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#include <stdio.h>
#include <stdint.h>

struct gdt_entry_t {
    uint16_t limit_low;
    uint16_t base_low;
    uint8_t base_middle;
    uint8_t access;
    uint8_t granularity;
    uint8_t base_high;
} __attribute__((packed, aligned(8)));


struct gdt_ptr_t {
    uint16_t limit;
    uint64_t base;
}__attribute__((packed));

extern void load_gdt();

struct gdt_entry_t gdt_entries[5];
struct gdt_ptr_t gdt_ptr;
struct gdt_ptr_t* gp;

static void gdt_set_gate(int32_t entry, uint32_t base, uint32_t limit, uint8_t access, uint8_t gran) {
    gdt_entries[entry].base_low = (base & 0xFFFF);
    gdt_entries[entry].base_middle = (base >> 16) & 0xFF;
    gdt_entries[entry].base_high = (base >> 24) & 0xFF;
    gdt_entries[entry].limit_low = (limit & 0xFFFF);
    gdt_entries[entry].granularity = (limit >> 16) & 0x0F;
    gdt_entries[entry].granularity |= gran & 0xF0;
    gdt_entries[entry].access = access;
}

void gdt_init() {
    gdt_ptr.limit = (sizeof(struct gdt_entry_t)*5) - 1;
    gdt_ptr.base = (uint64_t)&gdt_entries;
    gp = &gdt_ptr;

    gdt_set_gate(0, 0, 0, 0, 0); // Null segment
    gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF); //Code segment
    gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF); //Data segment
    gdt_set_gate(3, 0, 0xFFFFFFFF, 0xFA, 0xCF); //User mode code segment
    gdt_set_gate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF); //User mode data segment

    load_gdt();
}

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bits 64

global load_gdt
extern gp

load_gdt:
    CLI
    LGDT  [rel gp]
    PUSH 0x08                 ; Push code segment to stack, 0x08 is a stand-in for your code segment
    LEA RAX, [rel .reload_CS] ; Load address of .reload_CS into RAX
    PUSH RAX                  ; Push this value to the stack
    RETFQ                     ; Perform a far return, RETFQ or LRETQ depending on syntax

.reload_CS:
    ; Reload data segment registers
    MOV   AX, 0x10 ; 0x10 is a stand-in for your data segment
    MOV   DS, AX
    MOV   ES, AX
    MOV   FS, AX
    MOV   GS, AX
    MOV   SS, AX
    RET

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    gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF); //Code segment
    gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF); //Data segment
    gdt_set_gate(3, 0, 0xFFFFFFFF, 0xFA, 0xCF); //User mode code segment
    gdt_set_gate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF); //User mode data segment

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    gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xAF); //Code segment
    gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF); //Data segment
    gdt_set_gate(3, 0, 0xFFFFFFFF, 0xFA, 0xAF); //User mode code segment
    gdt_set_gate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF); //User mode data segment