R. H. KING ACADEMY
INFORMATION TECHNOLOGY
TEE2O COURSE
COMPUTER MEMORY AND STORAGE DEVICES
GLOSSARY OF TERMS
CACHE
Pronounced cash, a special high-speed storage mechanism. It can be either a reserved section of main
memory or an independent high-speed storage device. Two types of caching are commonly used in
personal computers: memory caching and disk caching. A memory cache, sometimes called a cache
store or RAM cache, is a portion of memory made of high-speed static RAM (SRAM) instead of the
slower and cheaper dynamic RAM (DRAM) used for main memory. Memory caching is effective because
most programs access the same data or instructions over and over. By keeping as much of this
information as possible in SRAM, the computer avoids accessing the slower DRAM. Some memory
caches are built into the architecture of microprocessors. The Intel 80486 microprocessor, for example,
contains an 8K memory cache, and the Pentium has a 16K cache. Such internal caches are often called
Level 1 (L1) caches. Most modern PCs also come with external cache memory, called Level 2 (L2)
caches. These caches sit between the CPU and the DRAM. Like L1 caches, L2 caches are composed of
SRAM but they are much larger. Disk caching works under the same principle as memory caching, but
instead of using high-speed SRAM, a disk cache uses conventional main memory. The most recently
accessed data from the disk (as well as adjacent sectors) is stored in a memory buffer. When a program
needs to access data from the disk, it first checks the disk cache to see if the data is there. Disk caching
can dramatically improve the performance of applications, because accessing a byte of data in RAM can
be thousands of times faster than accessing a byte on a hard disk.
EDO DRAM
Short for Extended Data Out Dynamic Random Access Memory, a type of DRAM that is faster than
conventional DRAM. Unlike conventional DRAM which can only access one block of data at a time, EDO
RAM can start fetching the next block of memory at the same time that it sends the previous block to the
CPU.
SDRAM
Short for Synchronous DRAM, a type of DRAM that can run at much higher clock speeds than
conventional memory. SDRAM actually synchronizes itself with the CPU's bus and is capable of running
at 133 MHz, about three times faster than conventional FPM RAM, and about twice as fast EDO DRAM
and BEDO DRAM. SDRAM is replacing EDO DRAM in many newer computers.
EPROM
Acronym for erasable programmable read-only memory, and pronounced ee-prom, EPROM is a special
type of memory that retains its contents until it is exposed to ultraviolet light. The ultraviolet light clears its
contents, making it possible to reprogram the memory. To write to and erase an EPROM, you need a
special device called a PROM programmer or PROM burner. An EPROM differs from a PROM in that a
PROM can be written to only once and cannot be erased. EPROMs are used widely in personal
computers because they enable the manufacturer to change the contents of the PROM before the
computer is actually shipped. This means that bugs can be removed and new versions installed shortly
before delivery.
EEPROM
Pronounced double-ee-prom or e-e-prom, short for electrically erasable programmable read-only
memory. EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge.
Like other types of PROM, EEPROM retains its contents even when the power is turned off. Also like
other types of ROM, EEPROM is not as fast as RAM. EEPROM is similar to flash memory (sometimes
called flash EEPROM). The principal difference is that EEPROM requires data to be written or erased
1
R. H. KING ACADEMY
INFORMATION TECHNOLOGY
TEE2O COURSE
one byte at a time whereas flash memory allows data to be written or erased in blocks. This makes flash
memory faster.
FLASH MEMORY
A special type of EEPROM that can be erased and reprogrammed in blocks instead of one byte at a time.
Many modern PCs have their BIOS stored on a flash memory chip so that it can easily be updated if
necessary. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in modems
because it enables the modem manufacturer to support new protocols as they become standardized.
FRAM
Short for Ferroelectric Random Access Memory, a type of non-volatile memory developed by Ramtron
International Corporation. FRAM combines the access speed of DRAM and SRAM with the non-volatility
of ROM. Because of its high speed, it is replacing EEPROM in many devices. The term FRAM itself is a
trademark of Ramtron.
