From 4dc73ef6bbefb1f5d108ab19a72349563c586e28 Mon Sep 17 00:00:00 2001
From: Jake Mannens <jake72360@gmail.com>
Date: Fri, 5 Oct 2018 03:28:43 +1000
Subject: Added code to enable gate A20 during startup.

---
 kernel/boot.s | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

(limited to 'kernel/boot.s')

diff --git a/kernel/boot.s b/kernel/boot.s
index d1f5606..8d07354 100644
--- a/kernel/boot.s
+++ b/kernel/boot.s
@@ -7,6 +7,7 @@ extern con_init
 extern fs_init
 extern kmain
 extern paging_init
+extern panic
 extern printk
 extern syscall_init
 extern timer_init
@@ -33,6 +34,7 @@ kboot:
   mov esp, 0x80000
   ; setup descriptor tables and enable paging
   call flush_gdt
+  call enable_a20
   call paging_init
   call flush_idt
   ; initialize the console
@@ -218,6 +220,60 @@ register_trap:
   pop ebp
   ret
 
+enable_a20:
+  push ebp
+  mov ebp, esp
+  ; test if the A20 gate was already enabled
+  call .testgate
+  jne .end
+  ; send read controller output port command
+  call .waitout
+  mov al, 0xD0
+  out 0x64, al
+  ; read controller output port
+  call .waitin
+  in al, 0x60
+  xchg ax, cx
+  ; send write controller output port command
+  call .waitout
+  mov al, 0xD1
+  out 0x64, al
+  ; write controller output port (enabling gate A20)
+  call .waitout
+  xchg ax, cx
+  or al, 0x02
+  out 0x60, al
+  ; make sure the gate is working (panic if not)
+  call .testgate
+  jne .end
+  push .msg
+  call printk
+  add sp, 4
+  call panic
+.waitin:
+  in al, 0x64
+  test al, 0x01
+  jz .waitin
+  ret
+.waitout:
+  in al, 0x64
+  test al, 0x02
+  jnz .waitout
+  ret
+.testgate:
+  ; test if the A20 gate is enabled by depositing
+  ; two words 1MB away from each other in memory.
+  mov word [.tmp+0x100000], 0xBEEF
+  mov cx, [.tmp]
+  mov dx, [.tmp+0x100000]
+  cmp cx, dx
+  ret
+.end:
+  pop ebp
+  ret
+.tmp: dw 0xDEAD
+.msg: db "Failed to enable A20 gate!", 10, 0
+
 pic_init:
   push ebp
   mov ebp, esp
-- 
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