5.3  The SamRam


    The SamRam contains a 32K static CMOS Ram chip, and some I/O logic for
    port 31.  If this port is read, it returns the position of the
    joystick, as a normal Kempston joystickinterface would.  If written to,
    the port controls a programmable latch chip (the 74LS259) which
    contains 8 latches:


       Bit    7   6   5   4   3   2   1   0
            +---+---+---+---+---+---+---+---+
       WRITE|   |   |   |   |  address  |bit|
            +---+---+---+---+---+---+---+---+


    The address selects on of the eight latches; bit 0 is the new state of
    the latch.  The 16 different possibilities are collected in the diagram
    below:

        OUT 31,   |  Latch  | Result
        ----------+---------+----------------------------------------
            0     |    0    | Switch on write protect of CMOS RAM
            1     |    "    | Writes to CMOS RAM allowed
            2     |    1    | turn on CMOS RAM (see also 6/7)
            3     |    "    | turn off CMOS RAM (standard Spec.  ROM)
            4     |    2    | -
            5     |    "    | Ignore all OUT's to 31 hereafter
            6     |    3    | Select CMOS bank 0 (Basic ROM)
            7     |    "    | Select CMOS bank 1 (Monitor,...)
            8     |    4    | Select interface 1
            9     |    "    | Turn off IF 1 (IF1 rom won't be paged)
           10     |    5    | Select 32K ram bank 0 (32768-65535)
           11     |    "    | Select 32K ram bank 1 (32768-65535)
           12     |    6    | Turn off beeper
           13     |    "    | Turn on beeper
           14     |    7    | -
           15     |    "    | -

    At reset, all latches are 0.  If an OUT 31,5 is issued, only a reset
    will give you control over the latches again.  The write protect latch
    is not emulated; you're never able to write the emulated CMOS ram in
    the emulator.  Latch 4 will pull up the M1 output of the Z80.  The
    Interface I won't page the ROM anymore then.