1 files changed, 55 insertions, 0 deletions
diff --git a/i686/gdt.h b/i686/gdt.h
new file mode 100644
index 0000000..9148c3d
--- /dev/null
+++ b/i686/gdt.h
@@ -0,0 +1,55 @@
+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+
+enum Ring { Ring0 = 0x0, Ring1 = 0x1, Ring2 = 0x2, Ring3 = 0x3 };
+
+struct __attribute__((packed)) Access {
+  bool accessed : 1;      // if 0, is set by processor when accessed
+  bool readwrite : 1;     // code seg: read toggle; data seg: write toggle
+  bool direction : 1;     // code seg: conforming bit; data seg: direction bit
+  bool executable : 1;    // executable bit
+  bool segment : 1;       // true for code/data; false for gates/tss
+  enum Ring privilege : 2; // descriptor privilege level
+  bool present : 1;       // true for every active segment
+};
+_Static_assert(sizeof(struct Access) == 1);
+
+static const struct Access null_access = {};
+static const struct Access ktext_access = {.readwrite = true, .executable = true, .segment = true, .present = true};
+static const struct Access kdata_access = {.readwrite = true, .segment = true, .present = true};
+
+// Segment Descriptor
+// A memory structure (part of a table) that tells the CPU the attributes of a given segment
+// |31|  |  |  |  |  |  |24|23|22|21|20|19|  |  |16|15|  |  |  |  |  |  | 8| 7|  |  |  |  |  |  | 0|
+// |   base_31_24          | G|DB|  | A| lim_19_16 |   access              |   base_23_16          |
+// |   base_15_0                                   |   limit_15_0                                  |
+// |31|  |  |  |  |  |  |  |  |  |  |  |  |  |  |16|15|  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0|
+// limit    size of segment - 1, either in bytes or in 4KiB chunks (check flags)
+// base     address of segment
+// access
+// flags    defines the segment chunks and 16/32 bit
+struct __attribute__((packed)) SegmentDescriptor_t {
+  uint16_t limit_15_0;      // low bits of segment limit
+  uint16_t base_15_0;       // low bits of segment base address
+  uint8_t base_23_16;       // middle bits of segment base address
+  uint8_t access;           // access byte
+  uint8_t limit_19_16 : 4;  // high bits of segment limit
+  // flags
+  bool a : 1;               // unused, available for software use
+  bool rsv : 1;             // reserved
+  bool db : 1;              // false => 16-bit seg; true => 32-bit seg
+  bool granularity : 1;     // limit scaled by 4k when set
+  uint8_t base_31_24;       // high bits of segment address
+};
+_Static_assert(sizeof(struct SegmentDescriptor_t) == 8);
+
+void SegmentDescriptor(struct SegmentDescriptor_t *self, unsigned base, unsigned limit, uint8_t access);
+
+void gdt_install();
+
+enum SegmentIndex {
+  ktextDescriptor = 2 * sizeof(struct SegmentDescriptor_t),
+  kdataDescriptor = 3 * sizeof(struct SegmentDescriptor_t),
+};