Memory Organization
Memory System
A memory system is a very simple system exhibits a wide range of technology
and types.
Random Access Memory (RAM)
RAM is the place in a computer where the operating system, application
programs, and data in current usage are kept so that they can be quickly
accessed by the processor. RAM reads and writes faster than the other kinds
of storage in a computer like the hard disk, floppy disk, and co-rom.
However, the data in RAM is stored temporarily till the computer is running.
When the power of the computer cut then RAM loses its data. When you switch
on your computer, the operating system and other files are once again loaded
into RAM, usually from your hard disk.
RAM is known as ‘random access memory’ because any storage location can be
accessed randomly. RAM is organized and controlled in a way that enables
data to be stored and retrieved directly to specific locations. In addition
to disk, floppy disk, and CD-ROM (Compact Disk – Read Only Memory) storage,
another important from of storage is read-only memory (ROM), a more
expensive kind of memory that retains data when the computer is turned off.
Every computer comes with a small amount of ROM that holds just enough
programming so that the operating system can be loaded into RAM each time
the computer is turned on.
The Size of RAM
RAM is small, both in physical size (it is stored in microchip modules) and
for the data that it can hold. It is much smaller than your hard disk. A
typical computer may come with 32 million bytes of RAM and a hard disk that
can hold 4 billion bytes. RAM comes in the form of ‘discrete’ (meaning
separate) microchip modules that are plugged in the holed in the computer’s
motherboard. These holes connect through a bus or set of electrical paths to
the processor. The hard disk, on the other hand, stores data on a magnetized
surface that looks like a phonograph record.
Today, personal computers are available with 16 or more megabytes of RAM,
usually increasing in multiples of 8 megabytes. Most personal computers are
designed to allow you to add additional RAM modules up to a certain limit
(for example, up to 64, 128, or 256 megabytes). Having more RAM in your
computer reduces the number of times that the computer processor has to read
data in from your hard disk, an operation that takes much longer than
reading data from RAM. (RAM access time is expressed in nanoseconds; hard
disk access time is expressed in milliseconds.).
Kinds of RAM
RAM is of only two kinds, i.e. Dynamic and static. Which stores every kind
of data and makes it quickly accessible to a microprocessor? However, there
is some special RAM that can be both dynamic and static like video RAM.
Which stores data intended for the display screen, enabling images to girt
display faster.
- Flash RAM
- Video RAM
- Static RAM (SRAM)
- Dynamic RAM (DRAM)
Dynamic RAM (DRAM)
Dynamic RAM uses a kind of capacitor that needs frequent power refreshing
to retain its charge. Because reading a DRAM discharges its contents, a
power refresh is required after each read. RDAM must be refreshed about
every 15 microseconds. DRAM is the least expensive kind of RAM. Synchronous
DRAM (SDRAM) is a generic name for various kinds of DRAM that are
synchronized with the clock speed that the microprocessor is optimized.
Synchronization tends to increase the number of instructions that the
processor can perform in a given time. The speed of SDRAM is rated in MHz
rather than in nanoseconds (ns). This makes it easier to compare the bus
speed and the RAM chip speed. You can convert the RAM clock speed to
nanoseconds (ns) by dividing the chip speed into 1 billion ns (which is
equal to one second.) For Ex-, an 83 MHz RAM would be equivalent to 12
ns.
Static RAM (SRAM)
Static RAM is more expensive, requires four times the amount of space for a
given amount of data than dynamic RAM. Unlike, dynamic RAM, it does not need
to be power-refreshed and is therefore faster to access. One source gives a
typical access time as 25 nanoseconds in contrast to an access time of 60
nanoseconds for dynamic RAM.
Video RAM
Video RAM as the name signifies is a general form of RAM used to store
image data for the video display monitor. All types of video TAM are special
arrangements of dynamic RAM (DRAM). Video RAM is really a buffer between the
processor and the display monitor and it’s often known as the frame buffer.
When images are to be sent to the display, they are first read by the
processor and then written to video TAM. From video TAM (the frame buffer),
the data ties converted by a RAM digital-to-analog converted (RAMDAC) into
analog signals that are sent to the display presentation mechanism such as a
cathode ray tube (CRT). Usually, video RAM comes in a 1 or 2 megabyte
package and is located on the video or graphics card in the computer.
