Memory FAQ
(frequently asked questions)
Q. What is System Memory?
Memory is a computer’s primary workspace. It
works in tandem with the Central Processing Unit, or microprocessor, to store data,
programs, and processed information that can be made immediately and directly accessible
to the CPU or to other system devices. Memory is central to a computer’s
operation because it forms the size and number of programs that can be run simultaneously,
and helps to optimize the capabilities of increasingly powerful microprocessors.
There are many different kinds of memory, each with its
own features and benefits. Unfortunately, with so many different types of memory
types of memory, it can be easy to get them confused.
Q. What are DRAM modules?
The most common type of memory chip is Dynamic RAM
(Random Access Memory). These memory modules require 'refreshing' - or else they
forget. They are also accessed radomly rather than sequential read and write
patterns.
RAM chips are the storage places where the binary bit
intelligence is held while the processor is doing its work. Once the power is turned off
on the computer - all that intelligence is lost. Compare this to ROM (Read only Memory) in
which the computer only reads what has permanently been placed inside the chip.
The original packaging of DRAM was in DIPs (Dual In line
Package or pins). Other methods of packaging include SMD (Surface Mounted Device) such as
SIMMs, SIPs, or DIMMs - note: packaging has changed but not the chip itself.
SIMMs (Single In line Memory Modules) - A plug in print
board that fits on the motherboard and holds the silicon chips called SOJs (small outline
joint or small outline j-bend) or TSOP (thin small outline package). The RAM chips are
soldered onto the print board. This stops 'chip creep' present in socketed systems.
There are two main sizes of print board, described by
the number of pins (actually edge connection); 30-pin and 72-pin. Most pentium boards
require 72 pin modules. There are converters to use 30-pin SIMMs HOWEVER - the data path
is lower, therefore, performance will suffer.
Q. How do I know what type of RAM my PC is using?
We used to be able to say that the majority of 486 and
Pentium PCs on the market use either the 30-pin or 72-pin SIMMs. Now, it's a little more
complicated.
Different boards with different chipsets are designed in
different manners. Some use 168 pin DIMMs (Dual In Line Memory Modules) that can be either
non-ECC SDRAM, ECC SDRAM, and PC100 compliant SDRAM. The 168-pin modules are longer than
72-pin modules and those are longer than the 30-pin.
The best thing to do is to read the manual for the
motherboard or at least the specification sheet posted on the manufacturer's site. If it's
an older system, then you may need to identify the type of chip.
To help identify the RAM chip, there are markings placed
on the top of each piece of silicon (the SOJ, remember?). Get out a magnifying glass to
read the small print, comparing them to a chart, and then be ready to be hopelessly
confused, you will be able to know what type of memory module is installed in your PC.
See
Upgrading a PC
Q. What does SOJ (Soldered on Joint) and SOP (Small
Outline Package) mean?
The SIMM module is composed of SOJ or TSOP DRAM packages
soldered onto a PCB board. The most common SOJ sizes used in today's PCs are the 1 x 4 or
the 4 x 4 SOJs. You might see some vendors refer to 8 chip 1 x 32 SIMMs versus 2 chip 1 x
32 SIMMs. The 2 chip version is usually less expensive but does not work with all PCs. The
2 chip version is made up of 2 chips (SOJs) sized as 1 x 16 (versus 1 x 4).
30-pin modules: First identify the SOJs by the markings
on the chip. Next identify if you have a 1 x 3 or 1 x 9 (referred to as "one by
threes" or "one by nines"). These are easy to distinguish because you just
count the number of chips on the board. If there are three chips then your PC has 1 x 3
modules. If there are nine chips then it is a 1 x 9.
72-pin modules are a little more tricky: 1 x 32 (64-bit)
or 1 x 36 (72-bit). The 32 or 36 tells you whether or not there is parity on the module,
notice it didn't say ECC (This is a different type of module!!). For example, if you count
9 chips on the board, eight the same size and in the middle is a larger package - then you
have a parity module. (4 chips 1 chip 4 chips).
