Almost nothing really wants A20 off today - so add a config option
(and default it off) to control turning A20 off during 16bit
bootup.
#define CONFIG_KEYBOARD 1
// Support calling int155f on each keyboard event
#define CONFIG_KBD_CALL_INT15_4F 1
+// Disable A20 on 16bit boot
+#define CONFIG_DISABLE_A20 0
// Support for int15c2 mouse calls
#define CONFIG_PS2_MOUSE 1
// If the target machine has multiple independent root buses, the
movw %ax, %fs
movw %ax, %gs
+#if CONFIG_DISABLE_A20
// disable a20
inb $PORT_A20, %al
andb $~A20_ENABLE_BIT, %al
outb %al, $PORT_A20
+#endif
// Jump to 16bit mode
ljmpw $SEG32_MODE16_CS, $1f