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Upgrading PC Ram: PCBUILDs Memory FAQ

by Bob Wright
1999/09/29

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.

 

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