Data Storage
• All computers require some form of memory and
storage.
• Memory is usually referred to as the internal
devices used to store data that the computer can
access directly.
• This is also known as primary memory. This
memory can be the user’s workspace, temporary
data or data that is key to running the computer.
• Storage devices allow users to store applications,
data and files.
• The user’s data is stored permanently and they
can change it or read it as they wish.
• Storage needs to be larger than internal memory
since the user may wish to store large files (such
as music files or videos). Storage devices can
also be removable to allow data, for example, to
be transferred between computers.
• Removable devices allow a user to store
important data in a different location in case of
data loss.
• However, all of this removable storage has
become less important with the advent of
technology such as ‘data drop’ (which uses
Bluetooth) and cloud storage.
Memory and storage devices can be
split up into three distinct groups:
• primary memory
• secondary storage
• off-line storage.
The diagram shows how these all link
together:
Here is a summary of the differences between
primary memory and storage devices:
Primary memory
• Primary memory is the part of the computer
memory which can be accessed directly from
the CPU; this includes random access memory
(RAM) and read only memory (ROM) memory
chips.
• Primary memory allows the CPU to access
applications and services temporarily stored in
memory locations.
Random access memory (RAM)
• These memory devices are not really random;
this refers to the fact that any memory location
in RAM can be accessed independent of which
memory location was last used.
• When you run an application or program, data is
retrieved from secondary storage and placed
temporarily into RAM.
• Access time to locate data is much faster in RAM
than in secondary or off-line devices.
Features of RAM include:
• can be written to or read from, and the data
can be changed by the user or the computer
(i.e. it is a temporary memory)
• used to store data, files, part of an application
or part of the operating system currently in
use
• it is volatile, which means memory contents
are lost when powering off the computer.
• In general, the larger the size of RAM the
faster the computer will operate.
• In reality, the RAM never runs out of memory;
it continues to operate but just gets slower
and slower.
• As the RAM becomes full, the processor has to
continually access the hard disk drive to
overwrite old data on RAM with new data.
• By increasing the RAM size, the number of
times this access operation is carried out is
reduced, making the computer run faster.
• RAM is much faster to write to or read from
than other types of memory; but its main
drawback is its volatility.
There are currently two types of RAM
technology:
• dynamic ram (DRAM)
• static RAM (SRAM)
Dynamic RAM (DRAM)
• Each DYNAMIC RAM (DRAM) chip consists of a
number of transistors and capacitors. Each of
these parts is tiny since a single RAM chip will
contain millions of transistors and capacitors. The
function of each part is:
- capacitor – this holds the bits of information (0
or 1)
- transistor – this acts like a switch; it allows the
chip control circuitry to read the capacitor or
change the capacitor’s value.
DRAM
Capacitor
Transistor
• This type of RAM needs to be constantly
REFRESHED (that is, the capacitor needs to be
recharged every 15 microseconds otherwise it
would lose its value).
• 1 second is equal to 1,000,000 microsecond
• If it wasn’t refreshed, the capacitor’s charge
would leak away very quickly, leaving every
capacitor with the value 0.
DRAMs have a number of advantages
over SRAMs:
• they are much less expensive to manufacture
than SRAM
• they consume less power than SRAM
• they have a higher storage capacity than
SRAM.
Static RAM (SRAM)
• A big difference between SRAM and DRAM is that
this type of memory doesn’t need to be constantly
refreshed.
• It makes use of flip flops, which hold each bit of
memory.
• SRAM is much faster than DRAM when it comes to
data access (typically, access time for SRAM is 25
nanoseconds and for DRAM is 60 nanoseconds).
• 1 microsecond is equal to 1,000 nanoseconds
• DRAM is the most common type of RAM used
in computers, but where absolute speed is
essential, then SRAM is the preferred
technology.
• For example, the processor’s MEMORY CACHE
is the high speed portion of the memory; it is
effective because most programs access the
same data or instructions many times.
• By keeping as much of this information as
possible in SRAM, the computer avoids having
to access the slower DRAM.
SRAM Memory
Differences between DRAM and SRAM
Read Only Memory (ROM)
• Another form of primary memory is read-only
memory (ROM). This is similar to RAM in that
it shares some of its properties, but the main
difference is that it cannot be changed or
written to.
The main features of READ ONLY MEMORY
(ROM) can be summarised as follows:
• they are non-volatile/permanent memories
(the contents of the memory remain even
when the power to the ROM is turned off)
• they are often used to store the start-up
instructions when the computer is first
switched on (for example, ROM might store
the basic input/output system (BIOS))
• the data or contents of a ROM chip can only
be read; they cannot be changed.
Here is a summary of the main
differences between RAM and ROM:
Application
We will now consider an application, other
than a computer, where both RAM and ROM
chips are used.
A remote-controlled toy car has a circuitry
which contains both RAM and ROM chips. The
remote control is a hand-held device.
We will consider the function of each
type of memory independently:
• ROM
- stores the factory settings such as remote
control frequencies
- stores the ‘start-up’ routines when the toy
car is first switched on
- stores the set routines; for example, how the
buttons on the hand-held device control
turning left, acceleration, stopping, and so on.
• RAM
- the user may wish to program in their own
routines; these new instructions would be
stored in the RAM chip
- the RAM chip will store the data/instructions
received from the remote control
