Table of Contents
Types of Memory
There are mainly two types of memory – primary or main memory and storage memory. R/WM (Read and Write Memory) and ROM (Read Only Memory) are primary memory. The microprocessor uses this memory to store and execute programs. The speed of operation of these memory should be so fast that it can match the speed of execution of the microprocessor. Therefore, it should be random access memory, that is, the microprocessor can access any register at the same speed. Types of memory in Hindi In this article we will discuss the different types of memory. Click here to read this article in Hindi
Another type of memory is storage memory, for example, disk, tape, etc. This memory is used to store the result obtained after the execution of a program. The information stored in these memories are non-volatile, that is, the information remains intact even when the system is turned off. The microprocessor cannot directly access the programs stored on these devices, for which it is necessary to first copy the program into R/W prime memory. Therefore, the size of the prime memory determines how large the program can be processed by the system. The size of the storage memory is unlimited. When one disc or tape is full, another can be used.
RAM or RW memory
In a semiconductor memory, there are many cells to store information. Each cell stores one bit of information. It takes the same amount of time to access any cell on the chip. For example, if there are 16384 cells in a memory, then the microprocessor takes the same time to access cell 1 and 16384th cell. This type of accessing is called random access. Information stored on magnetic tape cannot be ‘random accessed’. It can be accessed only in one order i.e. from the beginning (sequentially). Random access memory is represented by RAM and semiconductor RAM is widely used at present. Information can be written in the cells of these memory and information can be read from the cells.
A binary cell of memory made using semiconductor flip-flop is shown in the figure. An R-S flip flop and other gates are used in the cell. When a ‘1’ is applied to the ‘IN’ terminal, the flip-flop goes into 1-state if the Write signal is also in ‘1’ state. The output of a binary cell is available at the ‘OUT’ terminal when a read signal is supplied.
Static and Dynamic RAMs
There are two types of RAMs (or Read/Write) memory used in microprocessor systems. Static RAM (SRAM) and Dynamic RAM (DRAM). Static RAM is a group of flip-flops and stores the input as a voltage, while dynamic RAM is made up of MOS transistor gates. The main characteristic of static RAM is that after a bit of information is written to its cell, it retains that bit until another bit is overwritten (overwritten), or the chip Power supply should not stop.
Dynamic RAM chip cells are smaller than SRAM and information is stored as a charge on a capacitor (in DRAM). A typical DRAM can store 4 times as much information as SRAM in the same area. For this reason DRAMs have a lower cost per bit.
Because information in DRAM is stored in the form of charge on capacitors. Therefore, to retain the stored information, the memory has to be refreshed after some 1 millisecond. Refreshing requires additional circuitry, which makes interfacing dynamic RAM with a microprocessor more complex than interfacing with static RAMs. Generally only systems with large memory capacity use dynamic RAMs. This reduces the cost of memory. Dynamic RAMs are used as primary memory in most personal computers (PCs). Static RAMs are used when speed of operation is important, or the size of the memory is not very large, or the cost is not of great importance in the system design.
Read Only Memory ‘ROM’
ROM is a pre-programmed chip and the microprocessor can only read it. Thus once information is recorded in ROM, it can be used only for ‘READ’ work. It cannot be used for any other purpose. The information in ROM is usually recorded by the manufacturer of the memory itself. It is a non-volatile memory. The information stored in it is retained even when the power supply is off.
Two types of memory are given in the permanent ROM group in the picture-Masked ROM and PROM. Similarly, there are two types of memory in erasable group – EPROM and EE-PROMI.
The principle of memory ‘ROM’ can be explained by arranging semiconductor diodes in a matrix format as shown in the figure. The horizontal lines in the figure are connected to the vertical lines only by diodes. The eight horizontal rows can be thought of as eight binary registers with binary addresses from 000 to 111. Information is stored as 0 or 1 in registers. The presence of the diode stores a ‘1’ and the absence stores a ‘0’. When a register is selected, the voltage of that line increases and the output lines, where the diodes are connected, also go high. For example, when memory register 010 is selected, the data byte 00110110 (= 36H) can be read on data lines D7 – Do.
The diode equivalent of the ROM shown in the figure is actually a simplified version of the MOSFET memory cell. ROM manufacturers design the MOSFET matrix according to the information to be stored. Thus information is permanently recorded in ROM like music on a record.
The description of different types of ROMs is as follows-
Masked ROM
The bit pattern is permanently recorded in the mask ROM by the masking and metallization processes. Although this is an expensive and specialized process, the cost is justified when mass produced.
PROM | Programmable Read Only Memory
Programmable ROM in microprocessor is called PROM. We (the user) can program it only once. After being programmed, it behaves like a normal ROM. In this memory, nichrome or polysilicon wires are arranged in a matrix which can be considered like a diode or fuse. Whether the fuse is lit or not lit (retain) determines whether the cell has a 1 or a 0 stored in it. This process is called ‘burning the PROM’ and after this the data (bit pattern) is stored permanently.
EPROM | Erasable Programmable ROM
The PROM which can be erased and reprogrammed by ultraviolet rays is called EPROM. To erase the EPROM, the chip is taken out of the circuit and ultraviolet rays are inserted through the quartz window made in it for some time (about 15 to 66 minutes). The erasure time depends on the product of the intensity of the ultraviolet light and the length of time it is exposed to this light. For example, the Intel 27256 EPROM can be erased in 15-20 minutes if exposed to ultraviolet rays of intensity 15 W-s/cm2.
Since EPROM can be used many times, it is widely used in experimental work, research work and product development in industries.
‘EE-PROM | Electrically Erasable PROM
This memory is similar to EPROM in terms of function. The main drawback of EPROM is that it has to be taken out of its normal circuit for erasing. 1 EEPROM does not need to be taken out of its normal circuit of operation. It can be erased and programmed by giving an electrical signal while performing its normal functions. Software is often updated in microprocessor systems. If EEPROMs are used in the system, they can be erased and reprogrammed by a remote link, using a telephone line, to a central computer. For example, in process control systems based on microprocessors, information on changes to parameters can be updted by giving an electrical signal.
The erase time of a typical EEPROM is 10 ms while the erase time of an EPROM is 15 to 20 minutes.
Flash Memory
It is a form of EEPROM. The main difference between Flash and EEPROM is in the erasing method. In EEPROM, any of its registers can be erased whereas flash memory has to be erased completely.
In microprocessor-based systems, programs are usually written in ROM and data, which changes frequently, is stored in R/W memory. For example, in microprocessor-controlled railway ticket booking, programs related to the time of arrival or departure of a train at a station, distances between different stations, train numbers and fares, etc., are permanently stored in ROM and Other data, eg passenger’s name, age, berth number etc. are entered into the R/W memory by the key board.
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