In comparison to 10 years ago the processor scene has become drastically different. While in the period 1980--1990, the proprietary processors and in particular the vectorprocessors were the driving forces of the supercomputers of that period, today that role has been taken on by common off-the-shelf RISC processors. In fact there are only three companies left that produce vector systems while all other systems that are offered are based on RISC CPUs (except the Cray MTA-2). We think, therefore, that it is useful to give a brief description of the main processors that populate the present supercomputers and look a little ahead to the processors that will follow in the coming year.
The modern RISC processors generally have a clock frequency that is
lower than that of the Intel Pentium 3/4 processors or the
corresponding AMD Intel look-alikes. However, they have a number of
facilities that put them ahead in the speed of floating-point oriented
applications. Firstly, all RISC processors are able to deliver 2 or
more 64-bit floating-point results in one clock cycle. Secondly, all of
them feature out-of-order instruction execution, which enhances the
number of instructions per cycle that can be processed (although the
newer AMD processors also have 2-way floating-point instruction
issuing and out-of-order execution, they are limited by their
adherence to the Intel x86 instruction set). Thirdly, the
bandwidth from the processor to the memory, in case of a cache miss, is
larger than that of the Intel(-like) processors. Notwithstanding these
commonalities between the various RISC processors, there are also
differences in instruction latencies, number of instructions processed,
etc., which we will address below. We provide block diagrams for each of
the processors to give a schematic idea of their structure. However,
these figures do not reflect the actual layout of the devices on the
respective chips.