INT 15 - SYSTEM - later PSIBM PS/2, any model/2s - EXPANSION UNIT, RETURN CONFIGURATION NUMBER
	AX = 5101h
Return: CF set if successful
	    AH = 00h
	    AL = current configuration number
		00h system unit only
		FFh configuration not recognized
	    BX = status flag
		bits 0-14: reserved
		bit 15: additional data is available (location TBD)
	CF clear on error
	    AH = status
		01h expansion unit is not present
		86h function not supported
Note:	CF convention is the reverse of the standard convention for this
	  interrupt.  (Perhaps a typo in the IBMInternational Busiuness MachinesInternational Busiuness Machines) A hardware, software and other service technology company founded in 1911. BIOS(Basic Input/Output System) A set of standardized calls giving low-level access to the hardware.  The BIOS is the lowest software layer above the actual hardware and serves to insulate programs (and operating systems) which use it from the details of accessing the hardware directly. Tech Ref?)