Memory

Types of Memory There are several different technologies available today when it comes to memory. No longer can you just buy a SIMM and stick it in.        There are many types available. Let's discuss them here. ROM : This is read-only memory, memory that can only be read, but cannot be written to. ROM is used in situations where the data must be held permanently. This is due to the fact that it is non-volatile memory. This means the data is "hard-wired" into the ROM chip. You can store the chip forever and the data will always be there. Besides, the data is very secure. The BIOS is stored on ROM because the user cannot disrupt the information.

There are different types of ROM, too: Programmable ROM (PROM): This is basically a blank ROM chip that can be written to, but only once. It is much like a blank CD-R which can be written only once. Some companies use special machinery to write PROM's for special purposes. Erasable Programmable ROM (EPROM) : This is just like PROM, except that you can erasethe ROM by shining a special ultra-violet light into a sensor on top of the ROM chip for a certain amount of time. Doing this wipes the data out, allowing it to be rewritten. Electrically Erasable Programmable ROM (EEPROM) : Also called flash BIOS. This ROM can be rewritten through the use of a special software program. Flash BIOS operates this way, allowing users to upgrade their BIOS. ROM is slower than RAM, which is why some try to shadow it to increase speed. RAM : Random Access Memory (RAM) is what most of us think of when we hear the word memory associated with computers. It is volatile memory, meaning all data is lost when power is turned off. The RAM is used for temporary storage of program data, allowing performance to be optimum. Like ROM, there are different types of RAM: Static RAM (SRAM) : This RAM will maintain it's data as long as power is provided to the memory chips. It does not need to be re-written periodically. In fact, the only time the data on the memory is refreshed or changed is when an actual write command is executed. SRAM is very fast, but is much more expensive than DRAM. SRAM is often used as cache memory due to its speed.          There are a two types of SRAM:          1. Async SRAM : An older type of SRAM used in many PC's for L2 cache. It is asynchronous, meaning that it works independently of the system             clock. This means that the CPU found itself waiting for info from the L2 cache. Sync SRAM. This type of SRAM is synchronous, meaning it is             synchronized with the system clock. While this speeds it up, it makes it rather expensive at the same time.          2. Pipeline Burst SRAM : Commonly used. SRAM requests are pipelined, meaning larger packets of data re sent to the memory at once, and acted             on very quickly. This breed of SRAM can operate at bus speeds higher than 66MHz, so is often used.   Dynamic RAM (DRAM) : DRAM, unlike SRAM, must be continually re-written in order for it to maintain its data. This is done by placing the memory on a refresh circuit that re-writes the data several hundred times per second. DRAM is used for most system memory because it is cheap  and small.           There are several types of DRAM, complicating the memory scene even more:          1. Fast Page Mode DRAM (FPM DRAM). FPM DRAM is only slightly faster than regular DRAM. Before there was EDO RAM, FPM              RAM was the main type used in PC's. It is pretty slow stuff, with an access time of 120 ns. It was eventually tweaked to 60 ns, but FPM was              still too slow to work on the 66MHz system bus. For this reason, FPM RAM was replaced by EDO RAM. FPM RAM is not much used              today due to its slow speed, but is almost universally supported.          2. Extended Data Out DRAM (EDO DRAM) : EDO memory incorporates yet another tweak in the method of access. It allows one access              to begin while another is being completed. While this might sound ingenious, the performance increase over FPM DRAM is only around 30%.              EDO DRAM must be properly supported by the chipset. EDO RAM comes on a SIMM. EDO RAM cannot operate on a bus speed faster              than 66MHz, so, with the increasing use of higher bus speeds, EDO RAM is now taking the path of FPM RAM.          3. Burst EDO DRAM (BEDO DRAM) : Original EDO RAM was too slow for the newer systems coming out at the time. Therefore, a new              method of memory access had to be developed to speed up the memory. Bursting was the method devised. This means that larger blocks or              data were sent to the memory at a time, and each "block" of data not only carried the memory address of the immediate page, but info on the              next several pages. Therefore, the next few accesses would not experience any delays due to the preceding memory requests. This technology              increases EDO RAM speed up to around 10 ns, but it did not give it the ability to operate stably at bus speeds over 66MHz. BEDO RAM              was an effort to make EDO RAM compete with SDRAM.          4. Synchronous DRAM (SDRAM) : SDRAM is really the new standard for PC memory. Its speed is synchronous, meaning that it is directly              dependent on the clock speed of the entire system. Standard SDRAM can handle higher bus speeds. In theory, it can operate at up to              100MHz,although it has been found that higher quality DIMMs must be used for stable operation at such speeds. Hence PC 100 SDRAM              Although SDRAM is faster, many users don�t notice the speed difference due to the fact that the system cache masks it. Also, most users are              working on a relatively slow 66MHz bus speed, which doesn't use the SDRAM to its full capacity.          5. RAMBus DRAM (RDRAM) : This is a technology still being developed by Intel that may prove to surpass SDRAM. Its goal is to get rid             of the latency, which is the time taken to access memory. It does this by actually narrowing the bus path and treating the memory bus as a             separate communication channel.

