<< Chapter < Page Chapter >> Page >

Almost all fast memories used today are semiconductor-based. Magnetic core memory is still used in applications where radiation “hardness” — resistance to changes caused by ionizing radiation — is important. They come in two flavors: dynamic random access memory (DRAM) and static random access memory (SRAM). The term random means that you can address memory locations in any order. This is to distinguish random access from serial memories, where you have to step through all intervening locations to get to the particular one you are interested in. An example of a storage medium that is not random is magnetic tape. The terms dynamic and static have to do with the technology used in the design of the memory cells. DRAMs are charge-based devices, where each bit is represented by an electrical charge stored in a very small capacitor. The charge can leak away in a short amount of time, so the system has to be continually refreshed to prevent data from being lost. The act of reading a bit in DRAM also discharges the bit, requiring that it be refreshed. It’s not possible to read the memory bit in the DRAM while it’s being refreshed.

SRAM is based on gates, and each bit is stored in four to six connected transistors. SRAM memories retain their data as long as they have power, without the need for any form of data refresh.

DRAM offers the best price/performance, as well as highest density of memory cells per chip. This means lower cost, less board space, less power, and less heat. On the other hand, some applications such as cache and video memory require higher speed, to which SRAM is better suited. Currently, you can choose between SRAM and DRAM at slower speeds — down to about 50 nanoseconds (ns). SRAM has access times down to about 7 ns at higher cost, heat, power, and board space.

In addition to the basic technology to store a single bit of data, memory performance is limited by the practical considerations of the on-chip wiring layout and the external pins on the chip that communicate the address and data information between the memory and the processor.

Access time

The amount of time it takes to read or write a memory location is called the memory access time . A related quantity is the memory cycle time . Whereas the access time says how quickly you can reference a memory location, cycle time describes how often you can repeat references. They sound like the same thing, but they’re not. For instance, if you ask for data from DRAM chips with a 50-ns access time, it may be 100 ns before you can ask for more data from the same chips. This is because the chips must internally recover from the previous access. Also, when you are retrieving data sequentially from DRAM chips, some technologies have improved performance. On these chips, data immediately following the previously accessed data may be accessed as quickly as 10 ns.

Access and cycle times for commodity DRAMs are shorter than they were just a few years ago, meaning that it is possible to build faster memory systems. But CPU clock speeds have increased too. The home computer market makes a good study. In the early 1980s, the access time of commodity DRAM (200 ns) was shorter than the clock cycle (4.77 MHz = 210 ns) of the IBM PC XT. This meant that DRAM could be connected directly to the CPU without worrying about over running the memory system. Faster XT and AT models were introduced in the mid-1980s with CPUs that clocked more quickly than the access times of available commodity memory. Faster memory was available for a price, but vendors punted by selling computers with wait states added to the memory access cycle. Wait states are artificial delays that slow down references so that memory appears to match the speed of a faster CPU — at a penalty. However, the technique of adding wait states begins to significantly impact performance around 25?33MHz. Today, CPU speeds are even farther ahead of DRAM speeds.

The clock time for commodity home computers has gone from 210 ns for the XT to around 3 ns for a 300-MHz Pentium-II, but the access time for commodity DRAM has decreased disproportionately less — from 200 ns to around 50 ns. Processor performance doubles every 18 months, while memory performance doubles roughly every seven years.

The CPU/memory speed gap is even larger in workstations. Some models clock at intervals as short as 1.6 ns. How do vendors make up the difference between CPU speeds and memory speeds? The memory in the Cray-1 supercomputer used SRAM that was capable of keeping up with the 12.5-ns clock cycle. Using SRAM for its main memory system was one of the reasons that most Cray systems needed liquid cooling.

Unfortunately, it’s not practical for a moderately priced system to rely exclusively on SRAM for storage. It’s also not practical to manufacture inexpensive systems with enough storage using exclusively SRAM.

The solution is a hierarchy of memories using processor registers, one to three levels of SRAM cache, DRAM main memory, and virtual memory stored on media such as disk. At each point in the memory hierarchy, tricks are employed to make the best use of the available technology. For the remainder of this chapter, we will examine the memory hierarchy and its impact on performance.

In a sense, with today’s high performance microprocessor performing computations so quickly, the task of the high performance programmer becomes the careful management of the memory hierarchy. In some sense it’s a useful intellectual exercise to view the simple computations such as addition and multiplication as “infinitely fast” in order to get the programmer to focus on the impact of memory operations on the overall performance of the program.

Questions & Answers

Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
AMJAD
what is system testing
AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
Hello
Uday
I'm interested in Nanotube
Uday
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
Prasenjit
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Good
Got questions? Join the online conversation and get instant answers!
QuizOver.com Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, High performance computing. OpenStax CNX. Aug 25, 2010 Download for free at http://cnx.org/content/col11136/1.5
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'High performance computing' conversation and receive update notifications?

Ask