78 lines
1.9 KiB
C
78 lines
1.9 KiB
C
#include <kernel/x86/gdt.h>
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#include <stdint.h>
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#include <stdio.h>
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#ifdef __TESTING__
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#include <kernel/_kernel.h>
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#endif
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/**
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* What is this file?
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*
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* Well, to properly set up a lot of the system, we need something called a GDT
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* or, a Global Descriptor Table.
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*
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* This table, establishes a few things.
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*
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* Mainly it sets 4 segments,
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* A kernel code segment, with RING 0 permissions
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* A kernel data segment, with RING 0 permissions
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* A user code segment, with RING 3 permissions
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* A user data segment, with RING 3 permissions
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*
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* This allows for future userspace to properly segment code and data,
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* anything in userspace shouldn't have access to hardware like the kernel does
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* So by passing through this GDT, we can dish out authority to access certain data, functions,
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* etc, by going through the CPU permission system (RING 0 - 3)
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*/
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uint64_t gdt[GDT_SIZE];
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uint64_t create_descriptor(uint32_t base, uint32_t limit, uint16_t flag)
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{
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uint64_t descriptor;
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descriptor = limit & 0x000F0000;
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descriptor |= (flag << 8) & 0x00F0FF00;
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descriptor |= (base >> 16) & 0x000000FF;
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descriptor |= base & 0xFF000000;
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descriptor <<= 32;
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descriptor |= base << 16;
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descriptor |= limit & 0x0000FFFF;
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return descriptor;
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}
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#ifdef __TESTING__
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void dump_gdt(void)
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{
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for (int i = 0; i < GDT_SIZE; i++) {
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printf("GDT_ENTRY %d: %4 | %2\n", i, gdt[i], gdt[i]);
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}
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}
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#endif
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void gdt_init(void)
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{
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#ifdef __TESTING__
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kinfo("Initializing the GDT");
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#endif
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gdt[0] = create_descriptor(0, 0, 0); // null
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gdt[1] = create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL0)); // kernel code
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gdt[2] = create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL0)); // kernel data
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gdt[3] = create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL3)); // user code
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gdt[4] = create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL3)); // user data
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setGdt((sizeof(uint64_t) * GDT_SIZE) - 1, &(gdt[0])); // limit, base
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reloadSegments();
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#ifdef __TESTING__
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kinfo("Initialized the GDT");
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dump_gdt();
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#endif
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}
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