Memory and Storage

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Memory and Storage

• Aldon Tom

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What is Memory?
Memory is a solid-state digital device that
stores data values. Memory holds running programs
and the data that those programs use. It also
stores data that is not currently being used,
although most memory falls into the category of
temporary storage.
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The Key Aspects of Memory
There are two basic aspects of memory

• Technology

The properties of the underlying hardware
that are used to construct the memory system.

• Organization

The format of the memory system.
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Memory Volatility
Memory is considered volatile if the
contents of the memory disappears when the power
is shut off. RAM (not magnetic) is a common
example of volatile memory. Nonvolatile
memory is the exact opposite. Its contents remain
even after power is shut off. An example would be
your hard drive.
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Nonvolatile Memory (cont)
Nonvolatile memory is usually much larger than
volatile memory -Additionally nonvolatile memory
is in the form of rotating disks -hard disks,
floppy disks, CDs, etc -it is also usually
much slower, but much cheaper Data access
must wait for seek time, which is the
positioning of the head. It also must wait
for rotational latency.
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Types of Memory Access
The most common types of memory is random access.
That is to say that any value in the memory can
be accessed at any point in time. The
alternative to random access is sequential
access. Sequential access memory must be read in
the same order that they were inputted. An
example is the FIFO First-In-First-Out
method. There are also methods that combine both
forms of access in various ways.
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Just what is a memory chip made of? Well each
chip is an integrated circuit made up of millions
of transistors and capacitors. Capacitors are
paired with transistors to make a memory cell
which holds one bit and can thus store a 1 or 0
by filling or emptying the capacitor. There are
still many other forms of memory, but this is the
most common form of RAM.
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Each memory cell has an address. This is how it
can be randomly accessed. The cells are then
further divided into a grid layout. This is why
your RAM modules have something called RAS and
CAS delays. The delays are time it takes for the
Row Access Select and Column Access Select to
execute between cycles. Then the system fetches
what is stored at a particular address and then
it stores it to a register for use in
calculations.
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Permanence of Values
Memory is characterized by whether it can be
read, written, or both. The primary form of
memory can be both read and written at anytime.
There are some types of memory that are more
permanent, namely ROM Read Only Memory
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Types of ROM
The standard ROM contains values that can only
be accessed, but not changed. There is also
PROM Programmable ROM which allows data
values to be written only once, and then accessed
many times. Intermediate forms also exist. For
example, EEPROM Electronically Erasable
Programmable ROM. Although it is an oxymoron, it
allows data to be electronically burned into the
memory, but it takes a long time. A popular form
of EEPROM is Flash Memory, which is commonly
used in digital cameras. Although it is slow, the
memory works faster than a human can aim and
take a picture.
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Primary and Secondary Memory
Primary memory refers to the fast, volatile,
internal memory of the computer -RAM
-Cache Secondary memory refers to the slower,
nonvolatile, external storage usually provided
by electromechanical devices such as -Hard
Disk -CDs
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Memory Hierarchy
Primary and Secondary memory arose from the idea
of a Memory Hierarchy. This hierarchy came about
due to the economy of production. That is to say
the cost versus the performance. Research
revealed that using given budget, the best
performance is not achieved by using the same
type of memory throughout the system. It is most
efficient to use a very small amount of
the highest performance memory, which is backed
up by a slightly larger amount of a slightly
slower memory, and so on. This creates a sort of
pyramid or a funnel.
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The CPU accesses memory according to a
distinct hierarchy. Whether it comes from
permanent storage or input, it is all stored in
RAM first. The CPU then stores pieces of
information that it will need to access often in
the cache and then it maintains certain
instructions in registers.
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Virtual Memory
Disk Storage
RAM
CPU
Cache
In order to maximize performance with powerful
CPUs, the data has to be easy and quick to
access. If the CPU does not get the data it needs
it stops and waits for it, therefore you need
memory that can keep up with the CPU. The problem
is that memory fast enough to keep up with the
CPU is extremely expensive.
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The solution is to tier the memory as described
earlier. Using progressively cheaper memory to
back up each other provides maximal efficiency in
terms of price. From slowest to fastest -Hard
Drive -RAM -Cache
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Instruction and Data Store
The earliest computers used to have separate
memories for programs and data. Later, most
architects adopted the Von Neumann Architecture
which combines both into one memory. With the
advent of new memory technologies, the
separation of data and programs has come back
into use, but only in special-purpose systems.
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Memory used to hold a program is called
instruction store. Memory used to hold data is
called data store. The motivations for
separating data and instructions is
basically since the data for a program only needs
to be loaded less frequently, but the
instructions that utilize that data are
executed much more often. Therefore if both data
and instruction are both in the same memory, the
extra speed is useless for the data portion,
where as the instructions suffer from lack of
speed.
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By separating the data and instructions into
separate memories, faster memory can be used for
instructions, while slower memory can be used
for the data.
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Fetching and Storing
All memory uses a fetch-store architecture. That
is there are only two operations that memory
understands Read and Write This is also known
as a load-store architecture, or read-write,
but they all just refer to the same thing.
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Executing a Program
After the system loads the necessary data, all
that is really left is the instructions. The
system only retrieves new data a small percentage
of the time. The rest of the time is spent on
instructions -Each instruction is fetched from
memory -Decoded -Executed -Then next, where next
can be the next in the sequence or a jump