More on PC-100 SDRAM

We all know that, when it comes to memory, that SDRAM is the way to go. It is faster than EDO RAM, and supports higher bus speeds. EDO RAM is moving into the older systems,  mainly, while even the bargain PC's make the move to SDRAM. But, the world of SDRAM is not cut and dry. Standard SDRAM is great for "older" boards. Now, with the release of BX motherboards, and the Super 7 boards, standard SDRAM begins to cause problems. Why? Because even though it was originally said that SDRAM could go up to 100MHz, it really couldn't. In fact, some SDRAM even got unstable at the 83MHz bus speed.  Enter PC100 : Basically, PC100 is SDRAM, which meets a certain specification to work with  stability at 100MHz. This SDRAM usually operates at 10ns, although some is created that is faster. Since the only qualification for PC100 is the ability to operate at 100MHz, there is no rule as to the access time. 10ns is the minimum speed for stability at 100MHz. some companies advertise PC100 faster than this, say 6ns, but, a lot of times you will find this to be inaccurate.  All PC100 is not equal. While it all operates at 100MHz, when you get into higher bus speeds than that, the high-quality stuff starts to stand out. The reason is that the latency rating of the higher quality stuff is lower. The latency is a measurement of how long it takes for other hardware to return data to the RAM. The lower the latency rating, the better the chip, and the faster it will operate.  The most common, and cheaper, type of SDRAM chip uses GL or G8 chips. The "GL" or "G8" will be seen on the actual SDRAM chips on the memory circuit board, so you will know what you're looking at. The GL's use a CAS latency of 3, which is pretty standard. The better stuff uses "GH" chips, which uses a CAS latency of 2. To operate at 100MHz or 112MHz bus speeds, almost any of this PC100 will work. But, bump it up to 133MHz, and you'll need to get the better GH SDRAM with a CAS latency of 2.  Only with this will you get stable operation at such high front side bus speeds. Along with high quality PC100, one must take notice of the printed circuit board on which the chips are mounted. The quality of these boards, for the most part, is measured in the amount of layers. You can equate this to thickness. Obviously, the thicker the material of the board, the less chance you have of damaging it, the longer it will last, and the less  electrical noise you will get. So, the more layers the better. Run-of-the-mill, cheap SDRAM often used good quality chips, but the manufacturer would cut corners by using lower quality PCB�s (Printed Circuit Boards). Often they would use 4-layer PCB's. Well, part of the PC100 spec is a minimum of 6-layer PCB. This ensures a higher level of quality. But, some manufacturers use even better PCB's, such as 8-layer. Pay attention to this. The more layers the better.  