Computer architecture

Computer architecture is the theory behind the design of a computer. In the
same way as a building architect sets the principles and goals of a building
project as the basis for the draftsman's plans, so too, a computer architect
sets out the computer architecture as a basis for the actual design
specifications.

There are several usages of the term, which can be used to refer to:

   * The design of a computer's CPU architecture and instruction set and
     techniques such as SIMD and MIMD parallelism.
   * More general wider-scale hardware architectures, such as cluster
     computing and NUMA architectures.
   * The less formal usage refers to a description of the requirements
     (especially speeds and interconnection requirements) or design
     implementation for the various parts of a computer. (Such as memory,
     motherboard, electronic peripherals, or most commonly the CPU.)

Design Goals

The most common goals in a Computer Architecture revolve around the
tradeoffs between cost and performance (i.e. speed), although other
considerations, such as size, weight, and power consumption, may be a factor
as well.

1. Cost

Generally, cost is held constant, determined by either system or commercial
requirements, and speed and storage capacity are adjusted to meet the cost target.

2. Performance

Computer retailers describe the performance of their machines in terms of
clock speed (usually in MHz or GHz). This refers to the cycles per second of
the main clock of the CPU. However, this metric is somewhat misleading, as a
machine with a higher clock rate may not necessarily have higher
performance. Modern CPUs can execute multiple instructions per clock cycle,
which dramaticallly speeds-up a program. Other factors aid speed, such as
the mix of functional units, bus speeds, available memory, and the type and
order of instructions in the programs being run.

But, there are also different types of speed. Interrupt latency is the
guaranteed maximum response time of the system to an electronic event (e.g.
when the disk drive finishes moving some data). This number is affected by a
very wide range of design choices. Computers that control machinery usually
need low interrupt latencies, because the machine can't, won't or should not
wait. For example, computer-controlled anti-lock brakes should not wait for
the computer to finish what it's doing - they should brake. Low latencies
can often be had very inexpensively.

Benchmarking, tries to take all these factors into account by measuring the
time a computer takes to run through a series of test programs. Although
benchmarking shows strengths, it may not help one to choose a computer.
Often the measured machines split on different measures. For example, one
system might handle scientific applications quickly, while another might
play popular video games more smoothly.

The general scheme of optimization is to find the costs of the different
parts of the computer. In a balanced computer system, the data rate will be
constant for all parts of the system, and cost will be allocated
proportionally to assure this. The exact form of the computer system will
depend on the constraints and goals it was optimized for.