1.2 Computer systems  (Page 3/8)

Clock

A circuit in a processor that generates a regular sequence of electronic pulses used to synchronize operations of the processor's components. The time between pulses is the cycle time and the number of pulses per second is the clock rate (or frequency).

The execution times of instructions on a computer are usually measured by a number of clock cycles rather than seconds. The higher clock rate, the quicker speed of instruction processing. The clock rate for a Pentium 4 processor is about 2.0, 2.2 GHz or higher

Memory

Memory refer to computer components, devices and recording media that retain digital data used for computing for some interval of time. Computer memory includes internal and external memory.

Internal memory

The internal memory is accessible by a processor without the use of the computer input-output channels.It usually includes several types of storage, such as main storage, cache memory, and special registers, all of which can be directly accessed by the processor.

Cache memory : A buffer, smaller and faster than main storage, used to hold a copy of instructions and data in main storage that are likely to be needed next by the processor and that have been obtained automatically from main storage.

Main memory (Main Storage) : addressable storage from which instructions and other data may be loaded directly into registers for subsequent execution or processing.

Storage capacity of the main memory is the total amount of stored information that the memory can hold. It is expressed as a quantity of bits or bytes. Each address identifies a word of storage. So the capacity of the main memory depends on the number of bits allowed to address. For instance, a computer allows also 32-bit memory addresses; a byte-addressable 32-bit computer can address $2^{32}$ = 4,294,967,296 bytes of memory, or 4 gigabytes (GB). The capacity of the main memory is 4 GB.

The main memory consists of ROM and RAM.

• Random Access Memory (RAM): The primary storage is referred to as random access memory (RAM) because it is possible to randomly select and use any location of the memory directly store and retrieve data. It takes same time to any address of the memory as the first address. It is also called read/write memory. The storage of data and instructions inside the primary storage is temporary. It disappears from RAM as soon as the power to the computer is switched off.
• Read Only Memory (ROM): There is another memory in computer, which is called Read Only Memory (ROM). Again it is the ICs inside the PC that form the ROM. The storage of program and data in the ROM is permanent. The ROM stores some standard processing programs supplied by the manufacturers to operate the personal computer. The ROM can only be read by the CPU but it cannot be changed. The basic input/output program is stored in the ROM that examines and initializes various equipment attached to the PC when the switch is made ON.

External Memory

The external memory holds information too large for storage in main memory. Information on external memory can only be accessed by the CPU if it is first transferred to main memory. External memory is slow and virtually unlimited in capacity. It retains information when the computer is switched off and is used to keep a permanent copy of programs and data.