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#include <kernel/arch/generic.h>
#include <kernel/arch/i386/gdt.h>
#include <shared/mem.h>
#include <stdbool.h>
#include <stdint.h>
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 _unused0;
uint32_t esp0; // kernel mode stack pointer
uint32_t ss0; // kernel mode stack segment
uint8_t _unused1[0x5c];
} __attribute__((packed));
struct lgdt_arg {
uint16_t limit;
uint32_t base;
} __attribute__((packed));
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_fillout(struct gdt_entry* entry, uint8_t ring, bool code);
static void gdt_prepare(void);
static void gdt_load(void);
static void gdt_fillout(struct gdt_entry* entry, uint8_t ring, bool code) {
*entry = (struct gdt_entry) {
// set up the identity mapping
.limit_low = 0xFFFF,
.limit_high = 0xF,
.gran = 1, // 4KB * 0xFFFFF = (almost) 4GB
.base_low = 0,
.base_high = 0,
.ring = ring,
.code = code,
.accessed = 0,
.rw = 1,
.conforming = 0,
.codeordata = 1,
.present = 1,
.long_mode = 0, // ???
.available = 1, // ???
.x32 = 1,
};
}
static void gdt_prepare(void) {
GDT[SEG_null].present = 0;
gdt_fillout(&GDT[SEG_r0code], 0, true);
gdt_fillout(&GDT[SEG_r0data], 0, false);
gdt_fillout(&GDT[SEG_r3code], 3, true);
gdt_fillout(&GDT[SEG_r3data], 3, false);
// tss
memset(&TSS, 0, sizeof(TSS));
TSS.ss0 = SEG_r0data << 3; // kernel data segment
TSS.esp0 = (uintptr_t) &_bss_end;
GDT[SEG_TSS] = (struct gdt_entry) {
.limit_low = sizeof(TSS),
.limit_high = sizeof(TSS) >> 16,
.gran = 0,
.base_low = (uintptr_t) &TSS,
.base_high = ((uintptr_t) &TSS) >> 24,
.accessed = 1, // 1 for TSS
.rw = 0, // 1 busy / 0 not busy
.conforming = 0, // 0 for TSS
.code = 1, // 32bit
.codeordata = 0, // is a system entry
.ring = 3,
.present = 1,
.available = 0, // 0 for TSS
.long_mode = 0,
.x32 = 0, // idk
};
}
static void gdt_load(void) {
lgdt_arg.limit = sizeof(GDT) - 1;
lgdt_arg.base = (uintptr_t) &GDT;
asm("lgdt (%0)"
: : "r" (&lgdt_arg) : "memory");
asm("ltr %%ax"
: : "a" (SEG_TSS << 3 | 3) : "memory");
// update all segment registers
gdt_farjump(SEG_r0code << 3);
asm("mov %0, %%ds;"
"mov %0, %%ss;"
"mov %0, %%es;"
"mov %0, %%fs;"
"mov %0, %%gs;"
: : "r" (SEG_r0data << 3) : "memory");
}
void gdt_init(void) {
gdt_prepare();
gdt_load();
}
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