Flash RAM
Flash RAM sometimes called flash memory is a type of constantly powered
non-volatile memory that can be erased and reprogrammed in units of memory
called blocks. It is a variation of electrically erasable programmable
read-only memory (EEPEOM), which unlike flash memory, is erased and
rewritten at the byte level. Flash memory, is erased and rewritten at the
byte level. Flash memory is often used to hold control code such as the
basic input/output system (BIOS) in a personal computer. When BIOS needs to
be changed or rewritten, the flash memory can be written in block (rather
than byte) sizes, making it easy to update. On the other hand flash memory
is not useful as random access memory (RAM) because RAM needs to be addressable at the byte (not the block) level.
Flash memory derives its name because the microchip is organized in a way
so that a section of memory cells are erased in a single action or ‘flash’.
Flash memory is used in digital cellular phones, digital cameras, LAN
switches, PC cards for notebook computer, digital set-up boxes, embedded
controllers, and other devices.
Read Only Memory (ROM)
Read only memory is most commonly used to store system –level programs that
need to be available to the PC at all times? The most common example is the
system BIOS program, which is stored in a ROM and is used to boot up the
system. When you first turn on the PC the system memory is empty, so there
has to be something for the PC to use when it starts up. These programs are
stored at the manufacturing time and could not be changed later on.
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| DRAM SRAM Video RAM Flash RAM |
Kinds of ROM
The contents of a ROM cannot be changed, there are times when being able to
change the contents of a ROM becomes very useful. There are several ROM
variants that can be changed under certain circumstances: these can be
thought of as mostly read only. The following are the different types of ROM
with a description of their relative modifiable:
- Read Only Memory (ROM)
- Programmable ROM (PROM)
- Erasable Programmable ROM (EPROM)
- Electrically Erasable Programmable ROM (EEPROM)
Read Only Memory (ROM)
A regular ROM is constructed from hard-wired logic, encoded in the silicon
itself, similar to a processor. It is designed to perform a specific
function and cannot be changed. This is inflexible and so regular ROM are
only used generally for programs that are static (not changing often) and
mass-produced.
Programmable ROM (PROM)
A type of ROM that can be programmed using special equipment; it can be
written to, but only once. It is useful for companies that make their own
ROM for the software they write. This is because when they change their code
they can create new proms without requiring expensive equipment. This is
similar to the way s CO-ROM reorder works by letting you create or write
programs onto bland one and then letting you read from them many times, in
fact, programming a PROM is also called burning a CD-R, and it is comparable
in terms of its flexibility.
Erasable Programmable ROM (EPROM)
An EPROM is a ROM that can be erased and reprogrammed. A little glass
window is installed at the top of the ROM package, through which you can
actually see the chip that holds the memory. Ultraviolet light of a specific
frequency can be shined through this window for a specified period of time,
which will erase the EPROM and allow it to be reprogrammed again. Obviously,
RPROM is much more useful than a regular PROM, but it does require the
erasing light. This technology is analogous to a reusable CD-RW (Compact
Disk – Read Write).
Electrically Erasable Programmable ROM (EEPROM)
The next level of erase ability is the EEPROM, which can be erased under software control. This it’s the most flexible type of ROM, and it’s now commonly used for holding BIOS programs. The term flashes BIOS or doing BIOS upgrade by flashing refers to reprogramming the BIOS EEPROM with a special software program.
Question from memory organization chapter:
- What is memory organization?
- What is memory system?
- What is RAM?
- What is full form of RAM?
- What is random access memory?
- What is the size of ram?
- Types of ram
- How many types of RAM?
- How many types of random access memory?
- What dynamic ram
- What is full form of DRAM?
- What is dynamic random access memory?
- What is SDRAM?
- What is synchronous DRAM?
- What is synchronous dynamic random access memory?
- What is full form of SDRAM?
- What static ram
- What is full form of SRAM?
- What is static random access memory?
- What is video RAM?
- What is flash RAM?
- What is ROM?
- What is read only memory?
- Types of ROM
- How many types of ROM?
- How many types of read only memory?
- What is PROM?
- What is full form of PROM?
- What is programmable ROM?
- What is full form of programmable ROM?
- What is programmable read only memory?
- What is EPROM?
- What is full form of EPROM?
- What is erasable programmable ROM?
- What is full form of erasable programmable ROM?
- What is erasable programmable read only memory?
- What is EEPROM?
- What is full form of EEPROM?
- What is electrically erasable programmable ROM?
- What is full form of electrically erasable programmable ROM?
- What is electrically erasable programmable read only memory?