A short pointer is 32-bits, but the first two bits are used to define which Space Register is used to define the object intended. Thus a short pointer can address four quadrants of 1 gigabyte each. When a named file is opened with the short-mapped option it uses Space Register 6 (SR6) to reserve part of one quadrant for your file. Since you share this address space with everyone else on the system, only small files can be opened short-mapped. Unnamed short-mapped files are given space in Space Register 5 (SR5). So, while they are still limited to 4 megabytes, you can have as many as you want (until you run out of SR5 space).