Int 15 Fn 1013  - Topview - "getbit" - Define A 2nd-level Interrupt Handl  [Q]

   AX = 1013h
   ES:DI -> FAR service routine

Return: BX = bit mask indicating which bit was allocated
        0000h if no more bits available

See Also: 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)