0040-005F ---- PIT  (Programmable Interrupt Timer  8253, 8254)

                 XT & AT uses 40-43    PS/2 uses 40, 42,43,44, 47

0040   r/w PIT  counter 0, counter divisor       (XT, AT, PS/2)
0041   r/w PIT  counter 1, RAM refresh counter   (XT, AT)
0042   r/w PIT  counter 2, cassette & speaker    (XT, AT, PS/2)

0043   r/w PIT  mode port, control word register for counters 0-2
        bit 7-6 = 00  counter 0 select
            = 01  counter 1 select   (not PS/2)
            = 10  counter 2 select
        bit 5-4 = 00  counter latch command
            = 01  read/write counter bits 0-7 only
            = 10  read/write counter bits 8-15 only
            = 11  read/write counter bits 0-7 first, then 8-15
        bit 3-0 = 000 mode 0 select
            = 001 mode 1 select - programmable one shot
            = x10 mode 2 select - rate generator
            = x11 mode 3 select - square wave generator
            = 100 mode 4 select - software triggered strobe
            = 101 mode 5 select - hardware triggered strobe
        bit 0   = 0   binary counter 16 bits
            = 1   BCD counter

0044   r/w PIT  counter 3 (PS/2, EISA)
       used as fail-safe timer. generates an NMI on time out.
       for user generated NMI see at 0462.

0047   w   PIT  control word register counter 3 (PS/2, EISA)
        bit 7-6 = 00  counter 3 select
            = 01  reserved
            = 10  reserved
            = 11  reserved
        bit 5-4 = 00  counter latch command counter 3
            = 01  read/write counter bits 0-7 only
            = 1x  reserved
        bit 3-0 = 00

0048       EISA
004A       EISA
004B       EISA