From cd091e37bfaf56b3dbe168772c15bc3839e347f8 Mon Sep 17 00:00:00 2001
From: Tom-on64 <kubecekt@gmail.com>
Date: Mon, 9 Dec 2024 12:51:40 +0100
Subject: [PATCH] Enabled the A20 line :)

---
 boot/stage2.s | 48 +++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 47 insertions(+), 1 deletion(-)

diff --git a/boot/stage2.s b/boot/stage2.s
index b2c0e2d..fbcb3e9 100644
--- a/boot/stage2.s
+++ b/boot/stage2.s
@@ -163,7 +163,7 @@ vbeSetup:
     cmp ax, 0x4f
     jne error
 
-    jmp loadGDT
+    jmp checkA20
 
 .nextMode:
     mov ax, [_segment]
@@ -185,6 +185,51 @@ error:
     cli
     hlt
 
+;; Enables the A20 line
+enableA20:
+    in al, 0x92
+    or al, 2
+    out 0x92, al
+
+;; Check if the A20 line is enabled
+checkA20:
+    push ds
+    push es
+    push di
+    push si
+
+    cli
+    xor ax, ax  ; ax = 0
+    mov es, ax  ; es = 0
+    not ax      ; ax = 0xffff
+    mov ds, ax  ; ds = 0xffff
+    
+    mov di, 0x0500
+    mov si, 0x0510
+
+    mov al, byte [es:di]
+    push ax
+    mov al, byte [ds:si]
+    push ax
+
+    mov byte [es:di], 0x00
+    mov byte [ds:si], 0xff
+
+    cmp byte [es:di], 0xff
+
+    pop ax
+    mov byte [ds:si], al
+    pop ax
+    mov byte [es:di], al
+
+    mov ax, 0
+    jne enableA20
+
+    pop si
+    pop di
+    pop es
+    pop ds
+
 ;; Load the global descriptor table (GDT)
 loadGDT:
     cli
@@ -197,6 +242,7 @@ loadGDT:
     ; Far jump to 32 bit mode!!
     jmp CODE_SEG:protected_start
 
+
 ;;
 ;; Prints a hex byte
 ;; al - value to print