What is RAM and why do I need it?

What is RAM and why do I need it?
Adding memory (RAM) to your system is the cheapest and most effective way to make it run faster. You can think of RAM as an incredibly fast hard drive that stores information temporarily instead of permanently. When you start a program it is loaded from the hard drive into RAM. When a program is running in RAM it can run hundreds to thousands of times faster than it can if run directly from the hard drive. The problem is that the capacity of a standard hard drive is many times the size of a computer's RAM size, meaning it is possible to load so many programs that the RAM can no longer hold them. When that happens, your computer's virtual memory kicks in, and your system tries to read from your hard drive instead..
With the addition of more RAM, you'll be able to keep more programs open at once, speed up program launches, and experience fewer system crashes. Plus, it's a pretty easy upgrade to make, requiring little technical expertise.
So what is DDR RAM then?
DDR is the acronym for Double Data Rate Synchronous DRAM (SDRAM). DDR SDRAM memory technology has evolved from mainstream PC100 and PC133. This memory technology has enabled a new generation of higher performance computer systems, including desktops, workstations, servers, portables, as well as new communications products such as routers and switches. DDR memory technology is also widely used in high-performance graphic adapters.
DDR memory is clocked at the same speed as normal SDRAM (ie 100, 133mhz), The big difference between DDR and SDRAM memory is that DDR can transport double the amount of data it reads by utilizing both the rising and falling edges of the clock signal. SDRAM only carries information on the rising edge of a signal. Basically this allows the DDR module to transfer data twice as fast as SDRAM. For example, instead of a data rate of 133MHz, DDR memory transfers data at 266MHz. Double data rate (DDR) SDRAM gives you twice the bandwidth of standard SDRAM.
What is bandwidth?Actually, it's a pretty simple concept. Bandwidth is the amount of data you can use (reading, writing, etc.) during a given time period. Bandwidth is usually defined as the number of bits per second that your computer can handle.
Remember that your computer performs read and write operations redundantly. When you open a file, your computer doesn't just call it from the hard drive to your screen; it reads to and from the processor, RAM, cache, memory, and hard drive every time you access a document. Even if you're just using a word processing program to write a letter, you're moving a lot of data between the various parts of your computer that store information. Start working on Digital video editing or multiply this by hundreds of users and you can see why bandwidth becomes a much bigger deal for servers.
A P3 processor with a 133MHz front side bus (FSB) has a bandwidth of about 1.06GB (gigabytes) per second. If this was all that your memory had to support, PC133 SDRAM would probably be just fine. But, in addition to the supporting the processor, the memory also has to support the bandwidth of the Peripheral Connect Interface (PCI) bus as well as the newer high performance graphics cards using an Accelerated Graphics Port (AGPX4) and soon AGPX8 will be demanding a whopping 2133MB/s alone. . Newer CPU’s have increased this requirement even further, the AMD Athlon XP using a 266mhz FSB (DDR) and the P4 Northwood upto a 533mhz FSB (Quad-pumped) its not long before your memory is a huge bottleneck in your system and other components are sitting around waiting for it to catch up. This is why a higher bandwidth memory is required. Enter DDR SDRAM. The 266MHz DDR SDRAM (133MHz doubled) has the bandwidth of 2.1GB/sec and is referred to PC2100. The PC2700 2.7GB/s and PC3200 DDR upto 3.2GB/s. Future DDR standards aiming at upto 8.4GB/s (DDR533 Dual channel) with Quad channel also being worked on.
So how does it work?
As in standard SDRAM, DDR SDRAM is tied to the front-side bus clock in your system. The memory and bus execute instructions at the same time rather than one of them having to wait for the other. As bus speeds have increased, this has improved system performance. DDR modules, like their SDRAM predecessors, are called DIMMs. They use motherboard system designs similar to those used by SDRAM; however, DDR is not backward compatible with SDRAM-designed motherboards. DDR memory supports both ECC (error correction code, typically used in servers) and non-parity (used on desktops/laptops.)
DDR DIMMs currently support memory bus speeds of either 100 or 133MHz, with a faster memory bus speed of 166mhz reviewed by JEDEC and released by many of the leading memory manufacturers early in 2002. Since DDR technology can perform two data operations per clock cycle (versus SDRAM’s single operation per clock cycle), the effective data throughput of a DDR DIMM are doubled over an SDRAM DIMM. For example, a 200MHz DDR DIMM will support a 100MHz memory bus and a 266MHz DDR DIMM will support up to a 133MHz memory bus and 166mhz supporting 333mhz. A 200 MHz (400mhz DDR) has also been released but not yet Jedec approved.
