Che differenza c'è tra RAM di tipo DDR, di tipo DIMM o di tipo SDR? Ricordando che la memoria RAM (Read Access Memory) è di tipo volatile, in quanto il suo contenuto è svuotato allo spegnimento del PC, vari sono i tipi di memoria attualmente supportati dai computer. Dai primi moduli SIMM a 30 e 72 piedini, contraddistinti da alti tempi di accesso e capacità limitate, si è pervenuti allo standard DIMM (Dual Inline Memory Module), che definisce il supporto per memorie SDRAM (Synchronous DRAM), riconoscibili per avere 168 contatti (vedi figura) e un tempo di accesso variabile di solito tra 15 e 7 ms (equivalenti a frequenze da 66 a 133 MHz). La loro evoluzione è rappresentata dalle memorie SDRAM di tipo DDR (Double Data Rate), in grado di raddoppiare la velocità delle memorie SDRAM. Il loro tempo di accesso può arrivare a 3 ms, esprimendosi in frequenze da 200, 266, 333 MHz e oltre. Sono facilmente riconoscibili per avere 184 contatti.



RAM FAQ

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 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. 

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

What are the benefits of upgrading your computers memory?

Upgrading your memory is typically the easiest and least expensive way to upgrade your computer for a significant boost in performance. The computer's RAM memory is its workspace, or where all of the instructions it needs to act on are stored temporarily. Think of the RAM as the desk you use to sort through your work. If the size of that desk is small, your efficiency is limited in comparison to a larger desk that allows you to work more effectively and efficiently. Similarly, a computer with more RAM can work more efficiently because it does not need to retrieve information from the hard disk drive as often. A memory upgrade is particularly helpful for users who work with large files, have more than one program open at one time, or use memory-intensive applications such as games or graphics and video editing software. 

How do you know it's time for a memory upgrade?

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 physical memory is insufficient, the system uses Hard Disk Space as memory. This is called "Virtual Memory". Since access time of Physical memory is in tens of NanoSeconds and Access time of Hard Disk is in MilliSeconds, the system slows down considerably. 

What's the difference between buffered and unbuffered DIMMs?

High density DIMMs have lots of chips on them and therefore possess a higher capacitive load on the address and control signals in comparison to lower density DIMMs. Some designers use redrive buffers on the DIMM to boost the signals to reduce system loading when compared to the same high density module without buffers. But the buffers introduce a small delay into the electrical signal, so adding buffers to a standard density module would have the effect of slowing down the signal, compared to the same low density module without buffers. 

What is Virtual Memory?

This is a method of extending the available physical memory on a computer. In a virtual memory system, the operating system creates a pagefile, or swapfile, and divides memory into units called pages. Recently referenced pages are located in physical memory, or RAM. If a page of memory is not referenced for a while, it is written to the pagefile. This is called "swapping" or "paging out" memory. If that piece of memory is then later referenced by a program, the operating system reads the memory page back from the pagefile into physical memory, also called "swapping" or "paging in" memory. The total amount of memory that is available to programs is the amount of physical memory in the computer in addition to the size of the pagefile. 

Will adding more RAM make my Internet browsing faster?

Maybe. Internet browsing speed depends on a huge number of factors, including your connection speed, traffic on the site you're visiting, and the other components in your system. You will probably notice the biggest improvement from additional RAM if are viewing or working with large files (such as photos and digital audio and video) or if you switch between your browser and other applications often. 

What's the difference between RDRAM and SDRAM?

RDRAM stands for Rambus Dynamic Random Access Memory. SDRAM stands for Synchronous Dynamic Random Access Memory. The two memories are completely different memory technologies and are not compatible with each other. RDRAM is a unique design developed by a company called Rambus, Inc. RDRAM is extremely fast and uses a narrow, high-bandwidth "channel" to transmit data at speeds much faster than SDRAM. 

What is the difference between 72 bit and 64 bit memory?

72 bit memory is commonly known as ECC memory. It has an additional 8 bits for Error Correction Check 64 bit memory is non-ECC. 72 bit or 64 bit configuration are typically found in 168 pin DIMMs.