While hard drives are the long-term storage for a computer, the computer also needs a "scratch pad" so that it can temporarily store values and information without having to read & write from the hard drive - this is where RAM comes in. You see the benefit of RAM every time you use your computer. For example, think of how long it takes to save a Word or Excel document - RAM seems instantaneous as it takes nanoseconds to read from or write to, while a hard drive's accesses speed is measured in milliseconds.
RAM stands for Random Access Memory. It gets the word "random" because information can be accessed in non-sequential order. Though the data itself is stored together, it could be anywhere in the "container" or amount of RAM available. It's like having a plot of land, and you've buried a coffee tin with money in it - it's there, but somewhere out there. The amount of RAM is equivalent to the size of land you have - 8 MB could be thought of as 8 acres. RAM is measured in "bits", and 8 bits equals 1 byte. A kilobyte equals 1024 bits, and a megabyte equals 1024 kilobytes. When a program writes a character, that character takes up 1 byte. That 1 byte might be at the beginning, or could be two-thirds the way through the capacity of the RAM.
In order to find that character, RAM uses CAS and RAS to index the amount available. CAS stands for Column Access Strobe, which is a control signal that lets RAM know that the data on the address bus is for a column within RAM memory. RAS stands for Row Access Strobe - Though important in how RAM works, RAS isn't as much of a concern as the CAS value to most consumers. The CAS value, found in the CMOS setup of your PC data, is the number of clock cycles the memory controller will wait after placing the address data on the address bus before it sends the CAS signal. The CAS signal causes the contents of the memory address to be read. The lower the CAS value, the faster the RAM is. SDRAM (PC100/133) can have a CAS of 2 or 3, and DDR SDRAM has 2 or 2.5. RAS stands for Row Access Strobe - Though important in how RAM works, RAS isn't as much of a concern as the CAS value to most consumers. CAS Latency, often referred to as Latency, is the measure of how long it takes for your memory to respond to a request to read and when the information is then sent out from the RAM. Latency is measured in clock cycles, with the notation CL2 or CL3. CL stands for clock cycles, and the number refers to the number of clock cycles. To find out what your RAMs' CAS rating (and other information) is, there are a few options - one is to check your BIOS when booting, and the other is to download SiSoft Sandra. To find the information in Sandra, access "Mainboard Information" and have the Sandra Frequently Asked Questions (FAQ) page opened to "Memory" (found under the "Detection" heading).
SIMM and DIMM refer to the motherboard interface - SIMM stands for Single Inline Memory Module, while DIMM stands for Dual Inline Memory Module. The SIMM was replaced by the DIMM, and can be found on 386/486's, as well as some early Pentium (586) models. SIMM modules have the same pin contacts on both sides, while SD RAM DIMMs have different pin contacts on each side in a 20, 60, or 88 pin configuration making for 168 pins total.
Though DDR RAM is the same size as SD RAM, the pins are different to make sure that DDR and SD RAM are not compatible - DDR DIMMs have 184 pins as compared to SD RAM's 168. Also, SD RAM has two notches while DDR RAM has only one to make the modules physically incompatible. The word RIMM is actually licensed by Rambus, and does not refer to Rambus Inline Memory Module. RIMMs differ to DIMMs in motherboard circuitry - RIMMs connect into a series circuit, while DIMMs connect in parallel. That means that DIMMs are relatively unaffected if you have only one or all the slots filled, while RIMMs need a C-RIMM (Continuity RIMM) to complete the circuit. The series circuit of the RIMM affects performance as data has to travel through all the RIMMs and C-RIMM before it exits the RAM, creating latency. Rambus refers to the series circuit as "Transmission Line Technology".
RAM has changed over the years as advances in motherboards chipsets and CPU's require faster transfer rates. Here's a list of the types of RAM from the past few years:
Extended Data Output (EDO) RAM: Comes in two SIMM versions (30 and 72 pin), as well as a 168 pin DIMM version. SIMMs can be found on 386/486's, as well as some early Pentium (586) models. DIMMs are the current motherboard interface, though EDO DIMMs aren't terribly common.
Synchronous Dynamic (SD) RAM: Though not different physically, it has three versions - PC66, PC100, and PC133 DIMMs. PC133 refers to the industry standard for 133 MHz modules or "sticks". It can be found on Pentium and Pentium equivalent systems to current Pentium3 (P3) and AMD Athlon/Duron systems. SD RAM can have a CAS value of 2 or 3.
