1 files changed, 66 insertions, 0 deletions
diff --git a/src/gdt.h b/src/gdt.h
new file mode 100644
index 0000000..99a6da2
--- /dev/null
+++ b/src/gdt.h
@@ -0,0 +1,66 @@
+#pragma once
+
+#include <types.h>
+
+/* From OSDev wiki:
+ *
+ * Segment
+ * a logically contiguous chunk of memory with consistent properties (CPU's speaking)
+ *
+ * Segment Register
+ * a register of your CPU that refers to a segment for a special use (e.g. SS, CS, DS ...)
+ *
+ * Selector
+ * a reference to a descriptor you can load into a segment register; the selector is an offset of a descriptor table
+ * entry. These entries are 8 bytes long. Therefore, bits 3 and up only declare the descriptor table entry offset, while
+ * bit 2 specifies if this selector is a GDT or LDT selector (LDT - bit set, GDT - bit cleared), and bits 0 - 1 declare
+ * the ring level that needs to correspond to the descriptor table entry's DPL field. If it doesn't, a General
+ * Protection Fault occurs; if it does correspond then the CPL level of the selector used is changed accordingly.
+ *
+ * Descriptor
+ * a memory structure (part of a table) that tells the CPU the attributes of a given segment
+ */
+
+class GDT {
+public:
+  GDT();
+  ~GDT() = default;
+
+  struct Pointer {
+    uint16_t limit;
+    uint32_t base;
+  } __attribute((packed));
+
+  class SegmentDescriptor {
+  public:
+    SegmentDescriptor(uint32_t base = 0, uint32_t limit = 0, uint8_t type = 0);
+    [[nodiscard]] uint32_t base() const;
+    [[nodiscard]] uint32_t limit() const;
+
+  private:
+    /*
+     * |31|  |  |  |  |  |  |24|23|22|  |20|19|  |  |16|15|  |  |12|11|  |  | 8| 7|  |  |  |  |  |  | 0|
+     * | base (24~31)          | G|DB|  | A| lim(16~19)| P| DPL | S|    Type   |   base (16~23)        |
+     * | base (0~15)                                   | limit (0~15)                                  |
+     * |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
+     */
+
+    uint16_t limit_low;
+    uint16_t base_low;
+    uint8_t base_high;
+    uint8_t type;
+    uint8_t flags_limit_high;
+    uint8_t base_vhigh;
+  } __attribute__((packed));
+
+private:
+  SegmentDescriptor segments[256];
+};
+
+static_assert(sizeof(GDT::Pointer) == 6);
+static_assert(sizeof(GDT::SegmentDescriptor) == 8);