bootasm.S: remove redundant A20 enable code

code/lab1/Makefile

@@ -206,7 +206,7 @@ TERMINAL        :=gnome-terminal
 debug: $(UCOREIMG)
 	$(V)$(QEMU) -S -s -parallel stdio -hda $< -serial null &
 	$(V)sleep 2
-	$(V)$(TERMINAL) -e "gdb -q -x tools/gdbinit"
+	$(V)$(TERMINAL) -e "gdb -q -tui -x tools/gdbinit"
 	
 debug-nox: $(UCOREIMG)
 	$(V)$(QEMU) -S -s -serial mon:stdio -hda $< -nographic &

code/lab1/boot/bootasm.S

@@ -42,9 +42,6 @@ seta20.2:
     movb $0xdf, %al                                 # 0xdf -> port 0x60
     outb %al, $0x60                                 # 0xdf = 11011111, means set P2's A20 bit(the 1 bit) to 1
 
-    movb $0xdf, %al                                 # 0xdf -> port 0x60, do it again!
-    outb %al, $0x60                                 # 0xdf = 11011111, means set P2's A20 bit(the 1 bit) to 1
-
     # Switch from real to protected mode, using a bootstrap GDT
     # and segment translation that makes virtual addresses
     # identical to physical addresses, so that the