%% This section gives the hardware overview of *T-ng.  
%%
%%   - a brief description of the message passing hardware.
%%   - a brief description of ACD and cache coherent shared memory. 
%%
%%------------------------------------------------------------------------
\section{Overview of \start/}
%%------------------------------------------------------------------------
\label{sec:hdw-overview}
%%------------------------------------------------------------------------

The core of a \start/ site is a commercial PowerPC 620 SMP augmented with
special hardware for message-passing and for implementing shared memory.
The 620 is an aggressive 64-bit, 4-way superscalar processor with a
dedicated 128-bit wide L2 path.  \start/ will have 4 MBytes of L2 cache per
processor.  The \start/ SMP site uses a data crossbar rather than a data
bus to increase data throughput to the memory.  An SMP using the
PowerPC~601 and the same cross-bar and memory system technology is already
on the market.  The \start/ site has 4 processor card slots, each of which
contains 2 processors.  \start/ will replace one to four of the processor
cards with network-endpoint-subsystem (NES) cards each containing a single
620 processor, its L2 cache and a network interface unit (NIU).  The NIU
allows the 620 to communicate with an MIT-developed Arctic network router
chip\cite{Boughton94}.  The \start/ system delivered to MIT will have 4 NES
boards per site and will have a total of 8 sites.

One of the NES boards will have an address capture device
(ACD)\footnote{All NES boards will have an ACD, but only one will be
enabled.} which will allow a designated processor to watch and respond
to bus transactions.  The ACD will be used to implement globally
shared, cache-coherent memory.  The ACD can be disabled, making it
completely invisible to the system.


\begin{figure}
\illustrationpercent{jhoe-start}{40}
\caption{A \start/ site: the white areas comprise the base
SMP.  The grey areas are our additions.}
\label{fig:start-site}
\end{figure}

When a processor is running application code, that processor is called
an application processor (\ap/).  When global shared memory support is
desired, one 620 processor is dedicated to servicing the ACD and is
called a service processor (\sp/).  Common machine configurations are,
therefore, four \ap/'s or three \ap/'s and an \sp/.

The next two sections  describe the message-passing and
shared-memory support for \start/.

%%------------------------------------------------------------------------
%%                               END
%%------------------------------------------------------------------------

