From e46551eebeeed2e79f1784f1c2b5f48c65d95b1d Mon Sep 17 00:00:00 2001
From: Joursoir <chat@joursoir.net>
Date: Fri, 1 Oct 2021 12:19:08 +0000
Subject: x86/boot/head: check a20 line

---
 arch/x86/boot/head.s | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/boot/head.s b/arch/x86/boot/head.s
index a1bc6d1..d24e36c 100644
--- a/arch/x86/boot/head.s
+++ b/arch/x86/boot/head.s
@@ -23,5 +23,13 @@ _start:
 	mov $0x90000, %ebp			# Update stack position so it is right
 	mov %ebp, %esp				# at the top of the free space.
 
+check_a20:
+	mov $0x112345, %edi			# Odd megabyte address
+	mov $0x012345, %esi			# Even megabyte address
+	mov %esi, (%esi)			# If A20 line is disabled two pointers
+	mov %edi, (%edi)			# would point to the address 0x012345
+	cmpsd						# Compare values at addresses
+	je .						# If equivalent, loop forever
+
 	call kernel_main
 	jmp . 						# infinite loop
-- 
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