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- /* bkerndev - Bran's Kernel Development Tutorial
- * By: Brandon F. (friesenb@gmail.com)
- * Desc: Global Descriptor Table management
- *
- * Notes: No warranty expressed or implied. Use at own risk. */
- /* Defines a GDT entry */
- struct gdt_entry
- {
- unsigned short limit_low;
- unsigned short base_low;
- unsigned char base_middle;
- unsigned char access;
- unsigned char granularity;
- unsigned char base_high;
- } __attribute__((packed));
- struct gdt_ptr
- {
- unsigned short limit;
- unsigned int base;
- } __attribute__((packed));
- /* Our GDT, with 3 entries, and finally our special GDT pointer */
- struct gdt_entry gdt[3];
- struct gdt_ptr gp;
- /* This is in start.asm. We use this to properly reload
- * the new segment registers */
- extern void _gdt_flush();
- /* Setup a descriptor in the Global Descriptor Table */
- void gdt_set_gate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran)
- {
- /* Setup the descriptor base address */
- gdt[num].base_low = (base & 0xFFFF);
- gdt[num].base_middle = (base >> 16) & 0xFF;
- gdt[num].base_high = (base >> 24) & 0xFF;
- /* Setup the descriptor limits */
- gdt[num].limit_low = (limit & 0xFFFF);
- gdt[num].granularity = ((limit >> 16) & 0x0F);
- /* Finally, set up the granularity and access flags */
- gdt[num].granularity |= (gran & 0xF0);
- gdt[num].access = access;
- }
- /* Should be called by main. This will setup the special GDT
- * pointer, set up the first 3 entries in our GDT, and then
- * finally call gdt_flush() in our assembler file in order
- * to tell the processor where the new GDT is and update the
- * new segment registers */
- void gdt_install()
- {
- /* Setup the GDT pointer and limit */
- gp.limit = (sizeof(struct gdt_entry) * 3) - 1;
- gp.base = &gdt;
- /* Our NULL descriptor */
- gdt_set_gate(0, 0, 0, 0, 0);
- /* The second entry is our Code Segment. The base address
- * is 0, the limit is 4GBytes, it uses 4KByte granularity,
- * uses 32-bit opcodes, and is a Code Segment descriptor.
- * Please check the table above in the tutorial in order
- * to see exactly what each value means */
- gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
- /* The third entry is our Data Segment. It's EXACTLY the
- * same as our code segment, but the descriptor type in
- * this entry's access byte says it's a Data Segment */
- gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
- /* Flush out the old GDT and install the new changes! */
- _gdt_flush();
- }
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