Virtual Channel (VC) SDRAM: is made by NEC, and are proprietary modules. The performance of VC133 is said to be somewhere between that of CAS 2 and 3 of PC133 RAM.
Double Data Rate (DDR) RAM has five versions - PC1600 (2x 100 MHz), PC2100 (2x 133 MHz), PC2700 (2x 166 MHz), PC3200 (DDR400) and PC4000 (DDR500). DDR RAM gets those PC ratings from the theoretical amount of data they can move (their bandwidth). For instance, PC100 SD RAM has the ability to move 0.8 gigabytes per second (GB/sec). As DDR RAM can move information twice as fast as SD RAM because it reads on the rise and fall of the clock cycle, 200 MHz DDR (100 MHz doubled) is able to move 1.6 GB/sec. However, though DDR transfers data twice as fast as SD RAM, this doesn't mean your computer will run twice as fast. DDR RAM has a CAS value of 2.5 due to DDR RAM accessing the RAM on either end of the clock cycle.
RD RAM: Rambus RAM has three renditions - PC600, PC700 and PC800 on a 184 pin RIMM module. As mentioned earlier, RIMM is trademarked and they connect in a series circuit (thus needing a C-RIMM to complete the circuit if you are only using one module on your motherboard- a terminator really). RD RAM works just like DDR RAM in that they both transfer data on the rise and fall of the clock signal - however, where they differ is that RD RAM transfers less information more often in an effort to increase speed. As mentioned earlier though, the "series circuit" nature of RD RAM introduces latency as the signal needs to travel through all the RIMM modules before getting to the memory controller. Also a non-sequential access of Rambus memory requires 25 clock cycles before the next address can be read. Since applications have a tendency to frequently "branch" to different areas of RAM memory, any increases in bandwidth inherent in Rambus is lost due to its high initial access latency.
Dual Channel DDR currently has three versions: PC2-4200 (DDR2-400), PC2-4200 (DDR2-533) and PC2-5300 (DDR2-667). More pins that DDR, Dual Channel has 240 pins and is installed in pairs.
| RAM Type | # of pins | CAS Values | Type of Modual | | EDO (Extended Data Output) | 30, 72 & 168 | | SIMM | | SD (Synchronous Dynamic). Speeds: PC66, PC100, and PC133 | 168 | 2 or 3 | DIMM | | VC (Virtual Channel) SDRAM. Speeds: PC100 and PC133 | 168 | 2 or 3 | DIMM | | DDR (Double Data Rate) RAM. Speeds: PC1600, PC2100, PC2700, PC3200 & PC4000. | 184 | 2 and 2.5 | DIMM | | RD (Rambus) RAM. Speeds: From PC600 to PC1600 and higher. | 184 | | RIMMtm |
| Dual Channel DDR (DDR2) | 240 | 3-5 | DIMM |
- What is Buffered and Unbuffered?
- What does Registered and Unregistered refer to?
- What is parity, non-parity, ECC, and how do they relate?
Buffered refers to where the RAM contains a buffer to help the chipset with the large electrical load for when a computer has a lot of memory, while unbuffered lacks a buffer. They are two different types of modules, and are not interchangeable as some even use different printed circuit boards (PCBs). To determine if the RAM is buffered or not, check the leads next to the first notch. If evenly spaced the RAM is buffered. If you attempt to install the wrong type, the stick will not sit properly and be offset.
Registered RAM means that the RAM has a delay on information transferred to the stick or module by one (1) clock cycle. This type of RAM is meant for servers or workstations that serve data to multiple users to ensure the quality of the data being served. Also note that unbuffered and registered RAM is not interchangeable (See Table 1).
Table 1
| Buffered | Unbuffered | Registered | Unregistered | | Buffered | Yes | No | Yes | No | | Unbuffered | No | Yes | No | Yes | | Registered | Yes | No | Yes | No | | Unregistered | No | Yes | No | Yes |
Non parity RAM is "regular" RAM - it has one (1) bit for every bit stored, while parity adds a single bit to every eight (8) bits (1 Byte) for a very basic form of error detection - that means that a 64 MB module of parity RAM is actually 72 MB due to the extra bit added to every Byte.
