luxos/Kernel/hal/cpu/gdt.c

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/******************************************************************
* gdt.c - GLOBAL DESCRIPTOR TABLE *
* Contains function prototypes for setting up the GDT *
******************************************************************/
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#define MAX_DESCRIPTORS 5
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#include "gdt.h"
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/* Our GDT, with 3 entries, and finally our special GDT pointer */
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struct GdtEntry gdt[MAX_DESCRIPTORS];
struct GdtPointer gp;
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/* Setup a descriptor in the Global Descriptor Table */
void GdtSetGate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran)
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{
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/* Sanity check */
if (num >= MAX_DESCRIPTORS) return;
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/* 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;
}
struct GdtEntry* GdtGetGate(int num)
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{
if (num>MAX_DESCRIPTORS) return 0;
return &gdt[num];
}
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/* 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 GdtInstall()
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{
/* Setup the GDT pointer and limit */
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gp.limit = (sizeof(struct GdtEntry) * 3) - 1;
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gp.base = (unsigned int)&gdt;
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/* Our NULL descriptor */
GdtSetGate(0, 0, 0, 0, 0);
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/* 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 */
GdtSetGate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
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/* 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 */
GdtSetGate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
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/* User mode Code segment*/
GdtSetGate(3, 0, 0xFFFFFFFF, 0xFA, 0xCF);
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/* User mode data segment*/
GdtSetGate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF);
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/* Flush out the old GDT and install the new changes! */
GdtFlush();
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}