An additional benefit of DDR is that it only uses 2.5 volts per signal, as opposed to the 3.3 volts used in current SDR SDRAM. This may not seem like much, but the less power required to drive the memory, the less drain on your power source. Less power also means longer battery life in laptops!
JEDEC Specifications The specifications for DDR DIMM modules are finalized by JEDEC. JEDEC is the semiconductor engineering standardization body of the Electronic Industries Alliance (EIA) with about 300 active member companies representing every segment of the industry.
The naming convention recommended by JEDEC is as follows: Memory chips are referred to by their native speed: 200 MHz DDR SDRAM memory chips are called DDR200 chips, and 266 MHz DDR SDRAM memory chips are called DDR266. DDR DIMM modules are referred to by their peak bandwidth, which is the maximum amount of data that can be delivered per second: A 200 MHz DDR DIMM is called a PC1600 DIMM, and a 266 MHz DDR DIMM is called a PC2100 DIMM. To illustrate this on a 266 MHz DDR DIMM module: Each module is 64 bits wide, or 8 Bytes wide (each byte = 8 bits). To get the transfer rate, multiply the width of the DIMM module (8 Bytes) by the rated speed of the memory module (in MHz): (8 Bytes) x (266 MHz/second) = 2,128 MB/second or about 2.1 Gigabytes/second. Therefore, the memory module is often referred to as PC2100. Similarly, a PC2700 DIMM module utilizes memory chips rated at 333 MHz.
JEDEC is already finalizing the DDR II spec, which will double the data transfer once more, using the quad-pumped technology that is common in AGP4x bus and the Pentium 4 bus. Speeds of 600-800mhz for DDR are not far away. Why Use DDR Technology? DDR memory technology is an evolutionary improvement of mature PC100 and PC133 SDRAM technology. DDR memory chips are produced by semiconductor manufacturers using existing wafer fabs, processes and testers resulting in lower memory chip costs.
Major technology and chipset companies, including, but not limited to, Intel® Corporation, Advanced Micro Devices, VIA Technologies, Acer Labs (ALi), Silicon Integrated Systems (SIS), nVIDIA, ATI® and ServerWorks have supported DDR memory since its inception. Motherboards and systems supporting DDR memory technology were introduced in Q4, 2000, and have ramped into high volume in 2001. Now in 2002 its become hard to find a SDRAM based board.
DDR DIMMs have the same physical dimensions as SDRAM DIMMs, but have a different footprint that uses 184 pins compared to 168-pin SDRAM DIMMs. Therefore, DDR memory technology is not backward-compatible with SDRAM and requires motherboards and systems especially designed for DDR.

DDR - PC266184 Pin DIMM
Single Notched(or Keyed)
Two operationsper hertz
VS

SDRAM - PC133168 Pin DIMM
Double Notched(or Keyed)
One operationper hertz
DDR Question and Answer
Q. How much faster is DDR when compared with PC133 SDRAM?A. While many factors need to be considered when determining performance, applying a number of speed tests to DDR platforms running a 1.2 GHz CPU can show performance improvements of 30% over PC133 SDRAM.
Q. How much faster is DDR when compared with PC800 RDRAM?A. While again many factors need to be considered when determining performance, in some applications we have seen a 5 -10% improvement in speed. In others the RDRAM has shown to be up to 10% faster. The thing to note is that DDR generally is less than ½ the price of RDRAM
Q. Do I need a special motherboard for DDR?A. Yes, your motherboard must support DDR. Currently all motherboard manufacturers have DDR motherboards. The only question is which one and how fast do you want to run it.
Q. Do I need a special CPU to support DDR?A. Yes to take advantage of the performance benefits of DDR you must use a CPU that can take advantage of an increase in memory bandwidth. The pentium3 and Celeron processors do not show any improvement when used with DDR. The Intel P4 and AMD Duron / XP range have shown a marked increase in performance when matched with DDR RAM.
Q. What is the Cas Latency for DDR memory?A. DDR specifies a CL of 2.5 or 2. At this time 2.5 is the most common value used in most systems. Most quality ram we have found runs at Cas2 and is sold as such.