ECC stands for "Error Checking and Correction". To figure out whether you have ECC, parity, or non-parity RAM you need to count the number of chips on the stick or module. If the total number of chips is divisible by 3, then the RAM is ECC or parity. Otherwise, the stick or module is non-parity. To differentiate if the RAM is ECC or parity, look at the part numbers on the chips of the stick. If the part numbers are the same, then you have ECC. Otherwise, you have parity. What is the difference between parity and ECC? ECC and parity will both work in ECC mode, but ECC won't work in parity (see Table 2) because although ECC adds a bit just like parity for error checking, the bit isn't able to be individually accessed like in parity. When building a system, keep in mind that ECC is said to decrease your PC's performance by 2% as ECC is intended for servers or workstations that are serving data that needs such precautions.
On a side note: ECC and non-ECC RAM can't be mixed.
Table 2
| Error Code Checking (ECC) | Parity | | Error Code Checking (ECC) | Yes | Yes | | True Parity | No | Yes |
Motherboards can support SPD and non-SPD RAM. SPD stands for Serial Presence Detect, which is done by a small Read Only Memory (ROM) chip that is attached to SD RAM and it contains information about the RAM. This information includes: the number of row addresses, error detection and/or correction, number of column addresses, data width, and the interface standards to name a few. When your computer boots up, it accesses the SPD to set the column and row settings as well as the module timings based on the information that the SPD gives. SPD is required 66 MHz, PC100 and PC133 compliant SD RAM.
Brand name computers (Compaq, Hewitt-Packard, Dell, etc.) typically ship with a proprietary version of RAM (among other things). This means that this RAM will work in other computers of the same brand, but more then likely will not work in home-built systems or other brands of computers. You can add other brands of RAM to the brand name systems though - there are little (if any) reports of brands of RAM not being compatible, but be sure that the new RAM is supported by the motherboard!
To answer this, you need to have the brand of the motherboard and its' model. From there, you can check the brand website and usually download the manual (typically PDF format) - but the easiest way to find out is to use various RAM brands memory choosers. This is where the brands have made the supported RAM information available online - here's a few examples:
1. Q: Are some brands better then others? What about when considering over-clocking?
A: Yes. Quality name brands are suggested when buying regardless if considering over-clocking or not. Brands such as: Mushkin, Crucial, Infiniteon, Micron, and Kingston are suggested as the quality of RAM is better, and they have better return policies and customer service. Most offer for the lifetime that you own the RAM. If considering over-clocking, best to research your motherboard and know what the motherboard allows for specs before purchasing.
2. Q: Is RAM difficult to install?
A: Surprisingly no. For more information, check this article for DIMM installation instructions.
To install SIMMs, check the SIMM stick or module - There should be a notch on one side. To install the new SIMM match the notch to the side that the already installed RAM has them. You might need to remove the already installed SIMMs to tell where the notch is, but you can also look at the slot where the SIMM is to be installed and note where there's a small plastic rise in the SIMM slot to account for the notch.
3a. Q: Can SDRAM PC100 / PC133 RAM be mixed together?
3b. Q: Can DDR PC1600 / 2100 / 2700 / 3200 / 5400 RAM be mixed together?
A: Yes they can.
What happens is the memory will run at the slowest of the versions installed.
So if the motherboard is said to only run PC100 RAM, then the RAM that is PC133 will not run to it's full potential of PC133 - it'll be capped at PC100, but this will neither damage the RAM or the motherboard. Same thing for DDR - if you run say 1600 with 2100, the cap is 1600.
The only considerations to be aware of are if the new RAM needs to be un/buffered and un/registered - check your manual for which is required.
4. Q: How much RAM can a Win9x (Win95/98/SE/ME) Operating System use?
A: 256 MB is the maximum suggested unless running image-editing programs like PhotoShop or resource using programs like CAD. Applications like MS Word and Excel don't need anywhere near that much, and RAM has little effect on web browsing other then how fast the browser starts (when you click on the desktop icon to access IE or Netscape). Actual page loading (web surfing) is more dependent upon your bandwidth then RAM or CPU clock speed - Broadband options like Cable or DSL will make a difference as compared to using a dial-up modem.
In summary, I hope this answers many questions regarding the major aspects of RAM - how it works, the types available, and common questions. If you do have any, feel free to post your question(s) in Da Forum but be sure to list important information such as motherboard (brand and model number) and the type of RAM.
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