Page 210 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 11: Registers and Counters

Figure 11.33: Semiconductor Memory Notes

Figure 11.33 is an example of semiconductor memory technology used in computers.
With the rapid growth in the requirement for semiconductor memories there have been a number
of technologies and types of memory that have emerged. Names such as ROM, RAM, EPROM,
EEPROM, Flash memory, DRAM, SRAM, SDRAM, and the very new MRAM can now be seen
in the electronics literature. Each one has its own advantages and area in which it may be used.
11.5.1 Types of Semiconductor Memory

Electronic semiconductor memory technology can be split into two main types or categories,
according to the way in which the memory operates:

Random Access Memory (RAM)
As the names suggest, the RAM or random access memory is a form of semiconductor memory
technology that is used for reading and writing data in any order as required. It is used for such
applications as the computer or processor memory where variables and other stored and are
required on a random basis. Data is stored and read many times to and from this type of memory.
Read Only Memory (ROM)
A ROM is a form of semiconductor memory technology used where the data is written once and
then not changed. In view of this it is used where data needs to be stored permanently, even when
the power is removed—many memory technologies lose the data once the power is removed.
As a result, this type of semiconductor memory technology is widely used for storing programs
and data that must survive when a computer or processor is powered down. For example the
BIOS of a computer will be stored in ROM. As the name implies, data cannot be easily written
to ROM. Depending on the technology used in the ROM, writing the data into the ROM initially
may require special hardware. Although it is often possible to change the data, this gain requires
special hardware to erase the data ready for new data to be written in.
Both of these categories of semiconductor technology are widely used. Each type being used in
different areas and applications within microprocessor based electronics systems.
11.5.2 Semiconductor Memory Technologies

There is a large variety of types of ROM and RAM that are available. These arise from the variety
of applications and also the number of technologies available. This means that there is a large
number of abbreviations or acronyms and categories for memories ranging from Flash to MRAM,
PROM to EEPROM, and many more.

PROM
This stands for Programmable Read-Only Memory. It is a semiconductor memory which can only
have data written to it once the data written to it is permanent. These memories are bought in a
blank format and they are programmed using a special PROM programmer. Typically a PROM
will consist of an array of fusible links some of which are “blown” during the programming
process to provide the required data pattern.

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