Primary Memory - RAM and ROM

All modern computers use semiconductor memory as primary memory. One of the important semiconductor memories used in desktop computers is Random Access Memory (RAM). Here “random access” means that any storage location can be accessed (both read and write) directly. This memory is faster, cheaper, and provides more storage space in lesser physical area. These very large-scale integrated semiconductor memory chips are mounted on pluggable printed circuit boards (PCBs). Enhancement or replacement of memory with such PCB memory modules is easy. These characteristics have made semiconductor memory more popular and attractive.

The only drawback of semiconductor memory is that it is volatile, i.e., it loses its contents whenever power is switched off.

Random Access Memory (RAM)

RAM holds the data and instructions waiting to be processed by the processor. In addition to data and program’s instructions, RAM also holds operating system instructions that control the basic functions of a computer system. These instructions are loaded into RAM every time the computer is turned on, and they remain there until the computer is turned off.

There are two types of RAM used in computer systems– dynamic and static.

  1. Dynamic RAM (DRAM) is a type of RAM that employs refresh circuits to retain its content in its logic circuits. Each memory cell in DRAM consists of a single transistor. The junction capacitor of the transistor is responsible for holding the electrical charge that designates a single bit as logical 1. The absence of a charge designates a bit as logical 0. Capacitors lose their charge over time and therefore need to be recharged or refreshed at pre-determined intervals by a refreshing circuitry.
  2. A more expensive and faster type of RAM, Static RAM (SRAM), does not require such type of refreshing circuitry. It uses between four to six transistors in a special ‘flip- flop’ circuit that holds a 1 or 0 while the computer system is in operation. SRAM in computer systems is usually used as processor caches and as I/O buffers. Printers and liquid crystal displays (LCDs) often use SRAM to buffer images.

SRAM is also widely used in networking devices, such as routers, switches, and cable modems, to buffer transmission information. Both dynamic and static RAM are volatile in nature and can be read or written to. The basic differences between SRAM and DRAM are listed in Table.

Static RAM Dynamic RAM
It does not require
It requires extra electronic
circuitry that ‘‘refreshes’’
memory periodically; otherwise
its content will be lost.
It is more expensive than
dynamic RAM.
It is less expensive than static
It is lower in bit density. It holds more bits of storage in
a single integrated circuit.
It is faster than dynamic
It is slower than SRAM, due to


There are several popular types of dynamic RAM used in computers. They are :-

  1. SDRAM (Synchronous Dynamic RAM),
  2. RDRAM (Rambus Dynamic RAM) and
  3. DDR RAM (Double Data Rate RAM).

The SDRAM used to be the most common type of RAM for personal computers. It was reasonably fast and inexpensive. It is no more used in the present day for personal computers as much improved RAMs are available now.

The RDRAM was developed by Rambus Corporation and is its proprietary technology. It is also the most expensive RAM and is used mostly in video interface cards and high- end computers that require fast computation speed and data transfer. RDRAMs are preferred for high-performance personal computers.

The DDR RAM is a refinement of SDRAM. DDR stands for Double Data Rate. It gives faster performance by transmitting data on both the rising and the falling edges of each clock pulse. DDR 2, DDR3 are other higher-speed versions of DDR RAM. Another type of RAM, termed Video RAM (VRAM), is used to store image data for the visual display monitor. All types of video RAM are special arrangements of dynamic RAM (DRAM). Its purpose is to act as a data storage buffer between the processor and the visual display unit.

There is a persistent mismatch between processor and main memory speeds. The processor executes an instruction faster than the time it takes to read from or write to memory. In order to improve the average memory access speed or rather to optimize the fetching of instructions or data so that these can be accessed faster when the CPU needs it, cache memory is logically positioned between the internal processor memory (registers) and main memory. The cache memory holds a subset of instructions and data values that were recently accessed by the CPU. Whenever the processer tries to access a location of memory, it first checks with the cache to determine if it is already present in it. If so, the byte or word is delivered to the processor. In such a case, the processor does not need to access the main memory. If the data is not there in the cache, then the processer has to access the main memory. The block of main memory containing the data or instruction is read into the cache and then the byte or word is delivered to the processor. There are two levels of cache.
Level 1 (Primary) cache This type of cache memory is embedded into the processor itself. This cache is very fast and its size varies generally from 8 KB to 64 KB.
Level 2 (Secondary) cache Level 2 cache is slightly slower than L1 cache. It is usually 64 KB to 2 MB in size. Level 2 cache is also sometimes called external cache because it was external to the processor chip when it first appeared.

Read Only Memory (ROM)

Read Only Memory (ROM) It is another type of memory that retains data and instructions stored in it even when the power is turned off. ROM is used in personal computers for storing start-up instructions provided by the manufacturer for carrying out basic operations such as bootstrapping in a PC, and is programmed for specific purposes during their fabrication. ROMs can be written only at the time of manufacture.

Another similar memory, Programmable ROM (PROM), is also non-volatile and can be programmed only once by a special device. But there are instances where the read operation is performed several times and the write operation is performed more than once though less than the number of read operations and the stored data must be retained even when power is switched off. This led to the development of EPROMs (Erasable Programmable Read Only Memories).

