103 lines
3.0 KiB
C
103 lines
3.0 KiB
C
#include "gdt.h"
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// Internal method
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extern void gdt_flush(uint32_t); // Defined on cpu/gdt.asm
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static gdt_entry_t construct_null_entry();
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static gdt_entry_t construct_entry(gdt_access_t access);
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static void init_gdt();
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gdt_entry_t gdt_entries[5];
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gdt_ptr_t gdt_ptr;
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// This method will be called by the kernel
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void gdt_setup() {
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init_gdt();
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}
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static void init_gdt() {
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gdt_ptr.limit = (sizeof(gdt_entry_t) * 5) - 1;
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gdt_ptr.base = (uint32_t)&gdt_entries;
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gdt_entry_t null_segment = construct_null_entry();
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gdt_entry_t kernel_mode_code_segment = construct_entry(
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(struct gdt_access){
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.type = GDT_CODE_TYPE_EXEC_READ,
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.dt = GDT_CODE_AND_DATA_DESCRIPTOR,
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.dpl = GDT_RING0,
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.p = GDT_SEGMENT_PRESENT
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}
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);
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gdt_entry_t kernel_mode_data_segment = construct_entry(
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(struct gdt_access){
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.type = GDT_DATA_TYPE_READ_WRITE,
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.dt = GDT_CODE_AND_DATA_DESCRIPTOR,
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.dpl = GDT_RING0,
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.p = GDT_SEGMENT_PRESENT
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}
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);
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gdt_entry_t user_mode_code_segment = construct_entry(
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(struct gdt_access){
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.type = GDT_CODE_TYPE_EXEC_READ,
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.dt = GDT_CODE_AND_DATA_DESCRIPTOR,
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.dpl = GDT_RING3,
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.p = GDT_SEGMENT_PRESENT
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}
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);
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gdt_entry_t user_mode_data_segment = construct_entry(
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(struct gdt_access){
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.type = GDT_DATA_TYPE_READ_WRITE,
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.dt = GDT_CODE_AND_DATA_DESCRIPTOR,
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.dpl = GDT_RING3,
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.p = GDT_SEGMENT_PRESENT
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}
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);
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gdt_entries[0] = null_segment;
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gdt_entries[1] = kernel_mode_code_segment;
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gdt_entries[2] = kernel_mode_data_segment;
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gdt_entries[3] = user_mode_code_segment;
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gdt_entries[4] = user_mode_data_segment;
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gdt_flush((uint32_t)&gdt_ptr);
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}
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static gdt_entry_t construct_entry(gdt_access_t access) {
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gdt_entry_t entry = (struct gdt_entry_struct) {
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.base_low = GDT_BASE & 0xFFFF,
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.base_middle = (GDT_BASE >> 16) & 0xFF,
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.base_high = (GDT_BASE >> 24) & 0xFF,
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.limit_low = (GDT_LIMIT & 0xFFFF),
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.access = access,
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.granularity = (struct gdt_granularity) {
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.g = GDT_GRANULARITY_4K,
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.d = GDT_OPERAND_SIZE_32,
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.zero = 0,
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.seglen = GDT_SEGMENT_LENGTH
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}
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};
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return entry;
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}
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// The only difference is in the access
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static gdt_entry_t construct_null_entry() {
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gdt_entry_t null_entry = (struct gdt_entry_struct) {
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.base_low = 0,
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.base_middle = 0,
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.base_high = 0,
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.limit_low = 0,
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.access = (struct gdt_access) {
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.p = 0,
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.dpl = 0,
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.dt = 0,
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.type = 0
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},
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.granularity = (struct gdt_granularity) {
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.g = 0,
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.d = 0,
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.zero = 0,
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.seglen = 0
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}
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};
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return null_entry;
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} |