SPECIAL DEVICE REGISTER

	The Special Device configuration register is at $E80040, occupying
	the lower byte of that word.  Ordinarily, this is seen only by the
	boot ROM, never by the Amiga ROM.  This is the first autoconfig
	device seen after a full reset, and in some modes the first one
	seen after a CPU initiated reset.

	The register contents are as follows:

	        7  6  5  4  3  2  1  0
	                 |  |  |  |  |
	                 |  |  |  |  +--- Phantom ROM Low
	                 |  |  |  +------ Phantom ROM High
	                 |  |  +--------- ROM Configured
	                 |  +------------ "Johann Mode"
	                 +--------------- 68000 Mode

	The register comes up with all bits cleared.  The first write of any
	kind to the register will enable the CPU asserted reset onto the 
	expansion bus; prior to that, no CPU initiated resets will affect the
	A2000 reset line.  The actual register functions are as follows:

	Phantom ROM Low
	Phantom ROM High

		These bits control where the on-board ROM will show up.  If
	Phantom Low is 0, the ROM appears at $00000000, where it's used for
	the initial boot up.  If Phantom High is 0, the ROM shows up at the
	$00F80000 slot.  Setting either of these to 1 removes the ROM from 
	it's respective slot.

	ROM Configured

		The Special Device register can be written several times
	before going on, if desired.  Writing a 0 to this bit keeps the
	register in place, writing a 1 here removes this register, allowing
	the next autoconfig device to show up here instead (if any 32 bit
	memory is present, that will be the next autoconfig device).

	"Johann Mode"

		This mode controls what happens to this special device
	during a system reset.  When low, the special device will only
	come up after a full global reset.  If set high, the special
	device will come up after a CPU initiated reset as well.

	68000 Mode

		This bit allows the 68000 to resume execution, while
	shutting the 68020/68030 down completely.  It comes up cleared, and
	if set, the 68020/68030 goes into a permanent RESET state, and the
	board rearbitrates to allow the 68000 back on, until the next 
	register reset (which of course depends on how "Johann Mode" is set).