As a result, some DOS programs would no longer work. To maintain compatibility with the PC and XT behavior, the AT included an "A20 line" gate (Gate A20) that made memory addresses on the AT wrap around to low memory as they would have on an 8088 processor. This gate could be controlled, initially through the keyboard controller, to allow running programs which were designed for this to access an additional 65,520 bytes (64 KiB of memory in real mode. At boot time, the BIOS first enables A20 when counting and testing all of the system's memory, and disables it before transferring control to the operating system. Enabling the A20 line is one of the first steps a protected mode x86 operating system does in the bootup process, often before control has been passed onto the kernel from the bootstrap (in the case of Linux, for example).