So, if you find yourself buying a Super 7 or BX motherboard, you should pick up some PC100 SDRAM with it. The older stuff will work, but, without PC100, you are stuck with your new board's slower bus speeds. SDRAM Considerations As mentioned before, SDRAM is the new developing standard. But, in buying SDRAM for you system, there is some information you must consider. Speed: SDRAM chips are generally named in two different ways. The most common way is the nano second rating. It is said to have a "10 nano second" rating, which is the common speed for SDRAM. The second method is the MHz rating, like "100 Mhz". SDRAM is synchronous, meaning it is tied into the bus speed of the system. This means that the memory must be fast enough to work on the system you intend to put it on. Unlike older memory that used wait states to compensate for slowness, SDRAM does not use wait states. The memory, then, must be fast enough for the system, taking slack into account. It is really this reason why SDRAM was created in the first place: to make memory that could keep up with the system. But, for most systems, EDO RAM does just fine. At the standard 66MHz speed, EDO is a dream, as that is what it was really designed for. But, it was soon found that EDO RAM worked just fine at even higher speeds, such as 75MHz or 83MHz. Some people can get it up to 100MHz, although this is pretty flaky from what I hear. SDRAM was designed mainly to operate with stability at bus speeds such as 100MHz. The problem with this is that, until recently, we really had very few motherboards that could make it to 100MHz. Therefore, how are we to know that that expensive SDRAM will really do it? Simply put, SDRAM offers very little performance increase in real world use over EDO RAM. In fact, in most cases, the performance increase is only around 1 or 2%. For this reason, don't buy SDRAM only for performance. I'm not saying to steer clear of it. Due to the fact that many motherboard manufacturers are starting to abandon EDO RAM altogether, I would definitely recommend buying SDRAM, for nothing else but future compatibility. 2-clock vs. 4-Clock : Two types of SDRAM's are the 2-clock and the 4-clock. Structurally, they are the same, but they are accessed differently. A 2-clock SDRAM module is set up so that each clock cycle accesses two chips on the module. A 4-clock SDRAM setup accesses 4 chips per clock cycle. To choose what kind to get, you must look into the motherboard's documentation. 4-clock modules seem to be the popular choice. Serial Presence Detect Some SDRAM modules have a special EEPROM chip on it that holds information about the SDRAM module, such as speed settings. The motherboard then queries this chip for info and makes changes in the settings to work with the SDRAM. Basically, this allows the SDRAM module and the chipset to communicate, making the SDRAM more reliable on a larger number of motherboards. Some motherboards require this feature. You will have to look at the manual, once again. If your board requires it, make sure you have it, because SDRAM without this won't work. When choosing SDRAM for your computer, you need to know your motherboard and get exactly the type it requires. Serial Presence Detect : Some SDRAM modules have a special EEPROM chip on it that holds information about the SDRAM module, such as speed settings. The motherboard then queries this chip for info and makes changes in the settings to work with the SDRAM. Basically, this allows the SDRAM module and the chipset to communicate, making the SDRAM more reliable on a larger number of motherboards. Some motherboards require this feature. You will have to look at the manual, once again. If your board requires it, make sure you have it, because SDRAM without this won't work.  When choosing SDRAM for your computer, you need to know your motherboard and get exactly the type it requires.

