121 lines
2.7 KiB
C
121 lines
2.7 KiB
C
#include <kernel/tty.h>
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#include <debugging.h>
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#include <stdint.h>
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#include <stddef.h>
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#define NUM_ENTRIES 3
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struct SegmentDescriptor {
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uint32_t limit;
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uint32_t base;
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uint8_t access_byte;
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uint8_t flags;
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};
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struct GDT {
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uint16_t size;
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uint64_t* base;
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} __attribute__((packed));
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static uint64_t entries[NUM_ENTRIES];
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static size_t current_entry = 0;
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struct GDT gdt;
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static void encodeGdtEntry(uint8_t *target, struct SegmentDescriptor source)
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{
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// Check the limit to make sure that it can be encoded
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if (source.limit > 0xFFFFF) {
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terminal_writestring("WARNING: GDT entry has a limit (");
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print_hex_bytes(&source.limit, 4);
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terminal_writestring(") that is larger than accepted.");
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return;
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}
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// Encode the limit
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target[0] = source.limit & 0xFF;
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target[1] = (source.limit >> 8) & 0xFF;
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target[6] = (source.limit >> 16) & 0x0F;
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// Encode the base
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target[2] = source.base & 0xFF;
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target[3] = (source.base >> 8) & 0xFF;
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target[4] = (source.base >> 16) & 0xFF;
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target[7] = (source.base >> 24) & 0xFF;
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// Encode the access byte
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target[5] = source.access_byte;
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// Encode the flags
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target[6] |= (source.flags << 4);
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print_hex_bytes(target, 8);
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}
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static void add_entry(struct SegmentDescriptor entry)
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{
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if (current_entry == NUM_ENTRIES) {
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terminal_writestring("WARNING: More GDT entries than NUM_ENTRIES.\n");
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return;
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}
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uint64_t descriptor = 0;
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encodeGdtEntry((uint8_t*) &descriptor, entry);
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entries[current_entry] = descriptor;
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current_entry++;
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}
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static void load_gdt()
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{
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gdt.size = NUM_ENTRIES * 8 - 1;
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gdt.base = entries;
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for (int i = 0; i < NUM_ENTRIES; i++) {
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print_hex_bytes(gdt.base + i, 8);
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terminal_putchar('\n');
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}
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asm ( "lgdt %0" : : "m"(gdt) );
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}
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void gdt_init()
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{
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terminal_initialize();
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struct SegmentDescriptor null;
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null.limit = 0;
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null.base = 0;
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null.access_byte = 0;
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null.flags = 0;
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terminal_writestring("Kernel Null: ");
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add_entry(null);
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terminal_putchar('\n');
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struct SegmentDescriptor kernel_code;
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kernel_code.limit = 0x000003FF;
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kernel_code.base = 0x00000000;
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kernel_code.access_byte = 0b10011010;
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kernel_code.flags = 0b1100;
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terminal_writestring("Kernel Code: ");
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add_entry(kernel_code);
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terminal_putchar('\n');
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struct SegmentDescriptor kernel_data;
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kernel_code.limit = 0x000003FF;
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kernel_data.base = 0x00400000;
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kernel_data.access_byte = 0b10010010;
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kernel_data.flags = 0b1100;
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terminal_writestring("Kernel Data: ");
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add_entry(kernel_data);
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terminal_putchar('\n');
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load_gdt();
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}
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