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GDT Tutorial: Difference between revisions
some macroes 'Written by rizlah@freenode#osdev'
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The IDT will be one of the first things to be enabled in your kernel sequence, so that you can catch hardware exceptions, listen to external events, etc. See [[Interrupts for dummies]] for more information about the interrupts of X86 family.
==Some stuff to make your life easy==
Macros and functions written by rizlah@freenode#osdev
<source lang="c">
/*
* Written by rizlah@freenode#osdev
* Public Domain.
*/
/* Used for creating GDT segment entries in 64bit integer form.
Written by rizlah*/
#include <stdio.h>
#include <stdint.h>
// 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 SEG_TYPE(x) ((x) << 0x04) // Descriptor type (0 for system, 1 for code/data)
#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
#define SEG_DATA_RDA 0x01 // Read-Only, accessed
#define SEG_DATA_RDWR 0x02 // Read/Write
#define SEG_DATA_RDWRA 0x03 // Read/Write, accessed
#define SEG_DATA_RDEXPD 0x04 // Read-Only, expand-down
#define SEG_DATA_RDEXPDA 0x05 // Read-Only, expand-down, accessed
#define SEG_DATA_RDWREXPD 0x06 // Read/Write, expand-down
#define SEG_DATA_RDWREXPDA 0x07 // Read/Write, expand-down, accessed
#define SEG_CODE_EX 0x08 // Execute-Only
#define SEG_CODE_EXA 0x09 // Execute-Only, accessed
#define SEG_CODE_EXRD 0x0A // Execute/Read
#define SEG_CODE_EXRDA 0x0B // Execute/Read, accessed
#define SEG_CODE_EXC 0x0C // Execute-Only, conforming
#define SEG_CODE_EXCA 0x0D // Execute-Only, conforming, accessed
#define SEG_CODE_EXRDC 0x0E // Execute/Read, conforming
#define SEG_CODE_EXRDCA 0x0F // Execute/Read, conforming, accessed
#define GDT_CODE_PL0 SEG_TYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
SEG_PRIV(0) | SEG_CODE_EXRD
#define GDT_DATA_PL0 SEG_TYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
SEG_PRIV(0) | SEG_DATA_RDWR
#define GDT_CODE_PL3 SEG_TYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
SEG_PRIV(3) | SEG_CODE_EXRD
#define GDT_DATA_PL3 SEG_TYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
SEG_PRIV(3) | SEG_DATA_RDWR
void
create_descriptor(uint32_t base, uint32_t limit, uint16_t flag)
{
uint64_t descriptor;
// Create the high 32 bit segment
descriptor = limit & 0x000F0000;
descriptor |= (flag << 8) & 0x00F0FF00;
descriptor |= (base >> 16) & 0xFF0000FF;
// Shift by 32 to allow for low part of segment
descriptor <<= 32;
// Create the low 32 bit segment
descriptor |= base << 16;
descriptor |= limit & 0x0000FFFF;
printf("0x%.16llX\n", descriptor);
}
int
main(void)
{
create_descriptor(0, 0, 0);
create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL0));
create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL0));
create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL3));
create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL3));
return 0;
}
</source>
== See Also ==
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