Multiprocessing (MP) on networks of workstations (NOW) is a high-performance computing architecture of growing importance. In traditional MP's, wormhole routing interconnection networks use fixed-size flits and backpressure. In NOW's, ATM-one of the major contending interconnection technologies-uses fixed-size cells, while backpressure can be added to it. We argue that ATM with backpressure has interesting similarities with wormhole routing. We are implementing ATLAS I, a single-chip gigabit ATM switch, which includes credit flow control (backpressure), according to a protocol resembling Quantum Flow Control (QFC). We show by simulation that this protocol performs better than the traditional multi-lane wormhole protocol: high throughput and low latency are provided with less buffer space. Also, ATLAS I demonstrates little sensitivity to bursty traffic, and, unlike wormhole, it is fair in terms of latency in hot-spot configurations. We use detailed switch models, operating at clock-cycle granularity.