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=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)

See also: 151014

151013 - TopView - "GETBIT" - DEFINE A 2ND-LEVEL INTERRUPT HANDLER