Returns the current size of the overlay buffer.
function OvrGetBuf: Longint;
The size of the overlay buffer is set through a call to OvrSetBuf. Initially, the overlay buffer is as small as possible, corresponding to the size of the largest overlay. A buffer of this size is automatically allocated when an overlaid program is executed. The initial buffer size could be larger than 64K, because it includes both code and fix-up information for the largest overlay.