Add A20 line check

This commit adds a check for whether the A20 line is usable; checking
whether more than one megabyte of RAM can be used. If it isn't usable,
it currently isn't activated, but rather, the boot process is stopped.

i386/loader/src-asm/memory/a20.asm

@@ -0,0 +1,95 @@
+
+check_a20_wraparound:
+.prolog:
+    push dword esi
+    sub esp, 64
+    mov esi, esp
+
+    ; EAX is used for the return value,
+    ; so it doesn't have to be preserved.
+    mov [esi + (64 - 8)], ebx
+    mov [esi + (64 - 12)], edi
+
+.test_a20:
+    ; Save the values which are going to be overwritten
+    mov edi, 0x0800
+    mov eax, [edi]
+    mov edi, 0x00100800
+    mov ebx, [edi]
+
+    ; Write to two locations which, if A20 wraps around,
+    ; will both be written to by the second statement.
+    ; Write 0 to 0x08000 to avoid the unlikely case that
+    ; those bytes are there coincidentally and A20 is on.
+    mov edi, 0x0800
+    mov [edi], dword 0x00000000
+    mov edi, 0x00100800
+    mov [edi], dword 0xffaa00dd
+
+    ; Check the lower address (which should NOT contain)
+    ; this value for the value. If it does contain it, the
+    ; memory space wraps around to 0 every 2^19 bytes.
+    cmp [0x0800], dword 0xffaa00dd
+    je .return_a20_not_enabled      ; If the value at 0x0800
+                                    ; was written to instead
+                                    ; (or at the same time) of 
+                                    ; writing to 0x100800,
+                                    ; the A20 wasn't enabled.
+    ; Reset the values at the addresses to which the test
+    ; values were written znd return that A20 is enabled.
+    mov edi, 0x0800
+    mov [edi], eax
+    mov edi, 0x00100800
+    mov [edi], ebx
+
+.return_a20_enabled:
+
+    mov eax, 1
+
+    ; Restore the one used register which
+    ; doesn't contain the return value.
+    mov ebx, [esi + (64 - 8)]
+
+    add esp, 64
+    pop dword esi
+    ret
+
+.return_a20_not_enabled:
+    ; It's only necessary to write to one of the addresses
+    ; as both of them are equal either way.
+    mov edi, 0x0800
+    mov [edi], eax
+
+    mov eax, 0
+
+    ; Restore the one used register which
+    ; doesn't contain the return value.
+    mov ebx, [esi + (64 - 12)]
+    mov edi, [esi + (64 - 8)]
+
+    add esp, 64
+    pop dword esi
+    ret
+
+
+
+enable_a20_through_keyboard_controller:
+.prolog:
+    push dword esi
+    sub esp, 64
+    mov esi, esp
+
+    mov [esi + (64 - 8)], ebx
+    mov [esi + (64 - 12)], ecx
+    mov [esi + (64 - 16)], edx
+    mov [esi + (64 - 20)], edi
+
+.epilog:
+    mov ebx, [esi + (64 - 8)]
+    mov ecx, [esi + (64 - 12)]
+    mov edx, [esi + (64 - 16)]
+    mov edi, [esi + (64 - 20)]
+    
+    add esp, 64
+    pop dword esi
+    ret