78 lines
2.6 KiB
C
78 lines
2.6 KiB
C
#include <system.h>
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/* Defines a GDT entry. We say packed, because it prevents the
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* compiler from doing things that it thinks is best: Prevent
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* compiler "optimization" by packing */
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struct gdt_entry
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{
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unsigned short limit_low;
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unsigned short base_low;
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unsigned char base_middle;
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unsigned char access;
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unsigned char granularity;
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unsigned char base_high;
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} __attribute__((packed));
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/* Special pointer which includes the limit: The max bytes
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* taken up by the GDT, minus 1. Again, this NEEDS to be packed */
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struct gdt_ptr
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{
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unsigned short limit;
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unsigned int base;
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} __attribute__((packed));
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/* Our GDT, with 3 entries, and finally our special GDT pointer */
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struct gdt_entry gdt[3];
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struct gdt_ptr gp;
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/* This will be a function in start.asm. We use this to properly
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* reload the new segment registers */
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extern void gdt_flush();
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/* Setup a descriptor in the Global Descriptor Table */
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void gdt_set_gate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran)
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{
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/* Setup the descriptor base address */
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gdt[num].base_low = (base & 0xFFFF);
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gdt[num].base_middle = (base >> 16) & 0xFF;
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gdt[num].base_high = (base >> 24) & 0xFF;
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/* Setup the descriptor limits */
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gdt[num].limit_low = (limit & 0xFFFF);
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gdt[num].granularity = ((limit >> 16) & 0x0F);
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/* Finally, set up the granularity and access flags */
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gdt[num].granularity |= (gran & 0xF0);
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gdt[num].access = access;
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}
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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 gdt_flush() 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 limit */
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gp.limit = (sizeof(struct gdt_entry) * 3) - 1;
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gp.base = &gdt;
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/* Our NULL descriptor */
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gdt_set_gate(0, 0, 0, 0, 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(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
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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(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
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/* Flush out the old GDT and install the new changes! */
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gdt_flush();
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}
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