Define memory hierarchy. Explain different types of ROM in detail.

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Memory hierarchy is a concept that is necessary for the CPU to be able to manipulate data.

Memory Hiearchy | CSIT Guide

Computer memory is classified in the below hierarchy.

1 . Internal register: Internal register is for holding the temporary results and variables. Accessing data from these registers is the fastest way of accessing memory.

2. Cache: Cache is used by the CPU for memory which is being accessed over and over again. Instead of pulling it every time from the main memory, it is put in cache for fast access. It is also a smaller memory, however, larger than internal register.

Cache is further classified to L1, L2 and L3:

a) L1 cache: It is accessed without any delay.

b) L2 cache: It takes more clock cycles to access than L1 cache.

c) L3 cache: It takes more clock cycles to access than L2 cache.

3) Main memory or RAM (Random Access Memory): It is a type of the computer memory and is a hardware component. It can be increased provided the operating system can handle it.

4) Hard disk: A hard disk is a hardware component in a computer. Data is kept permanently in this memory. Memory from hard disk is not directly accessed by the CPU, hence it is slower. As compared with RAM, hard disk is cheaper per bit.

5) Magnetic tape: Magnetic tape memory is usually used for backing up large data. When the system needs to access a tape, it is first mounted to access the data. When the data is accessed, it is then unmounted. The memory access time is slower in magnetic tape and it usually takes few minutes to access a tape.

Types of Read-Only Memory (ROM):

  1. MROM (Masked read-only memory)
  2. PROM (Programmable read-only memory)
  3. EPROM (Erasable programmable read-only memory)
  4. EEPROM (Electrically erasable programmable read-only memory)

1. MROM (Masked read-only memory): We know that ROM is as old as semiconductor technology. MROM was the very first ROM that consists of a grid of word lines and bit lines joined together transistor switches. This type of ROM data is physically encoded in the circuit and only be programmed during fabrication. It was not so expensive.

2. PROM (Programmable read-only memory): PROM is a form of digital memory. In this type of ROM, each bit is locked by a fuse or anti-fuse. The data stored in it are permanently stored and can not be changed or erasable. It is used in low-level programs such as firmware or microcode.

3. EPROM (Erasable programmable read-only memory): EPROM also called EROM, is a type of PROM but it can be reprogrammed. The data stored in EPROM can be erased and reprogrammed again by ultraviolet light. Reprogrammed of it is limited. Before the era of EEPROM and flash memory, EPROM was used in microcontrollers.

4. EEPROM (Electrically erasable programmable read-only memory): As its name refers, it can be programmed and erased electrically. The data and program of this ROM can be erased and programmed about ten thousand times. The duration of erasing and programming of the EEPROM is near about 4ms to 10ms. It is used in microcontrollers and remote keyless systems. 

Source: geeksforgeeks

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