109 lines
4.0 KiB
C
109 lines
4.0 KiB
C
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// Used for creating GDT segment descriptors in 64-bit integer form.
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#include <stdio.h>
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#include <stdint.h>
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// Each define here is for a specific flag in the descriptor.
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// Refer to the intel documentation for a description of what each one does.
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#define SEG_DESCTYPE(x) ((x) << 0x04) // Descriptor type (0 for system, 1 for code/data)
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#define SEG_PRES(x) ((x) << 0x07) // Present
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#define SEG_SAVL(x) ((x) << 0x0C) // Available for system use
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#define SEG_LONG(x) ((x) << 0x0D) // Long mode
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#define SEG_SIZE(x) ((x) << 0x0E) // Size (0 for 16-bit, 1 for 32)
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#define SEG_GRAN(x) ((x) << 0x0F) // Granularity (0 for 1B - 1MB, 1 for 4KB - 4GB)
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#define SEG_PRIV(x) (((x) & 0x03) << 0x05) // Set privilege level (0 - 3)
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#define SEG_DATA_RD 0x00 // Read-Only
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#define SEG_DATA_RDA 0x01 // Read-Only, accessed
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#define SEG_DATA_RDWR 0x02 // Read/Write
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#define SEG_DATA_RDWRA 0x03 // Read/Write, accessed
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#define SEG_DATA_RDEXPD 0x04 // Read-Only, expand-down
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#define SEG_DATA_RDEXPDA 0x05 // Read-Only, expand-down, accessed
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#define SEG_DATA_RDWREXPD 0x06 // Read/Write, expand-down
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#define SEG_DATA_RDWREXPDA 0x07 // Read/Write, expand-down, accessed
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#define SEG_CODE_EX 0x08 // Execute-Only
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#define SEG_CODE_EXA 0x09 // Execute-Only, accessed
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#define SEG_CODE_EXRD 0x0A // Execute/Read
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#define SEG_CODE_EXRDA 0x0B // Execute/Read, accessed
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#define SEG_CODE_EXC 0x0C // Execute-Only, conforming
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#define SEG_CODE_EXCA 0x0D // Execute-Only, conforming, accessed
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#define SEG_CODE_EXRDC 0x0E // Execute/Read, conforming
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#define SEG_CODE_EXRDCA 0x0F // Execute/Read, conforming, accessed
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#define GDT_CODE_PL0 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
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SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
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SEG_PRIV(0) | SEG_CODE_EXRD
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#define GDT_DATA_PL0 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
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SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
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SEG_PRIV(0) | SEG_DATA_RDWR
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#define GDT_CODE_PL3 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
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SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
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SEG_PRIV(3) | SEG_CODE_EXRD
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#define GDT_DATA_PL3 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
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SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
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SEG_PRIV(3) | SEG_DATA_RDWR
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#define GDT_MAX_DESCRIPTORS 3
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struct gdt_entry {
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uint16_t limit_low;
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uint16_t base_low;
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uint8_t base_middle;
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uint8_t access;
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uint8_t granularity;
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uint8_t base_high;
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}__attribute__((packed));
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struct gdt_ptr {
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uint16_t limit;
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uint32_t base;
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}__attribute__((packed));
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struct gdt_entry gdt[GDT_MAX_DESCRIPTORS];
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struct gdt_ptr gp;
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extern void gdt_flush(struct gdt_ptr* gdt_ptr_addr);
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void
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create_descriptor(uint32_t base, uint32_t limit, uint16_t flag)
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{
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uint64_t descriptor;
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// Create the high 32 bit segment
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descriptor = limit & 0x000F0000; // set limit bits 19:16
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descriptor |= (flag << 8) & 0x00F0FF00; // set type, p, dpl, s, g, d/b, l and avl fields
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descriptor |= (base >> 16) & 0x000000FF; // set base bits 23:16
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descriptor |= base & 0xFF000000; // set base bits 31:24
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// Shift by 32 to allow for low part of segment
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descriptor <<= 32;
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// Create the low 32 bit segment
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descriptor |= base << 16; // set base bits 15:0
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descriptor |= limit & 0x0000FFFF; // set limit bits 15:0
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printf("0x%.16llX\n", descriptor);
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}
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void gdt_init() {
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gp.limit = (sizeof(struct gdt_entry) * GDT_MAX_DESCRIPTORS) - 1;
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gp.base = (uint32_t) &gdt;
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create_descriptor(0, 0, 0);
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create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL0));
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create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL0));
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create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL3));
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create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL3));
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/*
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gdt_set_gate(0, 0, 0, 0, 0);
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gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
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gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
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*/
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gdt_flush(&gp);
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}
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