Page 28 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Unit 2: Data Processing

Notes
Figure 2.5: ROM

2.3.1.3 Programmable Read Only Memory (PROM)

There is another type of primary memory in computer, which is called Programmable Read Only
Memory (PROM). You know that it is not possible to modify or erase programs stored in ROM,
but it is possible for you to store your program in PROM chip. Once the programs are written
it cannot be changed and remain intact even if power is switched off. Therefore, programs or
instructions written in PROM or ROM cannot be erased or changed.

2.3.1.4 Erasable Programmable Read Only Memory (EPROM)
This stands for Erasable Programmable Read Only Memory, which overcomes the limitations
of PROM & ROM. EPROM chip can be programmed time and again by erasing the information
stored earlier in it. Exposing the chip for some time to ultraviolet light erases information stored
in EPROM. The chip can be reprogrammed using a special programming facility. When the
EPROM is in use, information can only be read.

2.3.1.5 Cache Memory

The speed of CPU is extremely high compared to the access time of main memory. Therefore,
the performance of CPU decreases due to the slow speed of main memory. To minimize
the mismatch in operating speed, a small memory chip is attached between CPU and Main
memory whose access time is very close to the processing speed of CPU. It is called CACHE
memory. CACHE memories are accessed much faster than conventional RAM. It is used to
store programs or data currently being executed or temporary data frequently used by the
CPU. So cache memory makes main memory to work faster and larger than it really is. It
is also very expensive to have bigger size of cache memory and therefore it is available in
limited capacity generally Kilo Bytes.

2.3.1.6 Registers
The CPU processes data and instructions with high speed. There is also movement of data
between various units of computer. It is necessary to transfer the processed data with high speed.
So the computer uses a number of special memory units called registers. These are not part of
the main memory but they store data or information temporarily and pass it on as directed by
the control unit.
2.3.2 Secondary Storage

Secondary storage (also known as external memory or auxiliary storage), differs from primary
storage in that it is not directly accessible by the CPU. The computer usually uses its input/output

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