Lesson 4

Read the text: Goals in computer architecture

The most common goals in computer architecture revolve around the tradeoffs between cost and performance (i.e. speed), although other considerations, such as size, weight, reliability, feature set, expandability and power consumption, may be factors as well.

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

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 dramatically 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 -- for example, adding cache usually makes latency worse (slower) but makes other things faster. 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. Furthermore, designers have been known to add special features to their products, whether in hardware or software, features which permit a specific benchmark to execute quickly but which do not offer similar advantages to other more general computational tasks. Naïve users are apt to be unaware of such deceptive tricks.

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.

Exercise 1. Learn the following words and word-combinations:

tradeoff, power consumption, storage capacity, to execute, interrupt latency, take into account, benchmark

Exercise 2. Answer the questions:

1) What can modern CPUs execute?

2) What are the types of speed?

3) What should or should not computer controlled anti-lock brakes wait?

4) What does benchmarking show?

5) What is the general scheme of optimization?

Exercise 3. Fill in the blanks with the appropriate words:

1) Speed and storage capacity ______ _______ to meet the cost target.

2) Low latencies can often _______ _______ very inexpensively.

3) One system _______ ________ scientific applications quickly.

4) Naïve users ______ _____ to be unaware of such deceptive tricks.

5) The exact form of the computer system _______ _______ on the constraints and goals it was optimized for.

a) will; b) are; c) adjusted; d) depend; e) are; f) might; g) be; h) apt; i) handle; j) had

Exercise 4. Match English words with the Ukrainian ones:

a) response	1) обмеження
b) event	2) темп, швидкість
c) range	3) вартість, ціна
d) strength	4) ознака, властивість
e) measure	5) міцність, ефективність
f) feature	6) коло, простір
g) advantage	7) реакція
h) cost	8) подія, випадок
i) rate	9) розмір, ступінь
j) constraint	10) перевага, виграш

Exercise 5. Compose a story on one of the topics (up to 100 words):

1) Different types of speed.

2) The goals and constraints a computer system is optimized for.