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| Chapter 11: Exploiting Memory(RAM and ROM)
What is RAM and ROM?
RAM: RAM is a type of volatile memory. Data in RAM is not permanently written. When you
power off your computer the data stored in RAM is deleted.
ROM: ROM is a type of
non- volatile memory. Data in ROM is permanently written and is not
erased when you power off your computer.
What do we want in a
Memory?
What do we want in a
Memory?
What we REALLY want: A
BIG, FAST memory!
v
Keep everything within instant
access
We’d like to have a memory system
that
v
Performs like 2-10 GB of fast SRAM
typical SRAM sizes are in MB, not
GB
v
Costs like 1-4 GB of DRAM (slower)
typical DRAMs are order of
magnitude slower than SRAM
v
SURPRISE: We can (nearly) get our
wish!
Key Idea: Use a hierarchy of
memory technologies
Key Idea
v Key
idea: Exploit “Principle of Locality”
ü
Keep data used often in a small
fast SRAM
Ø
Called ‘CACHE’, often on the same
chip as the CPU
ü
Keep all data in a bigger but
slower DRAM
Ø
Called ‘main memory” , usually separate chip
ü
Access Main Memory only rarely ,
for remaining data
ü
The reason this strategy works :
LOCALITY
Ø
If you access something now, you
will likely access it again(or its neighbors) soon.
Cache Analogy
“A cache is like a desk.
Picking up a document from the desk takes very little time. Getting it from the
filing cabinet takes a lot longer.”
(by: Sri
Parameswaran et al)
"A cache is like a
smart librarian. When a patron asks for a Harry Potter book, the librarian goes
looking for it on the shelves (i.e., the hard drive) and comes back with it
along with the other Harry Potter books that were filed around it. This
material is kept in a special place behind the desk (cache memory) so that when
other patrons ask for a Harry Potter book, the librarian can retrieve it much
more quickly. It's an efficient system indeed."
(by: Larry Schneider)
Internal Memory
What is internal memory?
There are many types of circuits consisting of internal memory
computer, including RAM and ROM. Different from the disk used for data storage
and memory is often called hard type of memory function together to run a
computer system. The most common is the RAM or ROM that provides temporary
storage for the work it is current. Other types of memory is read-only.
In general, the
basic forms of memory are consists :
Ø physical
memory
Ø virtual
memory and
Ø caches.
v Memory
is a solid-state digital device that provides storage for data values.
v It
also known as memory cell which has properties:
v Exhibit
two stable states ( represent binary 1 and 0)
v Capable
of being written into (at least once), to set
v the
state
v Capable
of being read to sense the state
Exploiting the Memory Hierarchy
Approach 1 (Cray,
others): Expose Hierarchy
Ø Registers,
Main Memory, Disk each available as explicit
storage alternatives
Ø Tell
programmers: “Use them cleverly”
Approach 2: Hide
Hierarchy
Ø Programming
model: SINGLE kind of memory, single
address
space.
Ø Transparent
to programmer: Machine AUTOMATICALLY
assigns locations, depending on runtime usage patterns.
CPU speed is
dominated by memory performance
TRICK #1: Make slow MAIN MEMORY
appear faster
TRICK #2: Make small MAIN MEMORY
appear bigger
TYPICAL MEMORY REFERRENCE PATTERN
Memory
Trace
Ø A temporal sequence of memory references (addresses) from a real program.
Temporal
Locality
Ø If an item is referenced, it will tend to be referenced again soon
Spatial
Locality
Ø If an item is referenced, nearby items will tend to be referenced
soon.
Memory Cell Operation 
The functional
terminals listed below capable of
carrying an
electrical signal.
Select terminal
- to select a memory cell for a read or write operation
Control terminal - to
indicate read / write
Vollatile
and Non-volatile Memory
Ø Volatile
memory, also known as volatile storage, is computer memory that requires power
to maintain the stored information, in other words it needs power to reach the computer
memory. Volatile memory retains the information as long as power supply is on,
but when power supply is off or interrupted the stored memory is lost. Eg :
RAM;
Ø non-volatile
memory does not require a maintained power supply. It has been less popularly
known as temporary memory. Eg : flash memory, magnetic disk, optical disk
Semiconductor
Memory
Definition
Semiconductor memory is an electronic data storage device, often
used as computer memory, implemented on a semiconductor-based integrated circuit. There are many different
types of implementations using various technologies.
Type
v RAM (Random Access Memory)
Ø Misnamed as all semiconductor memory is random access
Ø Read/Write
Ø Volatile
Ø Temporary storage
Ø Static or dynamic
RAM - KEYWORD
v CMOS – Complementary Metal Oxide Semiconductor
v DRAM – Dynamic RAM
v FPD DRAM – Fast Page DRAM
v EDO DRAM - Extended Data Out DRAM
v SDRAM – Synchronous DRAM
v DDRAM – Double Data Rate DRAM
v SRAM – Static RAM
RAM- Brief Into 1 of 6
SRAM:
Static random access memory uses multiple transistors, typically four to
six,
for each memory cell but doesn't have a capacitor in each cell.
It is used primarily for cache.
DRAM:
Dynamic random access memory has memory cells with a paired transistor
and
capacitor requiring constant refreshing.
