There are two major classifications of memory devices, depending on the manner of their storage of memory. Programs can only be read in Read-Only Memory (ROM), while programs can either be written or read on Random Access Memory (RAM).
Below is a diagram of memory hierarchy, to help you visualize memory arrangement in a computer.
Read-Only Memory (ROM)
One major type of memory that is used in PCs is called read-only memory, or ROM for short. ROM is a type of memory that normally can only be read, as opposed to RAM, which can be both read and written.
ROM is very important for the following reasons:
1. Permanence: The values stored in ROM are always there, whether the power is on or not. This is also called non-volatile memory
2. Security: Since ROM cannot easily be modified it provides a measure of security against accidental (or malicious) changes to its contents. It is also for this reason why the BIOS of the computers are stored in ROM.
There are also different types of ROM, depending on their relative modifiability:
· Regular ROM: the program it contains is hard-wired, that is, it is soldered directly on the chip
· Programmable ROM (PROM): this type of ROM can be programmed only once, usually by the manufacturers
· Erasable Programmable ROM (EPROM): An EPROM is a ROM that can be erased and reprogrammed. Ultraviolet light erases the program installed inside through the glass window on the top of the chip.
· Electrically Erasable Programmable ROM (EEPROM): this quite similar to EPROM, except that the erasing and programming are both done electrically.
Random Access Memory (RAM)
RAM, or random access memory, is used for holding programs and data being executed. It differs with ROM because data or program can also be written to it, not only read from it.
There are also different types of RAM, depending on the frequency of memory refresh:
1. Static RAM (SRAM)
Static RAM is a type of RAM that holds its data without external refresh, for as long as power is supplied to the circuit. This is contrasted to dynamic RAM (DRAM), which must be refreshed many times per second in order to hold its data contents. SRAMs are used for specific applications within the PC, where their strengths outweigh their weaknesses compared to DRAM:
In contrast, SRAMs have the following weaknesses, compared to DRAMs:
These advantages and disadvantages taken together show that performance-wise, SRAM is superior to DRAM. Unfortunately, 32 MB of SRAM would be prohibitively large and costly, which is why DRAM is used for system memory. SRAMs are used instead for level 1 and level 2 cache memory, for which it is perfectly suited; cache memory needs to be very fast, and not very large.
2. Dynamic RAM (DRAM)
This type of RAM is called dynamic because the memory has to be refreshed several times in a second. A special circuit called the refresh circuit reads the content of the DRAM several times a second. This reading action refreshes the memory. Reading every "row" in the memory chip one row at a time does refreshing; the process of reading the contents of each capacitor in the memory circuit re-establishes the charge.
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