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#include "gdt.h"
#include "print.h"
extern void reloadSegments();
void encodeGDT(uint8_t* gdtEntry, struct GDT source) {
if ((source.limit > 65536) && ((source.limit & 0xFFF) == 0xFFF)) {
// Set the GDT to use paging
// To do this we need to make sure the limit is aligned to 4KiB
source.limit = source.limit >> 12;
// Granularity: 1 (use paging with 4KiB segments)
// Size: 1 (32 bit protected mode)
gdtEntry[6] = 0xC0;
}
else if (source.limit == 0) {
// Don't set any granularity for null entry
gdtEntry[6] = 0;
}
else {
// Granularity: 0 (1 byte segments)
// Size: 1
gdtEntry[6] = 0x40;
}
// Here we are encoding the limit
// Bits 0-15 encode the bottom 16 bits of the limit
gdtEntry[0] = source.limit & 0xFF;
gdtEntry[1] = (source.limit >> 8) & 0xFF;
// Bits 48-51 encode the last 4 bits of the limit
gdtEntry[6] |= (source.limit >> 16) & 0xF;
// Bits 16-39 encode the bottom 24 bits of the base
gdtEntry[2] = source.base & 0xFF;
gdtEntry[3] = (source.base >> 8) & 0xFF;
gdtEntry[4] = (source.base >> 16) & 0xFF;
// Bits 56-64 encode the last byte of the base
gdtEntry[7] = (source.base >> 24) & 0xFF;
// Bits 40-47 set the access byte, which is documented at https://wiki.osdev.org/GDT,
// where most of the ideas for this function are taken from shamelessly
gdtEntry[5] = source.type;
}
void initializeGDT() {
struct GDT nullEntry;
nullEntry.limit = 0;
nullEntry.base = 0;
nullEntry.type = 0;
struct GDT codeSection;
codeSection.limit = 0xFFFFFFFF;
codeSection.base = 0;
codeSection.type = 0x9A;
struct GDT dataSection;
dataSection.limit = 0xFFFFFFFF;
dataSection.base = 0;
dataSection.type = 0x92;
encodeGDT(gdt_entries , nullEntry );
encodeGDT(gdt_entries + 8 , codeSection);
encodeGDT(gdt_entries + 16, dataSection);
struct GDT_ptr gdt_ptr;
gdt_ptr.limit = 8*3;
gdt_ptr.base = (uint32_t)(&gdt_entries);
asm("lgdt (%0)" : :"r" ((uint8_t*)(&gdt_ptr)));
reloadSegments();
}
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