Know Your Ram
ECC - Error Correcting RAM - also called Parity Ram:
ECC RAM has an extra error-correcting bit (9 bits instead of 8) for higher reliability in servers and high-end
machines. Most forms of RAM (SIMMs, DIMMs, RDRAM and DDR RAM) are available in either ECC or Non-ECC. Most common
computers use Non-ECC (standard) memory. In order to use ECC RAM,
The machine's motherboard must support ECC, and all of the RAM in the machine must be ECC. If you install Non-ECC
and ECC together, the machine might work, but all of the RAM will revert to Non-ECC mode.
Although some machines can accept either ECC or Non-ECC, there is little advantage to installing ECC RAM in a general
purpose machine since it is more expensive and you won't reap the benefits.
If you have a machine that can take either ECC or Non-ECC and are unsure what to order, either check the RAM that
is already installed, or check your purchase invoice for the machine, which should indicate whether ECC was installed
or not initially, or just call us and we can help.
Reading memory chip part numbers:
You can sometimes read from the part number whether the RAM in your machine
is ECC (Parity). ECC RAM often has a number in the pat number that is a multiple of 9, where Non-ECC would have
a multiple of 8. Example, RAM that has x18 or x36 or x72 in the number would be ECC, compared to Non-ECC with x16
or x32 or x64.
Non-ECC RAM also called Non-Parity RAM
Unless specified otherwise, all standard RAM is Non-ECC.
SIMM - Single Inline Memory Module
Found in older desktops and many laser printers, in 72-pin packages, and less commonly 30-pin. Sometimes must be installed in pairs. Whether a SIMM is FPM or EDO memory (see below) is significant.
DIMM - Dual Inline Memory Module
Found in desktop machines and servers, PC66, PC100 and PC133 are all different speeds of DIMMs in a 168 pin package, there are also SDRAM DIMMs that predate the PC66 standard. DDR memory is also technically a DIMM format, but is seldom referred to as such.
SO-DIMM - Small Outline Dual Inline Memory Module
Most often found in laptops. Most SO-DIMMs are 144 pin packages - there are also DDR 200 pin SO-DIMMs and some rare 72 pin SO-DIMMs
Micro-DIMM
Rare, smaller than the SO-DIMM format, found in a few Sony laptops
RIMM, RAMBus, also called RDRAM
RAMBus memory module. Most often, RAMBus memory must be installed in pairs. 32-bit RAMBus moduled have been introduced which only need to be installed singley - the motherboard has to specifically support 32-bit RAMBus to use these.
Speeds are PC800 and PC1066. The PC600 and PC700 designations are obsolete and PC800 can be substituted.
DDR - Dual Data Rate RAM
Can be packaged in both DIMM (184-pin for Desktops) and SO-DIMM (200-pin for laptops) forms. Speeds are PC2100 (266 MHz), PC2700 (333 MHz). The standards for 400 MHz DDR are not completely finalized, although some manufacturers are offering high speed DDR modules.
FPM - Fast Page Mode and EDO - Extended Data Out
These two terms refer to older (c. 1998 and earlier) RAM produced before the introduction of the PC66 standard.
SIMMs, DIMMs and SO-DIMMs can be either EDO of FPM, in many cases they cannot be substituted for each other. EDO
modules are rising in price rapidly because the components are no longer being manufactured by most chipmakers.
It is not unusual for an EDO SO-DIMM for an older laptop to cost hundreds of dollars. The distinction between EDO
and FPM is irrelevant with PC66, PC100, PC133, RAMBus and DDR RAM types.
Ram Qualities:
Density:
When a RAM module is built, the manufacturer can design it using high density or low density chips. For example,
modules built with 4 x 32Mb chips, 8 x 16Mb chips or 16 x 8Mb chips all come out to the same amount of RAM. However,
the memory controller circuit in a computer must be able to address the chips on the module - if the memory controller
cannot address higher density chips, that computer will only function with modules built with lower density chips.
Profile:
The physical size of the RAM module, both in height and in thickness. Some machines, particularly laptops, have
physical space limitations that demand low-profile modules.
PC66, PC100, PC133:
These are standards for DIMMs and SO-DIMMs that specify memory bus speeds of 66 MHz, 100 MHz and 133 MHz. In broad
terms, you can substitute faster RAM into a machine (putting PC133 into a machine that specifies PC100, for example),
however there are still a significant number of earlier PC66 and PC100 machines that are incompatible with modern
PC133 modules for reasons of chip density and differences in the supporting chips on the module (buffer, EPROM,
Serial Presence Detect).
Clock:
To oversimplify a complex discussion, the timing of a RAM module is often described by a Clock value. Typical values
are CL3, CL2 and CL2.5. A CL2 (2-clock) module is not necessarily faster than a CL3 module - the effective speed
is determined by the memory controller on the motherboard of the computer, and in some cases a CL3 module matches
the motherboard timing better.
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