Introduction to Computer Memory

Computer memory is a physical device capable of storing information temporarily for instance like RAM or permanently like in ROM. Memory is used to store data and instructions being worked on. There are two types of computer memory – Primary Memory (RAM and ROM) and Secondary Memory (Hard disk drive, SSD, CD, USB Flash Drive, etc.)

Computer Memory
Computer Memory – Front and back of HyperX DDR3 RAM memory modules, By: Mr. Yukio [CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)]

There Are Two Main Types of Primary Memory Used In Computers

  1. RAM – Random Access Memory
  2. ROM – Read-Only Memory

Random Access Memory (RAM)

This is a temporary storage area where the programs and data required by the processor must reside. Memory storage is considered temporary because the data and programs remain there only as long as the computer has electrical power or is not rebooted. Before being shut down or restarted, any data that has been changed should be saved to a more permanent storage device (usually a hard disk drive) so it can be reloaded into memory in the future.

The Main memory is called RAM because you can randomly access any location in memory. This as opposed to sequential access where data is accessed in a predetermined ordered sequence. However, note that Read-Only Memory (ROM) is also randomly accessible. It is usually differentiated from the system RAM because it maintains data without power and can’t normally be written to.

A diagram of Computer Memory
Computer Memory

There Are Two Different Types of RAM

  1. DRAM (Dynamic Random Access Memory) and
  2. SRAM (Static Random Access Memory)

Dynamic Random Access Memory (DRAM)

Dynamic RAM (DRAM) is the type of memory chip used for most of the main memory in modern personal computers. The main advantage of Dynamic RAM is that it is very dense; hence a lot of bits can be packed into a very small chip. It is also inexpensive, which makes purchasing large amounts of memory affordable.

The memory cells in a DRAM chip are tiny capacitors that retain a charge to indicate a bit. A binary digit is either a zero or a one (absence or presence of a charge). The problem with DRAM is that it is dynamic. And because of the design, it must be constantly refreshed; otherwise, the electrical charges in the individual memory capacitors will drain and the data will be lost. Refresh occurs when the system memory controller takes a tiny break and accesses all the rows of data in the memory chips. Most systems have a memory controller (normally built into the North Bridge portion of the motherboard chipset).

Types of Dynamic Random Access Memory (DRAM)

  • Fast Page Mode DRAM – FPM DRAM
  • Extended Data Out DRAM – EDO DRAM
  • Burst Extended Data Out DRAM – BEDO DRAM
  • Synchronous DRAM – SDRAM
  • Double Data Rate SDRAM – DDR SDRAM
  • Double Data Rate 2 SDRAM – DDR2 SDRAM
  • Rambus DRAM – RDRAM

Static Random Access Memory (SRAM)

This is another distinct type of memory that is significantly faster than most types of DRAM. Static RAM does not need periodic refresh rates like DRAM. Because of how SRAMs are designed, they are much faster than DRAM and much more capable of keeping pace with modern processors.

SRAM cell is designed using six transistors. This represents each bit of storage. The use of transistors but no capacitors means that refresh rates are not necessary because there are no capacitors to lose their charges over time. As long as there is power, SRAM remembers what is stored.

Types of SRAM

  1. Async SRAM
  2. Sync SRAM
  3. Pipeline Burst SRAM
Static RAM Chip
Static Ram chip from a NES clone. 2K X 8 bit, Source: Satyamfifa [Public domain]

Read-Only Memory (ROM)

ROM is a type of computer memory that can permanently or semi-permanently store data. It is called read-only because it is either impossible or difficult to write to as opposed to RAM. Read-Only Memory is a non-volatile memory because any data stored in it remains there, even if the power is turned off. Thus, ROM is an ideal place to put the personal computers startup instructions. This is the software that boots the computer system. ROM is mostly used for firmware updates and it is found in computers and other electronic devices.

Erasable PROM type of computer memory
EPROM : The first INTEL EPROM, the 1702 (1971), Author: Poil 01:10, 17 Apr 2005 (UTC) [CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)]

ROM Chip Types

The four main types of ROM chips that have been used in PCs are as follows:

  • ROM – Read-Only Memory
  • PROM – Programmable ROM
  • EPROM – Erasable PROM
  • EEPROM – Electrically Erasable PROM, also sometimes called a flash ROM

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