INT 15 - TopView - "GETBIT" - DEFINE A 2ND-LEVEL INTERRUPT HANDLER

	AX = 1013h
	ES:DI -> FAR service routine
Return: BX = bit mask indicating which bit was allocated
	    0000h if no more bits available
SeeAlso: AX=1014h,AX=1015h
Note:	only a few TopView/DESQview API calls are allowed during a hardware
	  interrupt; if other calls need to be made, the interrupt handler
	  must schedule a 2nd-level interrupt with "SETBIT" (AX=1015h)