Initial commit

1 files changed, 36 insertions, 0 deletions

diff --git a/kernel/gdt.h b/kernel/gdt.h
new file mode 100644
index 0000000..03f575e
--- /dev/null
+++ b/kernel/gdt.h
@@ -0,0 +1,36 @@
+#ifndef _GDT_H_
+#define _GDT_H_
+
+// Structure of a GDT descriptor. There are 2 types of descriptors: segments and TSS.
+// Section 3.4.5 of Intel 64 & IA32 architectures software developer's manual describes
+// segment descriptors while section 6.2.2 describes TSS descriptors.
+typedef struct gdt_entry_st {
+    uint16_t lim15_0;
+    uint16_t base15_0;
+    uint8_t base23_16;
+
+    uint8_t type : 4;
+    uint8_t s : 1;
+    uint8_t dpl : 2;
+    uint8_t present : 1;
+
+    uint8_t lim19_16 : 4;
+    uint8_t avl : 1;
+    uint8_t l : 1;
+    uint8_t db : 1;
+    uint8_t granularity : 1;
+
+    uint8_t base31_24;
+} __attribute__((packed)) gdt_entry_t;
+
+// Structure describing a pointer to the GDT descriptor table.
+// This format is required by the lgdt instruction.
+typedef struct gdt_ptr_st {
+    uint16_t limit;    // Limit of the table (ie. its size)
+    uint32_t base;     // Address of the first entry
+} __attribute__((packed)) gdt_ptr_t;
+
+extern void gdt_init();
+extern void gdt_flush(gdt_ptr_t *gdt_ptr);
+
+#endif