RAM : 今日之内存 (2)

Today, there are FOUR varieties of memory in common use ( for Desktop Computers ): SDRAM (synchronous dynamic RAM), Direct RDRAM (Rambus dynamic RAM), DDR (double data rate) SDRAM and DDR2 ( DDR-II) SDRAM . All Four serve exactly the same role in a computer, but their internal designs are different, and memory module layouts are incompatible.

SDRAM was first used in 1996 with support for a PC processor bus (also called front-side bus or FSB) speed of 66 MHz, but by 1998 it had advanced to support the 100-MHz FSB. SDRAM certified to run at 100 MHz was dubbed PC100 SDRAM, and it is available in 168-pin dual in-line memory modules (DIMMs). Since each DIMM offers a 64-bit data bus, the peak bandwidth for SDRAM is 800 MBps (8 bytes x 100 MHz). This means a PC100 SDRAM DIMM can ideally pass up to 800 MBps between the DIMM and motherboard (though speeds rarely get this high in actual practice). By 1999, SDRAM was available to support a 133-MHz FSB speed, and this was termed PC133 SDRAM. With the same 64-bit data bus on a DIMM, PC133 SDRAM DIMM can theoretically handle up to 1.1 GBps (8 bytes x 133 MHz).
Please notice that PC133 RAM has single side RAM and Dounble Side RAM. Only old SDRAM based motherborads support Single-Side RAM. Newer version SDRAM based motherboards have to be filled by Double-Side SDRAM RAM.

Rambus (RIMM) introduced its memory technology in 1995. Rather than using the existing processor bus, as other memory technologies do, Direct RDRAM employs a small 16-bit data bus with a dedicated high-speed (300-MHz) clock. Since Rambus handles two operations per clock cycle, this effectively doubles the clock speed to 600 MHz. Rambus also supports dual channels, effectively doubling the data bus to 32 bits and providing a peak bandwidth of 2.4 GBps (300 MHz x 2 operations per clock x 16 bits x 2 channels). Rambus modules rated for 300-MHz operation are termed PC600 RIMMs (the 300-MHz clock x 2). By 1998, Rambus had advanced to dual 400-MHz channels, so the effective clock rate became 800 MHz (400 MHz x 2) with a 16-bit data bus, yielding a peak bandwidth of 1.6 GBps. This doubles to 3.2 GBps when you add a second channel. 184-pin Rambus modules rated for 800-MHz operation are called PC800 RIMMs. Today, Rambus is advancing to 1,066 MHz (PC1066) and 1,200 MHz (PC1200) for even higher performance.
Since the RDRAM supports Dual Channel features, most motherboards ask you to put RDRAM pair-by-pair. Each pair has to be two same size modules and with same speed. Each pair called a BANK. i.e. each bank has to be with same size and same speed modules. For different banks, you can put different size and different speed dodles.

DDR SDRAM . The issue with SDRAM is that each data line passes only one bit per clock cycle (resulting, for a 64-bit memory device, in 64 bits per clock). To compete more closely with RDRAM, SDRAM creators developed memory that would perform two operations per clock cycle. This memory is called double data rate or DDR SDRAM. For a 100-MHz FSB, DDR SDRAM provides twice the bandwidth (8 bytes x 100 MHz x 2) or 1.6 GBps. For a 133-MHz FSB, DDR SDRAM can reach a peak bandwidth of 2.1 GBps (8 bytes x 133 MHz x 2). With a 166-MHz FSB, DDR SDRAM can offer a peak bandwidth of 2.7 GBps (8 bytes x 166 MHz x 2). These are called DDR200, DDR266, and DDR333 for 100-MHz, 133-MHz, and 166-MHz speeds, respectively. But memory makers often name the modules based on their bandwidth, such as PC1600 (1.6 GBps), PC2100 (2.1 GBps), and PC2700 (2.7 GBps). A DDR SDRAM module uses 184 pins (like a RIMM). Since DDR SDRAM builds on well-established SDRAM technology, it's often cheaper than Direct Rambus modules.
Current Desktop Computers use DDR400 400-MHz FSB DDR RAM, called PC3200 RAM ( Bandwidth is 3.2 GBps). Other speed of DDR RAM, such as DDR533,... are overclocked. All DDR RAM do not support dual channel, therefore, each bank only filled by one module. Different bank can be filled by different size RAM model ( for most of motherboards).
The important thing to remember is that DDR400 and DDR333 are usually compitable, but DDR200, DDR266 are not compatiable with DDR400, even they have same pin 184 and same voltage 2.5v.

DDR2 SDRAM . The DDR2 is with double speed as DDR SDRAM. DDR2 has 240pin and with different voltage: 1.8v. Now DDR2 has 400mhz DDR2-400 (PC2-3200), 533mhz DDR2-533 ( PC2-4200) and 667mhz DDR2-667 ( PC2-667) front side bus. Other speeds are overclocked. Most of them are compatibale with the different speeds on same motherboards.
Since DDR2 support Dual Channel features, same as RDRAM, all DDR2 based motherboards have to be filled by Pair-by-Pair. Each pair called a BANK. Each bank has to be filled by two modules with same size and same speed. for different banks, you can use different size and different speed modules.

