1 files changed, 43 insertions, 0 deletions
diff --git a/boot/gdt.zig b/boot/gdt.zig
new file mode 100644
index 0000000..c428e2c
--- /dev/null
+++ b/boot/gdt.zig
@@ -0,0 +1,43 @@
+// we have to make our own gdt even though limine gives us cause the
+// spec says that the one they provide is only a temp one.
+// in long mode, segment base/limits are ignrored so all we really encode
+// is the access byte and the long mode bit.
+// see: https://wiki.osdev.org/Global_Descriptor_Table
+
+pub const KERNEL_CODE: u16 = 0x08;
+pub const KERNEL_DATA: u16 = 0x10;
+
+var gdt = [_]u64{
+    0x0000000000000000, // null
+    0x00209A0000000000, // 0x08 kernel code: present, ring 0, executable, L=1
+    0x0000920000000000, // 0x10 kernel data: present, ring 0, writable
+};
+
+const Gdtr = packed struct {
+    limit: u16,
+    base: u64,
+};
+
+var gdtr: Gdtr = undefined;
+
+// load the GDT, then reload CS (via a far return) and the data segments so the
+// CPU actually uses our descriptors instead of Limine's shit one.
+pub fn load() void {
+    gdtr = .{ .limit = @sizeOf(@TypeOf(gdt)) - 1, .base = @intFromPtr(&gdt) };
+    asm volatile (
+        \\ lgdt (%[gdtr])
+        \\ pushq $0x08
+        \\ leaq 1f(%rip), %rax
+        \\ pushq %rax
+        \\ lretq
+        \\ 1:
+        \\ movw $0x10, %ax
+        \\ movw %ax, %ds
+        \\ movw %ax, %es
+        \\ movw %ax, %fs
+        \\ movw %ax, %gs
+        \\ movw %ax, %ss
+        :
+        : [gdtr] "r" (&gdtr),
+        : .{ .rax = true, .memory = true });
+}