High-Performance Embedded Computing

294 Chapter 5 Multiprocessor Architectures

multicast an input to several selected outputs. Because of the rich connectivity,

we can multicast in any combination of nodes.

The downside of a crossbar is its size. If the crossbar has n inputs and

n outputs, then its area is proportional to n

2

. Because the wires and program-

ming points are fairly small, we can in fact afford crossbars for moderate num-

bers of inputs and outputs. For example, an 8 × 8 crossbar would be quite

reasonable on a modern VLSI chip, assuming that the words transferred through

the crossbar are not excessively wide. However, a 10,000 × 10,000 crossbar of

even one-bit wide words woul ...