Q. Can you run PC133 on DDR motherboards?A. No the connector on DDR is 184 rather than the 168 pins on the SDRAM. There are some “transition” boards available that have both SDRAM and DDR RAM sockets. These allow you to use your older SDRAM and upgrade to the faster DDR as your budget allows. You cannot use both at the same time.
Q. How do you know it's time for a memory upgrade? A. There are several signs indicating it may be time to upgrade your memory. If you see your mouse pointer turn into an hourglass for significant periods of time, if you hear your hard drive working, or if your computer seems to work more slowly than you expect, the reason is probably insufficient memory. When the memory is full, your system transfers data to the hard drive. This is called swapping. Since the hard drive is considerably slower than DRAM, your system seems slower altogether.
Q. What about static electricity? A. Before installing your memory, discharge any static electricity by grounding yourself to a metal object, such as the casing on your computer. An anti-static wrist strap is recommended for additional safety.
Q. What Memory timings, Cas Latency etc?A. If you look in your BIOS of your PC you will find a range of settings that you can adjust in relation to your memory. These can have a huge effect on not only system stability but also on overall performance. I have read many reviews that don’t take memory timings into account when benchmarking a system. You can make a very quick system very slow if these settings are set up incorrectly. Alternately you can make your system perform a lot better if you learn to tweak these settings. The most common are CAS Latency, RAS to CAS Delay and RAS Precharge. Remember if you set these to aggressively you can make your system unstable. CAS (Column Address Strobe) Latency - The time from the application of the memory module's CAS address to the appearance of data from the RAM Chips on the module. CAS 2 is therefore faster than CAS 2.5 as it only takes 2 clock cycles in lieu of 2.5.Active to Read or Write Delay - - The number of clocks between the activation of a particular row address inside the SDRAM's on the module and the issuance of a read or write command to that row. RAS Precharge - The minimum number of clocks between the completion of one memory access and the beginning of the next. [Bookmark]ECC or (Error Checking and Correction)
Q. What is the difference between ECC and non-ECC (non-parity) memory? A. If you are unsure of which type you have, just count the number of small black chips on one of your existing memory modules. If the number of chips divides by three evenly, then you need ECC. If the number of chips is does NOT divide evenly by three, you have non-ECC or non-parity memory. If you plan to use your system as a server or a similar mission critical type machine, you may want to take advantage of ECC. If you plan to use your system for regular home, office, or gaming applications, you are better off with non-ECC. Current technology DRAM is very stable and memory errors are rare, so unless you have a need for ECC, you are better served with un-buffered DDR SDRAM.
Q. What is meant by Registered memory? A. "Registered" is a means of "Buffering" the memory in SDRAM. This means that the clock signal is boosted across the entire array of memory chips so that the computer sees a clean sharp clock signal instead of a weakening clock signal as it progresses along the length of the memory path. Registered memory must be supported by the system board and cannot be mixed with "Un-buffered" modules. You must have only registered memory to use registered memory. This is a further means of eliminating the possibility of errors when reading the data in memory and is used when the data returned from memory is extremely critical, such as in scientific or financial calculations where the integrity of the data is CRITICAL. It also allows a module to be built using more chips, allowing for larger capacity modules to be made.
Q. What types of DDR memory are available? A. There are currently three types of DDR memory available. 1. PC1600 DDR provides a 1.6GB/second throughput of data.2. PC2100 DDR, provides a 2.1GB/second throughput of data.3. PC2700 DDR, provides a 2.7GB/second throughput of data4 PC3200 DDR , provides a 3.2GB/second throughput of data
Q. How Much Memory Is Enough?A. People will tell you that you can never have enough memory, but that's not necessarily always true. Some operating systems have issues with large amounts of RAM and your applications may not ever use it if you have excessive amounts. Most motherboards have limits on the amount of memory that your system can have or that the board can handle. If you're building a new PC, be sure to check this out with the manufacturer before purchasing a motherboard.
Q. What is Virtual Memory ?A. Virtual memory is simply your hard drive trying to act like a RAM chip. Your operating system creates a temporary storage area on your Hard drive and uses it to store some of the information it is using at a given point in time. Since the hard drive is so much slower than real memory, programs stutter and sometimes crash when the hard drive has to do a job it was never designed for. If you have enough physical memory your virtual memory is rarely needed. There are only two solutions to this problem: close some programs until virtual memory is no longer needed, or add more physical memory. With current memory prices are so low and affordable it, the latter solution is always preferable.