x86: add GDT header

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 */