1 files changed, 59 insertions, 0 deletions
diff --git a/src/loader/a20.c b/src/loader/a20.c
new file mode 100644
index 0000000..b59500a
--- /dev/null
+++ b/src/loader/a20.c
@@ -0,0 +1,59 @@
+// MIT License, Copyright (c) 2021 Marvin Borner
+// https://wiki.osdev.org/A20_Line
+
+#include <a20.h>
+#include <cpu.h>
+#include <def.h>
+#include <pnc.h>
+#include <rem.h>
+
+static u8 a20_check(void)
+{
+	if (mminw(0x7dfe) != mminw(0x7dfe + 0x100000))
+		return 1;
+
+	mmoutw(0x7dfe, ~mminw(0x7dfe));
+
+	if (mminw(0x7dfe) != mminw(0x7dfe + 0x100000))
+		return 1;
+
+	return 0;
+}
+
+void a20_enable(void)
+{
+	if (a20_check())
+		return;
+
+	// BIOS method
+	struct rem_regs r = { 0 };
+	r.eax = 0x2401;
+	rem_int(0x15, &r, &r);
+
+	if (a20_check())
+		return;
+
+	// Keyboard controller method
+	while (inb(0x64) & 2)
+		;
+	outb(0x64, 0xad);
+	while (inb(0x64) & 2)
+		;
+	outb(0x64, 0xd0);
+	while (!(inb(0x64) & 1))
+		;
+	u8 b = inb(0x60);
+	while (inb(0x64) & 2)
+		;
+	outb(0x64, 0xd1);
+	while (inb(0x64) & 2)
+		;
+	outb(0x60, b | 2);
+	while (inb(0x64) & 2)
+		;
+	outb(0x64, 0xae);
+	while (inb(0x64) & 2)
+		;
+
+	assert(a20_check());
+}