF;
GDT[1].gran = 1; // 4KB * 0xFFFFF = (almost) 4GB
GDT[1].base_low = 0;
GDT[1].base_high = 0;
GDT[1].accessed = 0;
GDT[1].rw = 1;
GDT[1].conforming = 0;
GDT[1].code = 0;
GDT[1].codeordata = 1;
GDT[1].ring = 0;
GDT[1].present = 1;
GDT[1].long_mode = 0; // ???
GDT[1].available = 1; // ???
GDT[1].x32 = 1;
// copy to r0 code
GDT[2] = GDT[1];
GDT[2].code = 1;
// r3 data & code
GDT[3] = GDT[1];
GDT[3].ring = 3;
GDT[4] = GDT[2];
GDT[3].ring = 3;
{ // tss
// TODO memset(&TSS, 0, sizeof(TSS));
TSS.ss0 = 1 << 3; // kernel data segment
TSS.esp0 = (uint32_t) &stack_top;
GDT[5].limit_low = sizeof(TSS);
GDT[5].base_low = (uint32_t) &TSS;
GDT[5].accessed = 1; // 0 for TSS
GDT[5].rw = 0; // 1 busy / 0 not busy
GDT[5].conforming = 0; // 0 for TSS
GDT[5].code = 1; // 32bit
GDT[5].codeordata = 0; // is a system entry
GDT[5].ring = 3;
GDT[5].present = 1;
GDT[5].limit_high = (sizeof(TSS) >> 16) & 0xf;
GDT[5].available = 0; // 0 for TSS
GDT[5].long_mode = 0;
GDT[5].x32 = 0; // idk
GDT[5].gran = 0;
GDT[5].base_high = (((uint32_t) &TSS) >> 24) & 0xff;
}
}
static void gdt_load() {
lgdt_arg.limit = sizeof(GDT) - 1;
lgdt_arg.base = (uint32_t) &GDT;
asm("lgdt (%0)" : : "b" (&lgdt_arg));
}
void gdt_init() {
gdt_prepare();
gdt_load();
// check if the GDT was set up correctly
tty_write("checking gdt...", 15);
asm("mov $8, %%eax;"
"mov %%eax, %%ds;"
: : : "%eax");
tty_write("ok", 2);
}
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