50 lines
1.9 KiB
C
50 lines
1.9 KiB
C
#include "../includes/gdt.h"
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/* Code taken from http://www.osdever.net/bkerndev/Docs/gdt.htm with some minor tweaks. */
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struct gdt_entry gdt_entries[3];
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struct gdt gdt_ptr;
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/* Should be called by main. This will setup the special GDT
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* pointer, set up the first 3 entries in our GDT, and then
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* finally call load_gdt() in our assembler file in order
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* to tell the processor where the new GDT is and update the
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* new segment registers */
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void gdt_install()
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{
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/* Setup the GDT pointer and the size of the table */
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gdt_ptr.size = sizeof(struct gdt_entry) * 3 - 1;
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gdt_ptr.address = (unsigned int) &gdt_entries;
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/* Our NULL descriptor */
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gdt_set_gate(0, 0, 0, 0, &gdt_entries[0]);
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/* The second entry is our Code Segment. The base address
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* is 0, the limit is 4GBytes, it uses 4KByte granularity,
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* uses 32-bit opcodes, and is a Code Segment descriptor.
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* Please check the table above in the tutorial in order
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* to see exactly what each value means */
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gdt_set_gate(0, 0xFFFFFFFF, 0x9A, 0xCF, &gdt_entries[1]);
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/* The third entry is our Data Segment. It's EXACTLY the
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* same as our code segment, but the descriptor type in
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* this entry's access byte says it's a Data Segment */
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gdt_set_gate(0, 0xFFFFFFFF, 0x92, 0xCF, &gdt_entries[2]);
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/* Flush out the old GDT and install the new changes! */
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load_gdt(&gdt_ptr);
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}
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void gdt_set_gate(unsigned long base, unsigned long limit, unsigned char access, unsigned char gran, struct gdt_entry* gdt_entry)
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{
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/* Setup the descriptor base address */
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gdt_entry->base_low = (base & 0xFFFF);
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gdt_entry->base_middle = (base >> 16) & 0xFF;
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gdt_entry->base_high = (base >> 24) & 0xFF;
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/* Setup the descriptor limits */
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gdt_entry->limit_low = (limit & 0xFFFF);
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gdt_entry->granularity = ((limit >> 16) & 0x0F);
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/* Finally, set up the granularity and access flags */
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gdt_entry->granularity |= (gran & 0xF0);
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gdt_entry->access = access;
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}
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