Bootloader: Try to enable A20 line if it is disabled

VirtualBox seems to have A20 disabled by default

bootloader/bios/CMakeLists.txt

@@ -3,6 +3,7 @@ cmake_minimum_required(VERSION 3.26)
 project(bootloader ASM)
 
 set(BOOTLOADER_SOURCES
+	a20_line.S
 	boot.S
 	command_line.S
 	disk.S

bootloader/bios/a20_line.S

@@ -0,0 +1,166 @@
+.code16
+.section .stage2
+
+# checks whether A20 line is enabled or disabled
+# return
+#	ax: 1 if enabled, 0 otherwise
+check_a20:
+	pushf
+	pushw %si
+	pushw %di
+	pushw %ds
+	pushw %es
+
+	cli
+
+	xorw %ax, %ax
+	movw %ax, %es
+	notw %ax
+	movw %ax, %ds
+
+	movw $0x0500, %di
+	movw $0x0510, %si
+
+	movb %es:(%di), %al
+	pushw %ax
+
+	movb %ds:(%si), %al
+	pushw %ax
+
+	movb $0x00, %es:(%di)
+	movb $0xFF, %ds:(%si)
+
+	cmpb $0xFF, %es:(%di)
+	
+	pop %ax
+	movb %al, %ds:(%si)
+
+	pop %ax
+	movb %al, %es:(%di)
+
+	movw $0, %ax
+	je .check_a20_done
+	
+	movw $1, %ax
+
+ .check_a20_done:
+	popw %es
+	popw %ds
+	popw %di
+	popw %si
+	popf
+	ret
+
+
+# Try to enable A20 using PS2 controller
+enable_a20_ps2:
+	pushf
+	pushw %ax
+
+	cli
+
+	# disable first port
+	call .enable_a20_ps2_wait1
+	movb $0xAD, %al
+	outb %al, $0x64
+
+	# read controller output
+	call .enable_a20_ps2_wait1
+	movb $0xD0, %al
+	outb %al, $0x64
+ 
+	call .enable_a20_ps2_wait2
+	inb $0x60, %al
+	pushw %ax
+
+	# write controller output
+	call .enable_a20_ps2_wait1
+	movb $0xD1, %al
+	outb %al, $0x64
+ 
+	call .enable_a20_ps2_wait1
+	popw %ax
+	orw $2, %ax
+	outb %al, $0x60
+
+	# enable first port
+	call .enable_a20_ps2_wait1
+	movb $0xAE, %al
+	outb %al, $0x64
+
+	call .enable_a20_ps2_wait1
+
+	popw %ax
+	popf
+	ret
+
+ .enable_a20_ps2_wait1:
+	inb $0x64, %al
+	test $2, %al
+	jnz .enable_a20_ps2_wait1
+	ret
+
+ .enable_a20_ps2_wait2:
+	inb $0x64, %al
+	test $1, %al
+	jnz .enable_a20_ps2_wait1
+	ret
+
+
+# Check if A20 line is disabled. If it is, try to enable it
+.global enable_a20
+enable_a20:
+	pushw %ax
+	pushw %si
+
+	call check_a20
+	testw %ax, %ax
+	jnz .enable_a20_done
+
+	movw $a20_line_disabled_msg, %si
+	call puts; call print_newline
+
+	# Try to enable A20 line using bios interrupt
+	movw $0x2401, %ax
+	int $0x15
+	call check_a20
+	testw %ax, %ax
+	jnz .enable_a20_done
+
+	# Try to enable A20 line using ps2 controller
+	call enable_a20_ps2
+	call check_a20
+	testw %ax, %ax
+	jnz .enable_a20_done
+
+	# Try to enable A20 line using fast A20 gate
+	inb $0x92, %al
+	testb $2, %al
+	jnz .enable_a20_fast_done
+	orb $2, %al
+	outb %al, $0x92
+ .enable_a20_fast_done:
+
+	call check_a20
+	testw %ax, %ax
+	jnz .enable_a20_done
+
+	movw $a20_could_not_enable_msg, %si
+	call print_and_halt
+
+ .enable_a20_done:
+	movw $a20_line_enabled_msg, %si
+	call puts; call print_newline
+
+	popw %si
+	popw %ax
+	ret
+
+a20_line_disabled_msg:
+	.asciz "A20 line disabled. Trying to enable it"
+
+a20_line_enabled_msg:
+	.asciz "A20 line enabled"
+
+a20_could_not_enable_msg:
+	.asciz "Could not enable A20 line"

bootloader/bios/boot.S

@@ -57,7 +57,11 @@ stage2_main:
 	movw $unreal_enter_msg, %si
 	call puts; call print_newline
 
+	call enable_a20
+
 	call get_memory_map
+
+	call print_newline
 	call read_user_command_line
 
 	call print_newline
@@ -129,7 +133,7 @@ enter_unreal_mode:
 	movw $0x10, %bx
 	movw %bx, %ds
 
-	andb 0xFE, %al
+	andb $0xFE, %al
 	movl %eax, %cr0
 	ljmpl $0x0, $.enter_unreal_mode_unreal