\begin{abstract}

\start/ is a parallel machine that supports both fine-grained
user-level message passing and cache-coherent distributed shared
memory (\ccdsm/).  A site in \start/ is an enhanced commercial PowerPC
620-based symmetric multiprocessor (SMP) connected to an MPP-class
network.  With the aid of a simple address capture device(ACD), a
designated processor at each site executes cache-coherence protocols.
Given the variety of memory models and ever growing complexity of
coherence protocols, \start/ should provide a relatively cheap, low
risk and flexible system to explore shared memory issues.  This paper
describes several difficulties we have encountered in implementing
CCDSM using commercial SMPs.  It offers some insights into the causes
of these difficulties and possible remedies.

\end{abstract}
