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Tuesday, 26 November 2019

RAM and ROM in Digital Electronics

Random Access Memory (RAM in Digital Electronics) is a type of memory device where the data can be accessed randomly. The term usually refers to random access read / write memory RAMs are basically sequential circuits (flip-flops). When the power is switches off, the data stored in a RAM is lost. Hence, RAMs are also called volatile memories. The name RAM comes from the fact that data can be stored into and retrieved from these cells at random.

RAM is the main memory of a computer. The speed of a computer CPU may be hundred of mega hertz. For example, Intel Pentium - IV chip operates at speed of 1500 MHz. However, this speed is limited by various factors, a major one among them being the speed of the RAM used in the computer. RAMs are read-write-erasable memories.
Read Only Memory (ROM in Digital Electronics) is a type of memory where data are permanently stored and can only be read, not written. ROM enclose permanently and semi-permanently accumulated data which can be read from the memory other than either cannot be changed at all or cannot be changed without specific equipment. A ROM stores data that are used repeatedly in system applications, such as tables, conversions, or programmed instructions for systems initialization and operation. ROMs retain stored data when the power is off and are therefore called non-volatile memories.




1. Data storage power is switched off.
Temporary, Vanishes when not affect data stored
Permanent, Power failure does
2. Data entry
Data can be entered fast. No special program-writers are required.
Special program-writers are required. So. data entry is normally slow. But this is being rectified.
3. Data read out time it is entered.
Data is read-out at the same entered.
Data is read-out only after it is
4. Repeatability of read out
Once the data is erased, it cannot be recovered.
Data is not erased; hence, it can be read out any number of times.
5. Erasability operation.
Easily erasable, very fast operation
Not easily erasable, slower
6. Packing density
Currently large
Currently low
7. Technology used
Bipolar, NMOS. CMOS (currently CMOS, mostly)
Bipolar, NMOS, CMOS (currently CMOS, mostly)
8. Static/dynamic operations
static and dynamic operations exist.
Only static ROMs exist.
9. Sense-amplifiers in many cases.
Sense amplifiers are required are required.
Normally, no sense amplifiers
10. Data-entry circuit
Separate address decoders are used for X and Y addressing.
Decoders are used for data entry (X-address), and multiplexers (Y-address) are used for data output.
11. Use of memory
Data entered in any computer is first stored in RAM. If the data is to be stored permanently it will be saved in the hard disk.
This memory stores data of a permanent nature such as operating-system commands, look-up tables, etc., which are not stored in hard disks.

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