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GDT Tutorial: Difference between revisions
Fixed bit shifting error as pointed out on the talk page, some comments and changed SEG_TYPE to SEG_DESCTYPE to differentiate segment and descriptor types
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(Fixed bit shifting error as pointed out on the talk page, some comments and changed SEG_TYPE to SEG_DESCTYPE to differentiate segment and descriptor types) |
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Line 214:
// Each define here is for a specific flag in the descriptor.
// Refer to the intel documentation for a description of what each one does.
#define
#define SEG_PRES(x) ((x) << 0x07) // Present
#define SEG_SAVL(x) ((x) << 0x0C) // Available for system use
#define SEG_LONG(x) ((x) << 0x0D) // Long mode
#define SEG_SIZE(x) ((x) << 0x0E) // Size (0 for 16-bit, 1 for 32)
#define SEG_GRAN(x) ((x) << 0x0F) // Granularity (0 for 1B - 1MB, 1 for 4KB - 4GB)
#define SEG_PRIV(x) (((x) & 0x03) << 0x05) // Set privilege level (0 - 3)
#define SEG_DATA_RD 0x00 // Read-Only
Line 239:
#define SEG_CODE_EXRDCA 0x0F // Execute/Read, conforming, accessed
#define GDT_CODE_PL0
SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
SEG_PRIV(0) | SEG_CODE_EXRD
#define GDT_DATA_PL0
SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
SEG_PRIV(0) | SEG_DATA_RDWR
#define GDT_CODE_PL3
SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
SEG_PRIV(3) | SEG_CODE_EXRD
#define GDT_DATA_PL3
SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
SEG_PRIV(3) | SEG_DATA_RDWR
void
Line 261:
// Create the high 32 bit segment
descriptor = limit & 0x000F0000; // set limit bits 19:16
descriptor |= (flag << 8) & 0x00F0FF00; // set type, p, dpl, s, g, d/b, l and avl fields
descriptor |= (base >> 16) &
descriptor |= base & 0xFF000000; // set base bits 31:24
// Shift by 32 to allow for low part of segment
Line 269 ⟶ 270:
// Create the low 32 bit segment
descriptor |= base << 16; // set base bits 15:0
descriptor |= limit & 0x0000FFFF; // set limit bits 15:0
printf("0x%.16llX\n", descriptor);
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