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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[SEG_end];
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_check();
static void gdt_prepare() {
GDT[SEG_null].present = 0;
GDT[SEG_r0data].limit_low = 0xFFFF;
GDT[SEG_r0data].limit_high = 0xF;
GDT[SEG_r0data].gran = 1; // 4KB * 0xFFFFF = (almost) 4GB
GDT[SEG_r0data].base_low = 0;
GDT[SEG_r0data].base_high = 0;
GDT[SEG_r0data].accessed = 0;
GDT[SEG_r0data].rw = 1;
GDT[SEG_r0data].conforming = 0;
GDT[SEG_r0data].code = 0;
GDT[SEG_r0data].codeordata = 1;
GDT[SEG_r0data].ring = 0;
GDT[SEG_r0data].present = 1;
GDT[SEG_r0data].long_mode = 0; // ???
GDT[SEG_r0data].available = 1; // ???
GDT[SEG_r0data].x32 = 1;
// copy to r0 code
GDT[SEG_r0code] = GDT[SEG_r0data];
GDT[SEG_r0code].code = SEG_r0data;
GDT[SEG_r3data] = GDT[SEG_r0data];
GDT[SEG_r3data].ring = 3;
GDT[SEG_r3code] = GDT[SEG_r0code];
GDT[SEG_r3data].ring = 3;
// tss
// TODO memset(&TSS, 0, sizeof(TSS));
TSS.ss0 = SEG_r0data << SEG_r3data; // kernel data segment
TSS.esp0 = (uint32_t) &stack_top;
GDT[SEG_TSS].limit_low = sizeof(TSS);
GDT[SEG_TSS].base_low = (uint32_t) &TSS;
GDT[SEG_TSS].accessed = 1; // 1 for TSS
GDT[SEG_TSS].rw = 0; // 1 busy / 0 not busy
GDT[SEG_TSS].conforming = 0; // 0 for TSS
GDT[SEG_TSS].code = 1; // 32bit
GDT[SEG_TSS].codeordata = 0; // is a system entry
GDT[SEG_TSS].ring = 3;
GDT[SEG_TSS].present = 1;
GDT[SEG_TSS].limit_high = (sizeof(TSS) >> 16) & 0xf;
GDT[SEG_TSS].available = 0; // 0 for TSS
GDT[SEG_TSS].long_mode = 0;
GDT[SEG_TSS].x32 = 0; // idk
GDT[SEG_TSS].gran = 0;
GDT[SEG_TSS].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));
}
static void gdt_check() {
tty_write("checking gdt...", 15);
asm("mov %0, %%ds;"
: : "r" (SEG_r0data << 3));
tty_write("ok", 2);
}
void gdt_init() {
gdt_prepare();
gdt_load();
gdt_check();
}
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