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| -rw-r--r-- | arch/x86/include/gdt.h | 89 |
1 files changed, 89 insertions, 0 deletions
diff --git a/arch/x86/include/gdt.h b/arch/x86/include/gdt.h new file mode 100644 index 0000000..b8dd2f0 --- /dev/null +++ b/arch/x86/include/gdt.h @@ -0,0 +1,89 @@ +#ifndef __X86_GDT_H +#define __X86_GDT_H + +#include <stdint.h> + +// Limit is 20-bit value +#define GDT_LIMIT_MAX (0x000FFFFF) + +// Flags (4 bits) +#define GDT_GRAN_4KB (1 << 3) +#define GDT_GRAN_BYTE (0 << 3) /* + * Granularity. Indicates the size the + * limit value is scaled by. + */ +#define GDT_64BIT (1 << 1) +#define GDT_32BIT (1 << 2) +#define GDT_16BIT (0 << 2) + +// Access flags (8 bits) +#define GDT_ACCESS_ACC (1 << 0) /* + * Accessed bit. The CPU sets 1 when + * the segment is accessed. + */ +#define GDT_ACCESS_RW (1 << 1) /* + * Readable/Writable bit. + * + * For code segments: readable bit. + * If set 1 read access is allowed. + * Write access is never allowed. + * + * For data segments: writable bit. + * If set 1 write access is allowed. + * Read access is always allowed. + */ +#define GDT_ACCESS_DC (1 << 2) /* + * Direction/Conforming bit. + * + */ +#define GDT_ACCESS_EX (1 << 3) /* + * Executable bit. + * 0 defines a data segment + * 1 defines a code segment + */ +#define GDT_ACCESS_S (1 << 4) /* + * Descriptor type bit. + * 0 defines a system segment (TSS, LDT) + * 1 defines a code/data segment + */ +#define GDT_ACCESS_PL0 (0 << 5) /* the highest privilege */ +#define GDT_ACCESS_PL1 (1 << 5) +#define GDT_ACCESS_PL2 (2 << 5) +#define GDT_ACCESS_PL3 (3 << 5) /* the lowest privilege */ +#define GDT_ACCESS_PRESENT (1 << 7) /* Must be set for any valid segments */ + +struct gdt_entry { + uint16_t limit_low; + uint16_t base_low; + uint8_t base_middle; + + union { + struct { + uint8_t type : 4; // EX + DC + RW + ACC + uint8_t s : 1; // 0 = system desc, 1 = regular desc + uint8_t dpl : 2; // desc privilege level + uint8_t p : 1; // present + }; + + uint8_t access; + }; + + union { + struct { + uint8_t limit_hi : 4; + uint8_t avl : 1; // available bit + uint8_t l : 1; // 64-bit segment + uint8_t d : 1; // default operation size. 0 = 16 bit, 1 = 32 bit + uint8_t g : 1; // granularity: 0 = byte, 1 = 4 KB + }; + + struct { + uint8_t limit_high : 4; + uint8_t flags : 4; + }; + }; + + uint8_t base_high; +} __attribute__((packed)); + +#endif /* __X86_GDT_H */ |