5. Answer the following questions.

1. What makes the binary number system the basis for representing data in computers?

2. In what terms is the capacity of computer’s memory expressed?

3. What is the difference between the computer and any adding machine?

4. What does the primary storage of your computer consist of?

5. What are the advantages of semiconductor memory?

6. What is the major disadvantage of semiconductor memory?

7. What types of semiconductor memory can you define?

Cache memory

The cache memory was introduced as the first attempt at using memories of different speeds. The problem was to increase speed of instruction execution. The analysis of programs showed that in the majority of programs only few variables are used frequently, so only few memory cells are frequently accessed. The solution was to store this frequently used data in a special memory with the higher speed. This type of memory is called a cache memory. For example, on a typical 100-megahertz system board, it takes the CPU as much as 180 nanoseconds to obtain information from main memory, compared to just 45 nanoseconds from cache memory.

When the program is executed, some of the variables are held in the cache memory. The control unit interprets the instruction and looks for the necessary data in the cache memory first. If the data is there, it is processed; otherwise the control unit looks for the data in RAM. A more sophisticated cache memory keeps acount of number of accesses made to each variable. These counts are compared at regular intervals and the most frequently used variables are moved to the cache.

The cache memory system is managed by an ’intelligent’ circuit called the cache memory controller. When a cache memory controller retrieves an instruction from RAM, it also takes back the next several instructions to cache. This occurs because there is a high probability that the adjacent instruction will also be needed.

To speed up the computers even more, some CPUs (e.g. 80486 and Pentiums) have got built-in cache memory. In this case, there will be two cache memories: one — built-in and another — external (to the CPU). The built-in cache memory is also referred to as level 1 cache or LI or primary cache. It is located inside the CPU. External cache is referred to as level

2. cache or L2 or secondary cache and is located on the motherboard. The capacity of built-in cache is between 8 and 32K, depending on the microprocessor. The capacity of external cache ranges in size from 64K to 1M.

When CPU chips do not contain internal cache, the external cache, if present, would actually be the primary (LI) cache. Some secondary caches can be expanded, some cannot.

Some advertisements specify the type of the secondary cache installed as write-back or associative.

Write-back cache holds off writing to the hard disk until there is a lull in CPU activity. This gives an advantage in speed but there is a danger that data can be lost if the power fails.

Associative cache describes an alternative architecture to direct mapped memory, and is generally faster than direct mapped cache.

Cache Speed and RAM Speed

In Pentium systems, 20ns cache SRAM is generally used for 50-60MHz system boards (using the Pentium 75/90/100/120), and 15ns cache SRAM is normally utilized for 66MHz system boards (using the Pentium 100/133). Cache SRAM at speeds up to 8ns has recently become available, although rare and expensive.

Comments:

cache memory кеш-пам'ять; над швидкодіюча оперативна пам'ять,

яка слугує для буферизації команд і/або даних із повільної пам'яті перед їхньою обробкою про­цесором або операціями вводу-виводу

variable змінна;у програмуванні — поіменована ділянка пам'я-

ті, якій програмно присвоюються різні значення. controller контролер; мікросхема, плата чи блок, які здійснюють

керування внутрішнім або периферійним пристроєм та обмін даними між ним і комп'ютером

associative cache асоціативний кеш; тип кеш-памяті, розроблений,

щоб усунути проблему зіткнень, характерну для кеша з прямим записуванням

SRAM (Static RAM) статичний ОПЗ; дорожчий і швидший (із часом до­ступу до 5 не) динамічний ОПЗ