Why is a 1 x 32 a 4 Megabyte module? Ok ... here's the
easiest way to determine the memory capacity. Divide 32 by 8 ( 8 bits makes a byte,
right?), which equals 4. Now 4 times 1 (because it is a 1 x 32 module) equals 4...
therefore 4 Megabytes. To continue that math game.... a 2 x 32 is an 8 Mb module...
because 32 divided by 8 equals 4 and 4 times 2 equals 8... And now you know that a 4 x 32
is a 16 Mb module... because 4 times 4 is 16... and 8 times 4 is 32 Mb (the 8 x 32 is a 32
Mb module). Tada, that was easy! Now that you know that, the manufacturer's are going to
change things.... Watch out DIMMs here we come.
Q. What are the differences between composite and
non-composite memory?
There are several ways to manufacturer a 72 pin SIMM;
i.e.using 4 x 4 SOJs or using 1 x 16 SOJs. 8 pieces of 4 x 4 will make a 4 x 32 module,
whereas 2 pieces of 1 x 16 also make 4 x 32 modules. Composite memory only uses the 1 x 4
and 4 x 4 SOJs. Non-composite memory utilizes the 1 x 16 SOJs.
Q. What is parity and ECC?
DRAMs usually operate in a set - usually 8 or 9. 8-bit
registers all work as one memory; the ninth acts as a parity or a 'health check' to make
sure the data in the first 8 are okay. ECC (Error Checking and Correcting) memory has a
10th bit. This memory can correct a single error once it is found. Notice, ECC memory and
not a functional ECC derived from a chipset such as the Triton II. In the case of the
Intel HX chipset, true parity memory is mounted on the board and the ECC function is from
the chipset.
Q. What is logic generated parity versus true
parity?
True parity is an actual DRAM module performing the
function of parity checking whereas the logic generated parity uses a TTL chip. Logic
generated is a method to appear to give the module parity checking when actually it is
nothing more than a device that says: "Yep all is well here"
Some people refer to logic generated parity as
"phantom" logic
Q. How can I identify logic generated modules?
Look for the following markings - BP or LP. BP
stands for "Brain Power". MA Labs has their own version of the generator.
Certain BIOS versions autodetect true parity
memory. Some advanced CMOS settings allow the user to disable parity checking. Packard
Bell users have a jumper setting on some revisions. Most motherboards now only use the
non-parity 72 pin RAM, simply because they are cheaper. If the PC is a file server or an
important workstation, we strongly suggest that you invest in true parity memory.
Pentium II systems should use the ECC/EDO memory.
Q. How do I know if my PC is using EDO or FPM?
EDO and FPM 72-pin memory look very similar unless you
read the markings on the top of the SOJ. Some BIOS versions from AWARD and AMIBIOS
autodetect the identity of SIMMs . AWARD will state EDO on its summary page that it
flashes at the top of the screen during the boot process. PCs using AMIBIOS can be checked
by entering the setup screen (Press F1 or F2).
Q. Can I mix EDO and FPM in the same PC?
Do not mix memory types, speeds, or even manufacturer's.
No matter how you look at this question please realize everything is
dependent upon the motherboard. Certain chipsets are designed so that different
"banks" of memory may be filled with different types of memory. On slower PCs
you can even mix different speeds of memory in different banks. Generally, though, you
should always try to match SIMM speeds. Just realize, some motherboards are very forgiving
while others have tighter controls. Wouldn't it be just your luck to own the only PC in
your neighborhood that requires proprietary memory? It happens.
Q. What do you mean by the speed of a SIMM?
Manufacturers refer to the access time of DRAM as speed.
Access times are measured in nanoseconds, abbreviated 'ns'.
"Access time is the time period between the CPUs command to the
memory that data should be read and these data being transferred to the processor."
One nanosecond is a billionth of a second. Light travels
over 11 inches in one nanosecond. 60 or 70 ns is a common 'speed' for today's SIMMs. This
may sound fast BUT sRAM (static DRAM, usually used as level two cache) is in the 15 to
20ns range and don't need to be refreshed. 50ns memory is becoming more common for
motherboards and CPUs running at peek performance.
There are several techniques manufacturers have used to
speed up the access time of Dynamic RAM. These operating modes are: page mode,
static-column mode, nibble mode and serial mode.