The important thing to remember is that you cannot mix SDRAM, Rambus, DDR SDRAM and DDR2 SDRAM on the same motherboard.

When you're adding memory, select the type specifically intended for your motherboard. SDRAM modules each have 168 pins. Rambus and DDR SDRAM modules have 184 pins ( some RDRAM has 200pin). And, DDR2 has 240pin. Although they are all similar in size, the difference in pin count and keying prevents you from using them in the wrong slots.

Workstations & Servers

You'll also need to check whether the memory in your system uses error-checking techniques such as Parity Checking and ECC (Error Correction Code) and whether it's Buffered or UnBuffered .

Most of Workstation motherboards use the ECC RAM, there are 4 categories of ECC RAM: SDRAM ECC, RAMBUS ECC, DDR ECC and DDR2 ECC. All of them are different and incompatitable each other.
If the motherboards ask ECC on specification, please use ECC RAM.
tips: Based on the expierence, the ECC RAM based motherboards can be filled by relatived RAM without ECC. But it is not gurranted in here.

If there are not any request on the specification on RAM with " buffered" or " registered", the means the RAM is "UnBuffered" or "Non-Registered" RAM on the market.

If the RAM said as " buffered" or " registered", the means the RAM is "Buffered" or "registered" on the market.
In theory, Buffer and Registered are two different concepts. But on PC Industrial & PC market, they are treated as same products. If the motherboards asked Buffered or Registered RAM, you have to fill the Rigistered or Buffered RAM in.

Usually, all Server motherboards use Registered RAM. And All commercial server motherboard support ECC on same time, therefore, the server motherboards ask ECC & Registered RAM on the same time. On market, ECC Registered RAM are very standard for servers.

Since servers ask reliability and Compatility first, performance is the 2nd, therefore the ECC REGISTERED RAM has to be Dual Channeled in most cases. i.e. All RAM has to be Paired. Each bank have two slots, Each bank has to be filled same size and same speed RAM modules. For Different Bank, you can use different size of the RAM modules, but the best is with same speed as other bank(s).

ECC REGISTERED RAM has FOUR categories: SDRAM ECC REGISTERED, RIMM ECC REGISTERED ( not populer on market ), DDR ECC REGISTERED and DDR2 ECC REGISTERED. Last two categories are very populer on server markets. Four categories of ECC REG. RAM are incompitabled.

By The Way, Some friends may realised DDR2 ECC REGISTERED RAM have two types: RANK-1 and RANK-2 .
The RANK is a concept related to RAM BANK. If you want to get over 2 banks of RAM on the server motherboards ( high-end Server motherboards has 4 banks, 8 banks even 16 banks on each motherboard, and some server can hold over 2 motherboards same time if with daughter Riser.... for 16-way CPU servers ... is not available for China Market,... can be installed in Western countries only )
e.g: your server motherboard has 4 BANKs, you have installed 4 modules in your server (2 banks) already, you want to install two more or Four more RAM modules in other ( one or two ) bank(s). Now you have to change your current RAM to RANK-2 RAM and the new RAM has to be RANK-2 RAM too, otherwise your motherboard doesn't recognize your 3rd bank and 4th bank RAM.
If you need help on your server or cluster( IBM, HP, SUN, SUPERMICRO, DELL, FUJITSU, NEC and Intel ...) please let me know... there are over 15 years server expierence for you to share.

RAM for Notebook

Notebook RAM are called SO-DIMM . There are THREE different categories of SO-DIMM: SDRAM SO-DIMM ( most of them are 144pin, there are 3.3v and 5v); DDR SO-DIMM ( There are 180pin and 200pin; most of them are 2.5v); DDR2 SO-DIMM 220pin 1.8v. Three SO-DIMM are incompitabled.

Some old notebook use SO-DIMM PC100, PC133. The RAM is very shortage on market --- too old. Some notebook use SO-DIMM DDR200, some use DDR333, they are incompatibled, even they have same pin and same voltage. Most DDR400 SO-DIMM based notebooks are universaled, i.e you can use any brand DDR400 SO-DIMM. For DDR2 SO-DIMM based on notebooks, the RAM speeds are not important, i.e you can use any speed RAM on it. But for reliability, best is to use Un-overclocked SO-DIMM.

Memory is one of the most popular PC upgrades. It's relatively inexpensive and easy to do, yet it can improve performance and allow the system to support more (and more sophisticated) applications. When planning a memory upgrade, you need to evaluate the amount of RAM you need and understand the memory types and characteristics that are appropriate for your system. We dot not list the installation process in here.

Please remember that: IT IS VERY IMPORTANT to Get the Correct type RAM for your system.