4. The system of claim 3, further including means for sequentially storing addresses of locations in said register means corresponding to negatively acknowledged transfers.
4. The system of claim 3, further including means for sequentially storing addresses of locations in said register means corresponding to negatively acknowledged transfers. 5. The system of claim 4, further including means for selecting for an output transfer one of the addresses sequentially stored, and means for thereafter identifying the location in said register means of the first received word for which no output transfer has been attempted.