Read Only Memory (ROM)

 

                READ ONLY MEMORY

There are random access memories in which data words are permanently written during fabrication. A word can later be read from the memory by specifying its address. The contents of the word cannot, however, be altered. Such a memory is called a Read Only Memory (ROM). Reading from a ROM should be non-destructive. The memory should also non-volatile. As important application of a ROM is to store tables which do not change. For example, table of trigonometric function. Suppose we want to store in sinθ. Then a ROM is fabricated to store the value of sinθ for various values of θ. To find sinθ for a given sinθ, the value of sinθ   is placed in the register of ROM and a Read-signal is issued. Sin θ stored in a word of the ROM is retrieved and put in the MDR of ROM.  Another application of ROM is for storing short programs for special applications. For example, a program to control the sequencing of operations of a washing machine may be stored in a ROM and interpreted by the processing unit. A ROM which has data written in it during manufacture in a factory is known as a factory programmed ROM. Such factory programming is feasible only in cases where the demand for such programmed ROMs is large. Example cited above would be suitable for factory programming.

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The Read Only Memory is following types:

 MROM:

It stands for “Masked ROM“. MROM has to contain the all pre-planned programs like as piece of instructions, and cheaper as well.

Programmable ROM (PROMs):

For more specialized uses where a user may like to store special functions or programs, it is preferable to use a ROM on which a user can write these. Such ROMs are available and are called programmable ROMs (PROMs). Programming is normally done using to special circuits. In such a case, the time taken to write data is long, but the read-time will be relatively small. PROM is not a flexible memory as the data in it cannot be altered. 

Erasable Programmable ROM (EPROM):

There are applications where one may like to store a program in a ROM which would normally not change, but under some unforeseen conditions, one may like to alter it. Erasable and reprogrammable ROMs have been made this requirement. Data in such a ROM is erased by shining strong ultraviolet light through a window. After the ROM is exposed to ultraviolet light, all bits are erased and become 0. The ROM may then be reprogrammed. Such a PROM is known as the EPROM (Erasable programmable ROM).

Electrically Erasable programmable Read Only Memory (EEPROM):

In other scheme, electrical pulses are used instead of ultraviolet light to erase a PROM. Such a ROM is known as Electrically Erasable Programmable Read Only Memory (EEPROM). Erasing a PROM with electrical voltage is convenient compared to using ultraviolet rays. Thus, EEPROMs are now dominant. In this memory, the write-time is much larger than read time. Strictly they are not ROMs- we should call them slow write-fast is much larger than read-time. Strictly they are not ROMs - we should call them slow write-fast read non- volatile random access memory.

1.     Flash Memory:

Flash memory is a variety of EEPROM. It does not use the same technology as the EEPROM and has different features. However, it also access use semiconductors, is non-volatile and is a fast read-slow write device just like EEPROM.

There are two types of flash memory: Nor flash and NAND flash. Each of these has different characteristics and different uses. Historically, NOR flash memory was the first one to be invented and was later followed by NAND flash. Currently, NAND flash is more widely used and is variously known as Pen drives, memory stick or flash disk. The memory system comes either as a long flat pluggable device (5 cm〤2 cm〤1.5 cm)or as a (3 cm 〤3 cm) flat disk. They have built-in controller for write/ read. The sizes given are indicative and not standard.

·        NOR flash memory

NOR flash memory is random access memory similar to EEPROM. We can thus address an individual word or a byte to read. However, we can erase a single byte unlike an EEPROM, where the entire contents are erased. Reading from NOR flash memory is fast, around 10ns/byte. The erase and write-times are about 100 times larger than the real -time. A NOR flash is suitable on a replacement of ROM, in which a fixed program can be stored. It can be erased 100,000 times without deterioration in performance. NOR flash manufactures provide spare blocks in case of write failure and provide a had block management system which keeps track of bad locks in case of write failure and provide a bad block management system which keeps track of bad blocks in the system and prevents writing new data in those blocks.

·        NAND flash memory

Unlike NOR flash, NAND flash memory is not random access memory. It is, however, non-volatile and fast read, slow write memory. A NAND flash is organized as a page where each page is typically 512-2048 bytes. Besides data types, each page is provided with a few bytes (12-16 bytes) to detect and current errors in the data stored in it. A set of around 32 pages is combined to form what is called a block. The minimum accessible unit of data to be read from NAND flash is a page. Thus, at a time 512 bytes are read (assuming page size as 512 bytes). Pages are addressable. Data is erased and written as a block of around 32 pages.

NAND flashes also have several bad blocks when manufactured and some blocks can become bad with use. Thus, manufacturers provide more block than what is specified as the capacity of the flash. Bad blocks are marked as non-usable and alternate blocks we provided. While using a NAND flash, the logical organization allows correction of single bit error in 256 bytes. If more error occurs, the data cannot be corrected and is usable. Block 0 is guaranteed to be readable. This block is thus used to store address a bad block table. The bad block table’s contents may vary with use.

The number of times data can be written and erased from NAND flashes is typically a million cycles. Thus, for all practical purposes, it can be used for several years. Data written in a NAND flash can be retained for around 10 years.

The capacity of NAND flash ranges from 32MB to 256 GB and is increasing every year. The speed of reading from NAND flashes ranges from 2 MB/s to 20 MB/s. very often they are quoted as 100x, 200x, etc., where x=150 KB/s.

Summary

•          In a random access memory may be fabricated with permanently stored data which cannot be erased. Such a memory is called a Read Only Memory (ROM).
•          A Rom in which data can be written permanently in the field is known as a Programmable ROM (PROM). It if can be erased and reprogrammed, it is known as an Erasable Programmable ROM (EPROM).
•          A flash memory, popularly known as a pen drive, is a type of electrically Erasable Programmable ROM (EEPROM).
•          Unlike a ROM, it is a random access memory but stores data in blocks of 512- 2048 bytes which is retrieved as a block.