file   : xtraprom.text
    date   : 15-March-1983
    author : Keith Ball

    Using the user proms

    A second set of proms, besides the Concept boot proms, can be
    installed on the Concept processor board.  These user proms, two
    2732 eproms, reside in the $20000 to $21FFF address space.
    Normally, they contain MacsBug, however, they can be used for any
    other user definable purpose.

    The boot proms will initialize and, under user control, execute
    the code in these proms.  For the boot proms to do so, the user
    proms must maintain the required interface.  The interface
    requires that the first 4 bytes of the address space ($20000 to
    $20003) have all zeros (0) in them.  The next 4 bytes ($20004 to
    $20007) must have the entry point address of the initialization
    code.  The initialization code must begin at address $2000C.  The
    next 4 bytes ($20008 to $2000B) must be the entry point address of
    the main code. This entry point can be at any even address within
    the user prom address space ($20000 to $21FFF, inclusive).

    The boot prom calls the user prom's intialization code during
    system cold starts.  The call occurs after the boot proms do the
    self-test and clear the static and dynamic RAM but, before it
    initializes the interrupt vectors and the boot prom's keyboard and
    display drivers, determines what peripherals are in the I/O slots
    and prints the boot message.  The main entry point is called only
    when the user has the boot switches set for the boot device prompt
    and then selects the "D" option.


    Note :

       Both proms are required.  One prom contains all the odd address
       bytes.  This is called the LOW prom.  The other prom contains
       all the even address bytes.  This is called the HIGH prom.