Memory Modules
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- What is memory and what's the difference
between DRAM and SRAM?
- What about memory speeds?
- What is EDO memory?
- Will EDO RAM work in a 486?
- Do I need 30-pin or 72-pin SIMMS?
- How many SIMMS are needed?
- Which one is better, SIMMS with gold or tin
contacts?
- Parity or nonparity SIMMS?
- What is ECC (Error Checking and Correction)?
What is memory and what’s the difference between DRAM and SRAM?
Generally speaking, memory works as a temporary storage medium. From the
operating system to an application program, everything is temporarily stored in
the computer’s memory while these applications are running. Memory is volatile;
meaning it loses its content when power to the computer is shut off. This type
of memory is called RAM or random access memory.
Memory in a computer is usually installed as SIMMs (single in-line memory
modules) and DIMMs (double in-line memory modules). SIMMs and DIMMs are small
printed circuit boards with memory chips soldered onto them. SIMMs can have 30
pins or 72 pins, and DIMMs have 168 pins. Pins are contacts that interface with
the computer. The more pins, the faster the interface, so DIMMs are faster than
SIMMs.
Some people confuse memory with storage, simply because memory and storage
use the same measurement numbers. Memory and storage are both measured in
megabytes. SIMMs come in various capabilities, for example 4MB, 8MB, 16MB, 32MB
and more. Today most computers come with an option of upgrading to 128MB and
more if required. Today’s powerful software programs require a minimum of 8 to
16 Megabytes of memory or more.
DRAM stands for dynamic random access memory. It is the most common form of
system memory capable of storing large amounts of data within a highly
integrated array of cells. "Dynamic" refers to the fact that DRAM can only hold
an electrical charge (data) for a short time. Since this memory in your computer
is constantly being refreshed, fast, high quality RAM is very important.
SRAM is static random access memory. This type of memory is used as system
cache, both on your motherboard and inside your CPU. It has transistors built
in, giving it faster speed, but less storage capacity.
What about memory speeds?
The speed rating marked on the chip indicates how long it takes for the
memory READ/WRITE to occur. A chip with a lower number is better because it
takes less time.
What is EDO memory?
Extended Data Out- A new chip technology that shortens the READ cycle between
the system’s main memory and the CPU. EDO chips allow the CPU to access memory
10 to 15 percent faster. EDO modules should be used in systems designed to take
advantage of EDO memory.
Will EDO RAM work in a 486?
Use of EDO RAM in a computer that is not designed to take advantage of it can
cause problems. Most 486 boards DO NOT support it. To be sure check your
manual or you may want to contact the manufacturer of the motherboard, 30-pin
SIMMs were the standard for most early edition 486 computers.
Do I need 30-pin or 72-pin SIMMs?
Most of the recent Pentium boards have 72-pin sockets. 286, 386 and older
processors use 30-pin modules. Some 486 boards take both 30-pin and 72-pin
modules in combination. 72-pin sockets are almost twice the size of 30-pin
sockets.
How many SIMMs are needed?
On 386 and 486 systems that use 30-pin modules, you must fill up a "bank" of
chips at one time. A bank is usually either 2 or 4 chips. Pentium boards use
72-pin SIMMs, which generally must be inserted two at a time. To upgrade your
Pentium board from 16 to 32MB, you will need to buy two 8MB SIMMs. Or you can
buy two 16MB SIMMs to upgrade from 16 to 48MB.
Which one is better SIMMs with gold or tin contacts?
For the best reliability, you should match the contact material of the SIMM
socket on your motherboard. However, this is not a critical issue and either
kind will work. Most Pentium boards have tin contacts, and almost all SIMMs
manufactured today use tin/lead alloy instead of gold.
Parity or nonparity SIMMs?
You should match whatever chips are currently on your board. Your motherboard
should tell you exactly what chips to buy to upgrade your memory. For 30-pin
SIMMs, count the number of chips on the module: 2 or 8 chips=nonparity, 3 or 9
chips=parity. For 72-pin SIMMs you can’t always be sure, but generally if they
have 4, 8, 16, or 32 chips on them, they have not parity.
What is ECC (Error Checking and Correction)?
The newest type of parity is called ECC, or Error Checking and Correction.
ECC uses a whole series of bits to ensure the bytes are transmitted correctly.
ECC module has each byte arranged like nonparity memory in 8-bit segments, e.g.,
0010 1100. The ECC bits are located at the end of the series of bytes in a
four-bit segment. You need a series of eight bits to perform ECC, which is why
you need two ECC modules used together to perform the ECC function. ECC DRAM
detects double-bit errors and corrects single-bit errors on the fly (without
needing to create an error message).
A nonparity module will work in a parity system if you disable the parity
function, but a parity module will not work in a nonparity system. The majority
of memory in a modern PCs is nonparity simply because memory errors are rare,
and a single bit error will most likely be harmless. ECC modules are used for
"mission critical" applications such as programming where a single error in your
code could cause major problems. A system that supports ECC can use a regular
parity module in place of a true ECC module by using the parity bits to make up
the ECC code. A parity system, however, can not use a true ECC module because
the ninth (parity) bit per byte is not associated with each byte.
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