1 files changed, 64 insertions, 0 deletions
diff --git a/kernel/include/gdt.h b/kernel/include/gdt.h
new file mode 100644
index 0000000..cd66676
--- /dev/null
+++ b/kernel/include/gdt.h
@@ -0,0 +1,64 @@
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+
+/*	The GDT is one of the x86's descriptor tables. It is used for memory segmentation.
+	Here, we don't use segmentation to separate processes from one another (this is done with paging).
+	We only use segmentation to make the difference between kernel mode (ring 3) and user mode (ring 0) */
+
+/* 	One entry of the table */
+struct gdt_entry {
+	uint16_t limit_low;
+	uint16_t base_low;
+	uint8_t base_middle;
+	uint8_t access;
+	uint8_t granularity;
+	uint8_t base_high;
+} __attribute__((packed));
+typedef struct gdt_entry gdt_entry_t;
+
+/*	Structure defining the whole table : address and size (in bytes). */
+struct gdt_ptr {
+	uint16_t limit;
+	uint32_t base;
+} __attribute__((packed));
+typedef struct gdt_ptr gdt_ptr_t;
+
+/*	The TSS is used for hardware multitasking.
+	We don't use that, but we still need a TSS so that user mode process exceptions
+	can be handled correctly by the kernel.	*/
+struct tss_entry {
+	uint32_t prev_tss;   // The previous TSS - if we used hardware task switching this would form a linked list.
+	uint32_t esp0;       // The stack pointer to load when we change to kernel mode.
+	uint32_t ss0;        // The stack segment to load when we change to kernel mode.
+	uint32_t esp1;       // Unused...
+	uint32_t ss1;
+	uint32_t esp2;
+	uint32_t ss2;
+	uint32_t cr3;
+	uint32_t eip;
+	uint32_t eflags;
+	uint32_t eax;
+	uint32_t ecx;
+	uint32_t edx;
+	uint32_t ebx;
+	uint32_t esp;
+	uint32_t ebp;
+	uint32_t esi;
+	uint32_t edi;
+	uint32_t es;         // The value to load into ES when we change to kernel mode.
+	uint32_t cs;         // The value to load into CS when we change to kernel mode.
+	uint32_t ss;         // The value to load into SS when we change to kernel mode.
+	uint32_t ds;         // The value to load into DS when we change to kernel mode.
+	uint32_t fs;         // The value to load into FS when we change to kernel mode.
+	uint32_t gs;         // The value to load into GS when we change to kernel mode.
+	uint32_t ldt;        // Unused...
+	uint16_t trap;
+	uint16_t iomap_base;
+} __attribute__((packed));
+typedef struct tss_entry tss_entry_t;
+
+void gdt_init();
+
+/* vim: set ts=4 sw=4 tw=0 noet :*/