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We discuss a preliminary design of a Rapid Single-Flux-Quantum (RSFQ) subsystem for general-purpose computers with petaflops-scale performance. The subsystem is being developed at Stony Brook within the framework of the Hybrid Technology MultiThreading (HTMT) project. COOL-0 design is based on 0.8-/spl mu/m RSFQ technology which enables the implementation of superconductor processing elements (SPELLs) operating at clock frequencies up to 100 GHz pipelined cryo-memory (CRAM) with 30 ps cycle time and interprocessor network (CNET) with a bandwidth of 30 Gbps per channel. The main architectural challenge is an almost 1,000-fold speed difference between the RSFQ processors and room-temperature SRAM comprising the second level of the HTMT memory hierarchy. The proposed solution to the problem is hardware support for two-level multithreading and block transfer techniques in SPELLs. Our preliminary estimates show that an RSFQ subsystem with 4 K SPELLs and a 4-Gbyte CRAM may be sufficient to achieve the performance close to 0.5 petaflops for computationally intensive program kernels. COOL-0 would occupy a physical space of about 0.5 m/sup 3/ and dissipate power as low as 250 Watts (at helium temperature). These numbers present a dramatic improvement compared to a hypothetical purely-semiconductor petaflops-scale computer.