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?)