A single shared bus is a common arrangement
on SMPs for PCs and workstations (Figure 17.5).With this arrangement, the single
bus becomes a bottleneck affecting the scalability (ability to scale to larger sizes)
of the design.The z990 copes with this problem in two ways. First, main memory is
split into multiple cards, each with its own storage controller that can handle
memory accesses at high speeds. The average traffic load to main memory is cut,
because of the independent paths to separate parts of memory. Each book includes
two memory cards, for a total of eight cards across a maximum configuration.
Second, the connection from processors (actually from L2 caches) to a single
memory card is not in the form of a shared bus but rather point-to-point links.
Each processor chip has a link to each of the L2 caches on the same book, and
each L2 cache has a link, via the MSC, to each of the two memory cards on the
same book.
Each L2 cache only connects to the two memory cards on the same book.The
system controller provides links (not shown) to the other books in the configuration,
so that all of main memory is accessible by all of the processors.
Point-to-point links rather than a bus also provides connections to I/O channels.
Each L2 cache on a book connects to each of the MBAs for that book. The
MBAs, in turn, connect to the I/O channels.