x86: add GDT code to the kernel

2 files changed, 65 insertions, 0 deletions

diff --git a/arch/x86/gdt.c b/arch/x86/gdt.c
new file mode 100644
index 0000000..d7e449a
--- /dev/null
+++ b/arch/x86/gdt.c
@@ -0,0 +1,63 @@
+#include <gdt.h>
+
+struct gdt_asm_t {
+	uint16_t size_minus_one;
+	struct gdt_entry *gdt_table;
+} __attribute__((packed));
+
+static struct gdt_entry gdt[8];
+
+static void gdt_set_entry(struct gdt_entry *g, uint32_t base, uint32_t limit, uint8_t access, uint8_t flags)
+{
+	// TODO: ASSERT(limit <= GDT_LIMIT_MAX);
+	// TODO: ASSERT(flags <= 0xf); // flags is 4 bits
+
+	g->base_low = (base && 0xffff);
+	g->base_middle = (base >> 16) & 0xff;
+	g->base_high = (base >> 24) & 0xff;
+
+	g->limit_low = (limit & 0xffff);
+	g->limit_high = (limit >> 16) & 0x0f;
+
+	g->access = access;
+	g->flags = flags & 0x0f;
+}
+
+static void load_gdt(struct gdt_entry *g, uint32_t entries_count)
+{
+	// TODO: ASSERT(interrupts are disabled);
+
+	struct gdt_asm_t table = {
+		(uint16_t)(entries_count * sizeof(struct gdt_entry) - 1),
+		g,
+	};
+
+	asm volatile (
+		"lgdt (%0)"
+		: // no output
+		: "p" (&table)
+		: "memory"
+	);
+}
+
+void init_segmentation(void)
+{
+	// NULL descriptor
+	gdt_set_entry(&gdt[0], 0, 0, 0, 0);
+
+	// Kernel code segment
+	gdt_set_entry(&gdt[1],
+		0,
+		GDT_LIMIT_MAX,
+		GDT_ACCESS_PRESENT | GDT_ACCESS_S | GDT_ACCESS_PL0 | GDT_ACCESS_RW | GDT_ACCESS_EX,
+		GDT_GRAN_4KB | GDT_32BIT);
+
+	// Kernel data segment
+	gdt_set_entry(&gdt[2],
+		0,
+		GDT_LIMIT_MAX,
+		GDT_ACCESS_PRESENT | GDT_ACCESS_S | GDT_ACCESS_PL0 | GDT_ACCESS_RW,
+		GDT_GRAN_4KB | GDT_32BIT);
+
+	load_gdt(gdt, 3);
+}
diff --git a/arch/x86/include/gdt.h b/arch/x86/include/gdt.h
index b8dd2f0..c352f32 100644
--- a/arch/x86/include/gdt.h
+++ b/arch/x86/include/gdt.h
@@ -86,4 +86,6 @@ struct gdt_entry {
 	uint8_t base_high;
 } __attribute__((packed));
 
+void init_segmentation(void);
+
 #endif /* __X86_GDT_H */