In the EPROM or Erasable Programmable Read Only Memory, data can be written electrically. The write operation, however, is not simple. It requires the storage cells to be erased by exposing the chip to ultraviolet light, thus bringing each cell to the same initial state. This process of erasing is time consuming. Once all the cells have been brought to the same initial state, the write operation on the EPROM can be performed electrically.

There is another type of Erasable PROM known as Electrically Erasable Programmable Read Only Memory (EEPROM). Like the EPROM, data can be written onto the EEPROM by electrical signals and retained even when power is switched off. The data stored can be erased by electrical signals. However, in EEPROMs the writing time is considerably higher than reading time. The biggest advantage of EEPROM is that it is non-volatile memory and can be updated easily, while the disadvantages are the high cost and the write operation takes considerable time.

Frequently Asked Questions

Ans: The abbreviated form of “ bi-nary dig-it ” is known as bit. When a “bit” is mentioned, it means a single binary digit, which may either be a “0” or “1”, is referred to. view more..
Ans: A binary number is made of 0s and 1s. In the binary number system only two symbols, 0 and 1, are used to represent numeric values. The symbol “0“ represents the value “zero” while the symbol “1” represents the value “one”. Since there are only two symbols in the binary number system, the value “two” is represented by placing the symbol “1” on the left–hand side of the symbol “0” resulting in the binary equivalent “10”. Next, the value “three” is represented by “11” in the binary number systemby replacing the “ 0 ” in “10” by the next higher value symbol “1”. view more..
Ans: To express the value of a given octal number as its decimal equivalent, add the octal digits after each digit has been multiplied by its associated weight. view more..
Ans: All modern computers use semiconductor memory as primary memory. One of the important semiconductor memories used in desktop computers is Random Access Memory (RAM). Here “random access” means that any storage location can be accessed (both read and write) directly. view more..
Ans: Secondary Memory And Secondary Storage Devices view more..
Ans: There are four main types of secondary storage devices available in a computer system: Disk drives CD drives (CD-R, CD-RW, and DVD) Tape drives USB flash drives view more..
Ans: Cache memory It is a special high-speed memory that allows a microprocessor to access data more rapidly than from memory located elsewhere on the system board. view more..
Ans: A microprocessor is an integrated circuit chip that contains all of the essential components for the central processing unit (CPU) of a microcomputer system. view more..
Ans: 1’s complement A number system that was used in some computers to represent negative numbers. To form 1s complement of a number, each bit of the number is inverted which means zeros are replaced with ones and ones with zero. view more..
Ans: 1’s complement A number system that was used in some computers to represent negative numbers. To form 1s complement of a number, each bit of the number is inverted which means zeros are replaced with ones and ones with zero. view more..
Ans: 2’s complement A number formed by adding 1 to the 1’s compliment of a number. The 2’s complement representation has become the standard method of storing signed binary integers. It allows the representation of an n-bit number in the range – 2n to 2n-1, and has the significant advantage of only having one encoding for 0. view more..
Ans: A Boolean or logic expression is a logic variable or a number of logic variables involved with one another through the logical operations ‘.’, ‘+’, and ‘–’. view more..
Ans: The AND gate is an electronic circuit that has two or more inputs and only one output. It gives a HIGH output (1) only if all its inputs are HIGH. view more..
Ans: The OR gate is an electronic circuit that has two or more inputs and only one output. It gives a HIGH output if one or more of its inputs are HIGH. view more..
Ans: NOT GATE also called INVERTER.The inverter is a little different from AND and OR gates as it has only one input and one output. Whatever logic state is applied to the input, the opposite state will appear at the output. The NOT function is denoted by a horizontal bar over the value to be inverted. view more..
Ans: The Exclusive-OR or XOR gate is a two-input circuit that will give a HIGH output if either, but not both, of the inputs are HIGH. The XOR function is an interesting and useful variation of the basic OR function. Its function can be stated as ‘Either A or B, but not both’. The XOR gate produces a logic 1 output only if the two inputs are different. If the inputs are the same, the output is a logic 0. XOR is also called an anti-coincidence gate or inequality detector. view more..
Ans: The Exclusive-NOR gate is a XOR gate followed by a NOT gate. XNOR gate is a two-input and one-output logic gate circuit. In the gate, the output is HIGH if both inputs are either LOW or HIGH view more..
Ans: A programming language can be defined formally as an artificial formalism in which algorithms can be expressed. It is composed of a set of instructions in a language understandable to the programmer and recognizable by a computer. Computer languages have been continuing to grow and evolve since the 1940’s. Assembly language was the normal choice for writing system software like operating systems, etc. But, C has been used to develop system software since its emergence. The UNIX operating system and its descendants are mostly written in C. Application programs are designed for specific computer applications. Most programming languages are designed to be good for one category of applications but not necessarily for the other. For an instance, COBOL is more suitable for business applications whereas FORTRAN is more suitable for scientific applications. view more..

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