Page Mode RAM: Special RAM chips that combine features
of both dynamic and static memory. These chips divide their total address range into
'pages' (usually in kilobit size). Each of these pages can be accessed repeatedly without
wait states. This is distinct technology from static-column RAM which divides the chip
into ROWS and COLUMNS.
Interleave Memory: a technique in which the total system
RAM is divided into 'banks' - Sequential bits are held in alternate banks.
Q. What are the types of memory on the market?
Fast-page mode, EDO, Burst EDO, EDRAM, SDRAM, CDRAM,
RDRAM, MDRAM.
EDO - Extended Data Output, also called "standard
EDO" (by whom.. dunno) supported by many chipsets. To help identify whether or not
your motherboard can accept EDO, your first task is to identify the chipset. You can
expect about a 1-5 percent increase in performance between using FPM (Fast Page Mode) and
EDO memory. Actually increasing the level two cache on the motherboard has greater impact
on performance.
Burst EDO - Pipelined nibble mode memory. An extension
of EDO. This module is also an attempt to move consumers to the faster SDRAM (not sRAM
which is static RAM versus sDRAM which is Synchronous DRAM)
Q. What is Synchronous DRAM?
Technically there are two internal memory arrays
decoding in parallel. It is an architecture that allows read and write performance of
X-1-1-1.
Q. What does major-on-third mean?
"Major on Major" RAM brand - the DRAM chip
(such as an SOP or SOJ package) and the PCB board are from the same 'major' manufacturer
ie Goldstar, Micron, etc. A "Major on Third" generic memory module is the SOJ
mounted on a third party PCB board.
Q. Are there different quality issues I should be
aware of before buying memory?
Absolutely.
There is Grade A, B, C, all the way to toy grade memory.
Yes, memory you find in toys.
Check the markings on the top of the SOJs. If a SIMM has
been pulled, remarked or remanufactured the clues can be seen on the top of the chips.
First, make sure that all the dates are the same on each module. Second, hold the SIMM to
the light, there is generally a round dot that shines. This area helps to ensure no one
has ground down other labels and remarked the chips.
Finally, manufacturer's label their SOJs based on a
'grade'; i.e. grade A eggs obviously taste better and you may also grade A SOJs are the
best the manufacturer has to offer. Other grades have failed certain tests, and have been
passed down to Grade B, C, or even D.
Some manufacturer's end their markings with a DJ.
This does not mean it is a grade D SOJ.
Q. Ok, so I call a competitor and they don't know
what kind of memory they are selling?
In the computer industry sales people work hard. Please
don't give them a bad time. If they don't know the manufacturer it doesn't mean they are
dumb. Besides, with honey you can catch more flies, maybe you just didn't ask politely.
Some of our team will always answer with a "dunno" before they can figure out
how much you know. Generally the conversation progresses based on your experience. Note:
Anyone who states "I've been in this industry for X years" losses the argument
first.
Computer memory is arranged as a matrix of cells.
Think of this matrix as a spreadsheet in which each cell is addressed with unique column
and row designations. When an initiator (Bus master) communicates with DRAM, the
address is received in two steps:
CAS, column address strobe
RAS, row address stobe
Actually it's row first then column address, designated
RB4C, where the intersection of the two represent the exact location the initiator wants
to communicate. You will see this option in the system BIOS - RAS only or RAS/CAS.
The newer technology is the later, RAS/CAS.
RAM is refreshed about every 15 ms - as determined by
the oscillator on the motherboard. For the overclockers, notice that the RAM timing
is dependent on the oscillator, right?
Good test for determining a module failure -
"walking bit test" - All eight bits are 'cleared' or set to zero. Then the first
bit is set to 1 (0000 0001) and the other 7 bits are checked to see if they changed. Then
the second bit is set to one and the other seven are checked - until the test program has
'walked' through all the memory locations.
Shadow RAM - During the POST (Power On Self Test) the
microprocessor copies the system board ROM contents into 'shadow RAM'. This is done
because RAM is accessed in nanoseconds versus the slower access of ROM.
Most of the newer motherboards shadow the BIOS as a default. |
