Set cpu speed

        Entry:
          AH = F0h
          AL = speed
               00h equivalent to 6 MHz 80286 (COMMON)
               01h equivalent to 8 MHz 80286 (FAST)
               02h full 16 MHz (HIGH)
               03h toggles between 8 MHz-equivalent and speed set by system board
                  switch (AUTO or HIGH)
               08h full 16 MHz except 8 MHz-equivalent during floppy disk access
               09h specify speed directly
                  CX = speed value, 1 (slowest) to 50 (full), 3 ~= 8088