MAIN MEMORY
Refers to physical memory that is internal to the computer. The word main is used to distinguish it from
external mass storage devices such as disk drives. Another term for main memory is RAM. The computer
can manipulate only data that is in main memory. Therefore, every program you execute and every file
you access must be copied from a storage device into main memory. The amount of main memory on a
computer is crucial because it determines how many programs can be executed at one time and how
much data can be readily available to a program. Because computers often have too little main memory
to hold all the data they need, computer engineers invented a technique called swapping, in which
portions of data are copied into main memory as they are needed. Swapping occurs when there is no
room in memory for needed data. When one portion of data is copied into memory, an equal-sized
portion is copied (swapped) out to make room. Now, most PCs come with a minimum of 32 megabytes of
main memory. You can usually increase the amount of memory by inserting extra memory in the form of
chips.
MASS STORAGE
Refers to various techniques and devices for storing large amounts of data. The earliest storage devices
were punched paper cards, which were used as early as 1804 to control silk-weaving looms. Modern
mass storage devices include all types of disk drives and tape drives. Mass storage is distinct from
memory, which refers to temporary storage areas within the computer. Unlike main memory, mass
storage devices retain data even when the computer is turned off. The main types of mass storage are:
 floppy disks : Relatively slow and have a small capacity, but they are portable,
inexpensive, and universal.
 hard disks : Very fast and with more capacity than floppy disks, but also more expensive.
Some hard disk systems are portable (removable cartridges), but most are not.
 optical disks : Unlike floppy and hard disks, which use electromagnetism to encode data,
optical disk systems use a laser to read and write data. Optical disks have very large
storage capacity, but they are not as fast as hard disks. In addition, the inexpensive
optical disk drives are read-only. Read/write varieties are expensive.
 tapes : Relatively inexpensive and can have very large storage capacities, but they do
not permit random access of data.
Mass storage is measured in kilobytes (1,024 bytes), megabytes (1,024 kilobytes), gigabytes (1,024
megabytes) and terabytes (1,024 gigabytes).
2
R. H. KING ACADEMY
INFORMATION TECHNOLOGY
TEE2O COURSE
MEMORY
Internal storage areas in the computer. The term memory identifies data storage that comes in the form
of chips, and the word storage is used for memory that exists on tapes or disks. Moreover, the term
memory is usually used as a shorthand for physical memory, which refers to the actual chips capable of
holding data. Some computers also use virtual memory, which expands physical memory onto a hard
disk. Every computer comes with a certain amount of physical memory, usually referred to as main
memory or RAM. You can think of main memory as an array of boxes, each of which can hold a single
byte of information. A computer that has 1 megabyte of memory, therefore, can hold about 1 million bytes
(or characters) of information.
There are several different types of memory:





RAM (random-access memory): This is the same as main memory. When used by itself,
the term RAM refers to read and write memory; that is, you can both write data into RAM
and read data from RAM. This is in contrast to ROM, which permits you only to read data.
Most RAM is volatile, which means that it requires a steady flow of electricity to maintain
its contents. As soon as the power is turned off, whatever data was in RAM is lost.
ROM (read-only memory): Computers almost always contain a small amount of readonly memory that holds instructions for starting up the computer. Unlike RAM, ROM
cannot be written to.
PROM (programmable read-only memory): A PROM is a memory chip on which you
can store a program. But once the PROM has been used, you cannot wipe it clean and
use it to store something else. Like ROMs, PROMs are non-volatile.
EPROM (erasable programmable read-only memory): An EPROM is a special type of
PROM that can be erased by exposing it to ultraviolet light.
EEPROM (electrically erasable programmable read-only memory): An EEPROM is a
special type of PROM that can be erased by exposing it to an electrical charge.
RAM
Pronounced ramm, acronym for random access memory, a type of computer memory that can be
accessed randomly; that is, any byte of memory can be accessed without touching the preceding bytes.
RAM is the most common type of memory found in computers and other devices, such as printers. There
are two basic types of RAM:
 dynamic RAM (DRAM)

static RAM (SRAM)
The two types differ in the technology they use to hold data, dynamic RAM being the more common type.