RAM – Brief Info 2 of 6
PM DRAM:
Fast page mode dynamic random access memory was the original form of
DRAM.
It waits through the entire process of locating a bit of data by column
and
row and then reading the bit before it starts on the next bit.
Maximum transfer rate to L2 cache is approximately 176 MBps.
EDO DRAM:
Extended data-out dynamic random access memory does not wait
for all of the processing of the first bit before continuing to the next
one.
As soon as the address of the first bit is located,
EDO DRAM begins looking for the next bit.
It is about five percent faster than FPM.
Maximum transfer rate to L2 cache is approximately 264 MBps.
SDRAM:
Synchronous dynamic random access memory takes advantage
of the burst mode concept to greatly improve performance.
It does this by staying on the row containing the requested bit and
moving rapidly through the columns, reading each bit as it goes.
The idea is that most of the time the data needed by the CPU will be in
sequence.
SDRAM is about five percent faster than EDO RAM and
is the most common form in desktops today.
Maximum transfer rate to L2 cache is approximately 528 MBps
RAM – Brief 3 of 6
DDR SDRAM:
Double data rate synchronous dynamic RAM is just like SDRAM except that
is has higher bandwidth, meaning greater speed.
Maximum transfer rate to L2 cache is
approximately 1,064 MBps (for DDR SDRAM 133 MHZ).
RDRAM:
Rambus dynamic random access memory is a radical departure from the
previous DRAM architecture.
Designed by Rambus, RDRAM uses a Rambus in-line memory module (RIMM),
which is similar in size and pin configuration to a standard DIMM.
What makes RDRAM so different is its use of a special high-speed data bus
called the Rambus channel.
RDRAM memory chips work in parallel to achieve a data rate of 800 MHz,
or 1,600 MBps.
Since they operate at such high speeds, they generate much more heat.
To help dissipate the excess heat Rambus chips are fitted with a heat
spreader,
which looks like a long thin wafer.
Just like there are smaller versions of DIMMs,
there are also SO-RIMMs, designed for notebook computers.
RAM – Brief Info 4 of 6
Credit Card Memory:
Credit card memory is a proprietary self-contained DRAM memory module
that plugs into a special slot for use in notebook computers.
PCMCIA Memory Card:
Another self-contained DRAM module for notebooks, cards type are not
proprietary
and should work with any notebook computer
whose system bus matches the memory card's configuration.
CMOS RAM:
CMOS RAM is a term for the small amount of memory used by your computer
and some other devices to remember things like hard disk settings
This memory uses a small battery to provide it with the power it needs
to maintain the memory contents.
VRAM:
VideoRAM, also known as multiport dynamic random access memory (MPDRAM),
is a type of RAM used specifically for video adapters or 3-D
accelerators.
The "multiport" part comes from the fact that VRAM normally
has two independent access ports instead of one, allowing the CPU and
graphics processor
to access the RAM simultaneously.
VRAM is located on the graphics card and comes in a variety of formats.
The amount of VRAM is a determining factor in the resolution and color
depth of the display.
VRAM is also used to hold graphics-specific information
such as 3-D geometry data and texture maps.
True multiport VRAM tends to be expensive, so today, many graphics cards
use
SGRAM (synchronous graphics RAM) instead.
Performance is nearly the same, but SGRAM is cheaper.
RAM – Variation 6 to 6
SIMM,
Single in-line memory module.
This memory
board used a 30-pin connector and about 3.5 x .75 inches in size
(about 9 x 2
cm).
In most
computers, had to install SIMMs in pairs of equal capacity and speed.
DIMM. Dual
in-line memory module
With 168-pin or
184-pin connector and a size of 5.4 x 1 inch (about 14 x 2.5 cm),
DIMMs range in
capacity from 8 MB to 1 GB per module and can be installed singly
instead of in
pairs.
SODIMM. Small
outline dual in-line memory module
Many brands of
notebook computers use proprietary memory modules, but
several
manufacturers use RAM based on the configuration.
SODIMM cards are
small, about 2 x 1 inch (5 x 2.5 cm), and have 144 or 200 pins.
Capacity ranges
from 16 MB to 1 GB per module.
To conserve
space, the Apple iMac desktop computer uses it
Instead of the
traditional DIMMs
Type
of ROM
This type of ROM can be re-programmed
by using a special device called a PROM programmer. Generally, a PROM can only
be changed/updated once.
This type of ROM can have its contents
erased by ultraviolet light and then reprogrammed by an RPROM programmer. This
procedure can be carried out many times; however, the constant erasing and
rewriting will eventually render the chip useless.
This type of ROM works in a similar way
to Flash memory in that it can its contents can be 'flashed' for erasure ad
then written to without having to remove the chip from its environment. EEPROMs
are used to store a computer system's BIOS, and can be updated without
returning the unit to the factory. In many cases, BIOS updates can be carried
out by computer users wishing a BIOS update.
Cache Memory
v Cache Memory
·
The level of the memory hierarchy
closest to the CPU
v Given accesses X1, …, Xn–1, Xn
before reference to the Xn
after the reference to Xn
Direct Mapped Cache
v Location
determined by address
v Direct
mapped: only one choice
§ (Block
address) modulo (#Blocks in cache)
Cache
Example
v 8-blocks, 1 word/block, direct mapped
v Initial state
(1)
(2)
(3)
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