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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)