This post will look at the different aspects of storage devices.
Memory Storage Media
Memory provides storage for data, instruction and the result of processing. Memory is characterised by the ability to retain data when the electricity to the computer stops and the ability to change the information already stored.
Programs and data can be held on a memory chip temporarily or permanently. The categories include: RAM, ROM, Cache Memory and Buffer.
Random Access Memory (RAM)
RAM is primary storage. Whenever a CPU writes data, instructions or information to RAM, it wipes out the previous contents of RAM.
Ram performs the following functions:
- Stores data for processing
- Stores instructions for the processing of data
- Stored processed data that is waiting to be sent to an output or secondary storage device.
There are two types of RAM used in computer systems: Dynamic RAM (DRAM) and Static RAM (SRAM).
DRAM is memory based on capacitor technology and requires the contents of each storage cell within the echip to be periodically refreshed. SRAM is based on transistor technology, and does not require refreshing.
Read Only Memory (ROM)
This is a memory chip that stores instructions and data permanently. Information on a ROM chip was stored there by manufactures and cannot be modified by a user. A CPU can read and retrieve the instructions and data from the ROM chip but it cannot change the contents in the ROM; information in ROM is only for computer use.
The programs on a ROM chip are called firmware and could include: the operating system or the part of the operating system needed to start up the computer and utility programs, including anti-virus programs and disk management programs.
There are three different types of ROM:
- Programmable Read-Only Memory (PROM): This chip is left blank by the manufacturer and written to the customer. Once written to it becomes ROM.
- Erasable Programmable Read-Only Memory (EPROM): EPROM is a reusable PROM-chip that can be erased by a special ultraviolet light. EPROM holds its content until erased and new instructions can be written to it. To reprogram an EPROM chip it has to be removed from the computer.
- Electrically Erasable Programmable Read Only Memory (EEPROM): EEPROM-chip is similar to an EPROM-chip except that it is erased by applying electrical pulses to the chip. making it possible to reprogram the chip without removing it from the computer.
Cache Memory
This is a type of high speed memory designed to supply te processor with the most frequently requested instructions and data. Instructions and data located in cache memory can be accessed many times faster than instructions and data located in main memory. The more instructions and data the processor can access directly from cache memory, the faster the computer runs as a whole.
When a request from the CPU is found in cache it is referred to as a cache hit. When it is not found it is referred to as a cache miss.
Buffers
This is an internal memory area used for temporary storage of data records during input or output operations.