Read Only Memory | ROM

Read Only Memory (ROM) is a type of memory used in many electronic devices, including computers. The key feature of ROM is that it allows unlimited reads but limited writes. This means data can be stored once, usually during manufacturing, or only a few times. However, this data can be read as many times as needed. ROM retains its information even when the power is off, making it a permanent, non-volatile memory device.

Inside, ROM consists of a grid of rows and columns connected by diodes based on the programming bit received, making it a hard-wired memory device. This means ROM doesn’t need frequent refresh cycles to retain data, unlike RAM. The single-write, multiple-read nature of ROM makes it ideal for storing boot-up programs and firmware that need few updates. ROM is used in devices like calculators, laser printers, washing machines, and gaming consoles.

Various kinds of ROMs in existence are

1. 
   

   Masked Read Only Memory (MROM)

   
   

   These were the first set of ROMs developed which are hard-wired and read-only in nature. MROM is programmed by the manufacturer during fabrication after which the contents of the memory can never be changed. This is because, MROMs are fabricated using masks (integrated circuits which have definite opaque and transparent areas) which either allow or block the passage of light (like that of UV) during photolithography. These devices are compact than any other kind of semiconductor memories for a given bit and are preferred when the need is for mass production.

   
   
2. 
   

   Programmable Read Only Memory (PROM)

   
   

   PROMs in the market are blank ROMs and contain no pre-recorded instructions like those of MROMs. These allow the user to program it only once, using PROM programmers, after which their contents can never be changed. This is because, a typical PROM with all 1’s in it will be programmed to have 0’s by blowing-off the fuses using high voltage pulses, wherever required. As this is an irreversible process, PROMs are one-time programmables.

   
   
3. 
   

   Erasable and Programmable Read Only Memory (EPROM)

   
   

   These devices comprise of an array of transistors characterized by floating-gates, which are programmed one-by-one with the help of high-voltage pulses. However, such a programmed EPROM can be erased by exposing them to strong ultra-violet (UV) light for certain duration, after which they can be programmed once again. Nevertheless, multiple-erase operations cause wearing out of the device due to which they lack the unlimited reprogramming ability. These kind of memory devices are generally used to store firmware programs which demand frequent upgrades.

4. Electrically Erasable and Programmable Read Only Memory (EEPROM)

   EEPROMs can be programmed more than once, as they are capable of being erased electrically. This means that they need not be removed from the system inorder to carry on the process of erasing and programming. However writing into an EEPROM is a slow process when compared to reading from it. Moreover they offer the flexibility of erasing byte-by-byte or selected areas only, instead of entire chip as in the case of EPROMs.
   Depending on this ability they can be classified as

   1. 
      

      Electrically Alterable Read Only Memory (EAROM)

      
      

      This kind of EEPROM allows bit-wise modification. However writing process is extremely slow and requires higher voltage than that for reading. Thus these are used to store set-up information of the system or for similar applications which require rare and/or partial rewriting.

      
      
   2. 
      

      Flash Read Only Memory (FROM)

      
      

      These can be erased and reprogrammed much faster than EEPROMs and allow the user to access multiple locations of the memory at the same time.