Memory Module Identification

These are examples of the most common types of memory modules found in computers.   72 PIN SODIMM approx. 2.375� x 1� 144 PIN SODIMM approx. 2.625� x 1� SODIMMs are typically used in laptop computers. 100 PIN DIMM approx. 3.5� x 1.25� 100 PIN DIMMs are primarily used in printers. 30 PIN SIMM approx. 3.5� x .75�   30 PIN SIMMs are typically found in older desktop computers. 72 PIN SIMM approx. 4.25� x 1� 168 PIN DIMM approx. 5.375� x 1� 72 PIN SIMMs and 168 PIN DIMMs were used in most desktop systems manufactured .     184 PIN DIMM approx. 5.375� x 1� 184 PIN DIMMs are used in most desktop systems manufactured today.

Installing RAM

First there are some things you need to watch out for when getting RAM.    Buy memory that fits into the sockets on your motherboard. You'll know what kind it is because of the length of the socket.  Get memory that is the right speed for your computer so that it can use the memory without tripping. Look on the SIMM. Somewhere, it should say how fast it is. Look for a number after a dash, like "-100" or "-70". This is the speed in nanoseconds. Consult your motherboard's manual to see what kinds of memory it can handle. In general, you don't want to get memory that is any slower than the memory currently on your computer.  Make sure you don't exceed the amount of memory your motherboard can handle. Different boards have different limits. Again...consult your board's manual for thisinfo. Also pay attention to chipset limitations, such as the TX's 64MB cacheable memory limit. Now I'll give you instructions to put it in. The actual installation is as easy as could be. The problem usually comes from not finding compatible memory, but even this is not a huge problem. 1) Turn off the computer, unplug it, and take off the case cover. 2) Get your memory. Here is where you make sure the above guidelines are met. 3) Install the RAM. First some info, though. Your computer organizes its SIMM  sockets into groups called banks. Some boards say that two sockets make a bank. Some say that one is a bank. Nevertheless, a bank must be full. A half full bank will drive your computer nuts. Also, you can't mix two different kinds of memory in a single bank. For example, you can't put a 4MB SIMM and an 8MB SIMM in one bank and expect to get 12 MB of RAM. Also, many systems require you to put the memory in pairs. Therefore, if you want 32 megs of RAM, you have to stick 2 16's in instead of one 32. Here's a shortcut which is almost always true. An older computer with a 386 or an early 486 chip usually has a 4 socket bank of 30-pin SIMM modules. A later model 486 requires only one socket of 72-pin modules. Pentium machines have two socket banks of 72 pin modules, meaning you must install RAM in pairs. In all of these systems, the bank must be full for your system to operate. Following these guidelines, lets say you want to add 16MB of RAM to your Pentium machine. You could buy one 16MB SIMM, but this won't work because you will have a partially filled bank. You must buy two 8MB SIMMS instead, and install them in a pair. Many newer motherboards have the 168-pin slots for SDRAM. Each SDRAM slot is a bank, so one SDRAM chip will work fine. Other motherboards have both SIMM and DIMM slots. Usually, each DIMM slot is a bank, just like normal. The SIMM slots right next to them are usually paired in a bank, just like the normal Pentium bank setup. Now that that is out of the way, lets install the RAM. First, ground yourself. Then pick up the SIMM and look for the notched end. That notch will only let the thing go in one way, so if it goes in, it�s right. Push the SIMM down into the socket. Tilt the SIMM slowly until the little spring snaps into place and holds it in. With DIMM chips, the setup is slightly different. There are little levers that hold the DIMM in place. When installing, you open the levers. They flip to the side. You then push the DIMM in place. The notch is off-center, so it will only allow the chip in the right way. Once it is in all the way, close the levers. 4) Consult the manual for any jumpers or DIP switches you need to mess with. Flip the ones you need to flip. Most boards do not have such jumpers, but this step is included for those of you that do have such hardware. 5) Turn the system on. Your computer should either greet you with an error message or just boot up normally. If it boots normally, just watch it count up the memory to see if it counts the new memory. If you get an error message, your computer has found the memory and wants you to confirm that you did indeed put more in it. This all happens in the CMOS. Every setup program is a little different. If your machine doesn't seem to be working at all, go over the checklist and make sure you put the right kind of memory in. 6) You're done!

Memory Problems

There are a lot of problems out there to be had with memory. That's why it isn't too fun. Most of the time, these errors can be traced simply to faulty SIMMs or DIMMs. Nevertheless, let's look at some common memory errors, just so you know and you can impress your friends. Two very common memory errors are: "NMI error at [address]" or "Memory parity  interrupt at [address]".  If you have Phoenix BIOS, it goes on : "Type (S)hut off NMI, (R)eboot,(I)gnore". An NMI is a nonmaskable interrupt. This means that your computer isn't allowed to mask this one while finishing another task. When an NMI occurs, you'll know. The whole computer will lock up and will stay dead until you fix the problem.  Most PC's reserve a little chunk of memory so that it can test the rest of the memory. Almost all modern computers use non-parity memory. These computers will not generate an NMI if it finds only one bit or so bad during the test. With these non-parity computers, then, you may get a few random errors in your work due to these bad bits that the memory test hides. It�s not a major problem.  So, what causes this parity error? Most likely you have a bad RAM chip. Either that, or the voltage to the memory dropped and your computer forgot everything. These two possibilities are most likely. Otherwise,  (1) you have bad address logic to the memory chips, making the computer confuse one chip for another, or  (2) a bad chip that reports a problem but is just joking with you because there is none. These last two usually result in getting another motherboard, because the chips that are complaining are irreplaceable alone. To find error sources, though, usually requires a software memory test. There are several out there. Hopefully these programs can point out the bad chip so you can just replace it. If you're lucky, the SIMM just slipped out of the socket somehow.

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