Dynamic RAM needs to be refreshed thousands of times per second. Static RAM does not need to be
refreshed, which makes it faster; but it is also more expensive than dynamic RAM. Both types of RAM are
volatile, meaning that they lose their contents when the power is turned off. In common usage, the term
RAM is synonymous with main memory, the memory available to programs. For example, a computer
with 8M RAM has approximately 8 million bytes of memory that programs can use. In contrast, ROM
(read-only memory) refers to special memory used to store programs that boot the computer and perform
diagnostics. Most personal computers have a small amount of ROM (a few thousand bytes). In fact, both
types of memory (ROM and RAM) allow random access. To be precise, therefore, RAM should be
referred to as read/write RAM and ROM as read-only RAM.
REGISTER
A, special, high-speed storage area within the CPU. All data must be represented in a register before it
3
R. H. KING ACADEMY
INFORMATION TECHNOLOGY
TEE2O COURSE
can be processed. For example, if two numbers are to be multiplied, both numbers must be in registers,
and the result is also placed in a register. (The register can contain the address of a memory location
where data is stored rather than the actual data itself.) The number of registers that a CPU has and the
size of each (number of bits) help determine the power and speed of a CPU. For example a 32-bit CPU is
one in which each register is 32 bits wide. Therefore, each CPU instruction can manipulate 32 bits of
data. Usually, the movement of data in and out of registers is completely transparent to users, and even
to programmers. Only assembly language programs can manipulate registers. In high-level languages,
the compiler is responsible for translating high-level operations into low-level operations that access
registers.
ROM
Pronounced rahm, acronym for read-only memory, computer memory on which data has been
prerecorded. Once data has been written onto a ROM chip, it cannot be removed and can only be read.
Unlike main memory (RAM), ROM retains its contents even when the computer is turned off. ROM is
referred to as being nonvolatile, whereas RAM is volatile. Most personal computers contain a small
amount of ROM that stores critical programs such as the program that boots the computer. In addition,
ROMs are used extensively in calculators and peripheral devices such as laser printers, whose fonts are
often stored in ROMs. A variation of a ROM is a PROM (programmable read-only memory). PROMs are
manufactured as blank chips on which data can be written with a special device called a PROM
programmer .
SRAM
Short for static random access memory, and pronounced ess-ram. SRAM is a type of memory that is
faster and more reliable than the more common DRAM (dynamic RAM). The term static is derived from
the fact that it doesn't need to be refreshed like dynamic RAM. While DRAM supports access times of
about 60 nanoseconds, SRAM can give access times as low as 10 nanoseconds. In addition, its cycle
time is much shorter than that of DRAM because it does not need to pause between accesses.
Unfortunately, it is also much more expensive to produce than DRAM. Due to its high cost, SRAM is often
used only as a memory cache.
PROM
Short for programmable read-only memory, a memory chip on which data can be written only once.
Once a program has been written onto a PROM, it remains there forever. Unlike RAM, PROMs retain
their contents when the computer is turned off. The difference between a PROM and a ROM (read-only
memory) is that a PROM is manufactured as blank memory, whereas a ROM is programmed during the
manufacturing process. To write data onto a PROM chip, you need a special device called a PROM
programmer or PROM burner. The process of programming a PROM is sometimes called burning the
PROM. An EPROM (erasable programmable read-only memory) is a special type of PROM that can be
erased by exposing it to ultraviolet light. Once it is erased, it can be reprogrammed. An EEPROM is
similar to a PROM, but requires only electricity to be erased.
USB flash drive
A small, portable flash memory card that plugs into a computer’s USB port and functions as a portable
hard drive with up to 2GB of storage capacity. USB flash drives are touted as being easy-to-use as they
are small enough to be carried in a pocket and can plug into any computer with a USB drive. USB flash
drives have less storage capacity than an external hard drive, but they are smaller and more durable
because they do not contain any internal moving parts.
USB flash drives also are called pen drives, key drives or simply USB drives.
Source: http://pcwebopedia.com/TERM/m/main_memory.html
4