e4cc638a8d
This commit adds GDT initialization with proper code/data segments and reloads CS. It also adds the initial IDT structure and loads an empty IDT. Build configuration updated to disable SSE/MMX to prevent compiler generation of unsupported instructions in early boot.
69 lines
2.3 KiB
C
69 lines
2.3 KiB
C
#include "gdt.h"
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#include <stdint.h>
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/* GDT Pointer Structure */
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struct gdt_ptr gp;
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/* GDT entries */
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struct gdt_entry gdt[5];
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extern void gdt_flush(uint32_t);
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/* Setup a descriptor in the Global Descriptor Table */
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void gdt_set_gate(int32_t num, uint32_t base, uint32_t limit, uint8_t access, uint8_t 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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* call gdt_flush() in our assembler file in order to tell
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* the processor where the new GDT is and update the
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* internal registers */
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void init_gdt()
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{
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/* Setup the GDT pointer and limit */
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gp.limit = (sizeof(struct gdt_entry) * 5) - 1;
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gp.base = (uint32_t)&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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/* 0x9A = 1001 1010
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P=1, DPL=00, S=1 (Code/Data), Type=1010 (Code, Exec/Read) */
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/* 0xCF = 1100 1111
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G=1 (4k), DB=1 (32bit), L=0, AVL=0, LimitHigh=0xF */
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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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/* 0x92 = 1001 0010
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P=1, DPL=00, S=1, Type=0010 (Data, Read/Write) */
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gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
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/* User mode code segment */
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gdt_set_gate(3, 0, 0xFFFFFFFF, 0xFA, 0xCF);
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/* User mode data segment */
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gdt_set_gate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF);
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/* Flush out the old GDT and install the new changes! */
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gdt_flush((uint32_t)&gp);
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}
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