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COMPUTERS: Adding memory To your PC Troubleshooting Your PC; Pt.4 Want to add some more memory to your computer but don’t know what type to use? This article will help you decide which type of memory is right for you. By BOB DYBALL Selecting RAM for your system used to be simple. Neglecting the video card, there were just two choices: static RAM for the cache memory and regular DRAM, or dynamic RAM, for the main memory. Unfortunately, it’s no longer quite so easy. As computer speeds have increased, so the memory choices have become more complicated. There are now several different types of DRAM and it’s important to select the correct type if you are adding extra memory to your computer, otherwise it may not work correctly. As for video RAM, well that’s a whole new ballgame again. But that’s not the end of the story. 40  Silicon Chip The system motherboard will also have an EPROM (erasable programmable read only memory), which contains all the BIOS settings so that the machine can boot up. And some of the expansion cards may include an EPROM or a ROM as well. As you might expect with computers, memory has it’s fair share of buzzwords and jargon to confuse the uninitiated. In this article, we’ll sort out what the jargon really means so that you can decide what sort of RAM is best for your PC. RAM versus ROM As most people know, the word RAM stands for “Random Access Memory”. This type of memory is known as read/write memory be­cause you can both read data from it and write data to it (although not necessarily at the same time). This contrasts with ROM, which is read only memory except when it is initially being programmed. The main differences between Static RAM, Dynamic RAM and ROM chips are summarised in Table 1. It’s worth noting that there are a number of different types of DRAM and ROM. To make matters more interesting, when someone says “ROM”, it might be a mask programmed ROM (ie, one programmed when it was made in the factory) or it could be an EPROM or Flash ROM and they are simply using “ROM” as a generic term. Most motherboards now have a Flash ROM for their BIOS. Older PCs either had a PROM or an EPROM for the system BIOS. A PROM (Programmable Read Only Memory) is designed to be programmed just once. An EPROM (erasable PROM), on the other hand, has a small window which allows you to erase the contents by exposing the chip to UV light for a short time. A special UV lamp is normally used for this but it is also possible that a chip might be erased, or partly erased, when exposed to sunlight or artificial light over a longer period of time. For this reason, the window is usually covered with an opaque sticker to prevent accidental erasure. The E2ROM is an electrically erasable PROM. This means that the memory can be erased electrically, which makes it more con­venient to reprogram. A Flash ROM is an E2ROM and most mother­boards now use this type of device to store the system BIOS since it is easy to update it with new BIOS versions. All you have to do is run a small DOS utility to load the new version code into BIOS. A word of warning here – you must be careful when repro­ gramming a Flash ROM because if you mess things up and the new code is crook, the computer might not boot. Similarly you’ll be in for trouble if you interrupt the writing to a Flash ROM. It may be possible to “hot-swap” the crook unit with the Flash ROM from an identical motherboard after boot up and then re-burn it but you cannot rely on this somewhat dodgy procedure to work all the time. If you don’t have a couple of similar PCs around to try this trick, then you will need to remove the Flash ROM and have it reprogrammed in a special EPROM programmer instead. Table 1: RAM vs. ROM Feature Retains information when power is removed? Power Consumption Dynamic RAM (DRAM) Static RAM (SRAM) ROM No No Yes Moderate H i gh Low Access Speed Fast Very Fast S l ow Main Use In PC System RAM L2 Cache RAM BIOS ROM of capacitors etched into the silicon of the IC. Like all capacitors, these “leak” and, after a couple of seconds, will “forget” what was in them. To prevent this, the memory cells are refreshed every few millisec­ onds (during the “refresh cycle”), so that the charges on the tiny capacitors in the chip are constantly topped up. The obvious drawback of this type of memory is the overhead dictates of the refresh cycle. Essentially, for a part of the overall time, the system will be busy doing nothing except re­freshing the memory. Most BIOSes, however, do have options to modify what happens in DRAM. For example, some have a “hidden refresh” that’s carried out while the PC is also busy elsewhere. Other BIOSes can allow a change in the time between refreshes. Note, however, that while it’s often possible to squeeze an extra ounce of performance out of a PC by increasing the refresh time, you should be careful here. Too long a time between re­freshes can result in errors as Alzheimer's sets in – the memory literally “forgets” the data written to it before the next re­fresh cycle comes along. Video RAM Do you remember “CGA snow?” Back before VGA, in the days of EGA, MGA and CGA video cards, you would often see some flickering on the screen. This was caused by interruptions to the video card when it was scanning the video RAM to produce its screen output. This problem was overcome in time by better video drivers, better video cards and faster memory, especially through newer “dual-ported” memory chips (more on this shortly). By the way, if the RAM on your VGA card can be expanded, check upgrade costs Static RAM or SRAM The memory cells or individual storage locations in static RAM are made up of TTL gates. Its main advantage is that it is fast but there are drawbacks. The larger the chip the more power it consumes and the more heat it must dissipate. And that adds considerably to the cost, as well as placing some restraint on the size of the memory chip. Because SRAM is considerably more expensive than DRAM, it is used in places that need moderate amounts of high-speed RAM, such as the motherboard level 2 cache (or secondary cache). Dynamic memory or DRAM The memory cells in DRAM consist The RAM used on video cards is often dual-ported which means that it can be written to and read from at the same time to speed up operation. This 3-year old Diamond Stealth card uses 2Mb of VRAM, with provision for another 2Mb to be plugged into adjacent sockets. August 1998  41 Table 2: Matching RAM To Your Motherboard CPU Memory Bus S peed Non Parity? Parity? DRAM Type 486 (1) 33MHz Yes (5) Yes (5) FP DRAM Pentium (2) 66MHz Yes (5) Yes (5) FP DRAM Pentium (3) Up to 83MHz Yes (6) N/A ED O D R A M Pentium II (4) 66-125MHz Yes (6) N/A SDRAM (7) Notes: (1). With the exception of a couple of 486 chipsets de­signed for EDO, you should use FP DRAM to avoid instability in these PCs. (2). Older Pentium PCs will usually use FP DRAM. (3). If supported, EDO RAM will be faster than FP DRAM, though EDO RAM might prove unstable at 83MHz (or higher) memory bus speeds. (4). SDRAM DIMMs are the memory of choice for faster Pentium based PCs. (5). Some older 486 PCs have no option to disable parity, so memory upgrades require parity SIMMs. More recent 486 PCs and most Pentiums that support parity will often have an option to disable it (this is saved in CMOS and changed in the BIOS setup options). (6). Many motherboard chipsets, including the popular Intel Triton, no longer support parity and default to a non-parity mode. You can easily mix parity SIMMs and non-parity SIMMs in these PCs because parity is ignored anyway. (7) Make sure you specify that these are for a Pentium II, since they will usually need to have “Serial Presence Detect” EEPROMs. If you don’t use this type of SDRAM module in a Pentium II ma­chine, the memory won’t work. before you buy. Buying the extra RAM with the card is often cheaper than buying it separately later on. As you increase the resolution and number of colours with newer VGA cards, using ordinary DRAM on the card can create all sorts of problems. Between the DRAM needing to be refreshed, your system trying to write to it and the VGA card trying to write to the screen to display the picture, things can either get slow or ugly, or both! The factor here is called memory bandwidth – there is simply not enough of it to spare with the slower types of RAM at higher resolutions. So, to overcome this problem, special types of RAM have been introduced to cater for the needs of VGA cards. These include VRAM, WRAM and SGRAM. means that it can read and write data at the same time. This makes VRAM ideal for use in video cards because the RAMDAC (ie, the digital to analog converter) can read the data it needs to display the next pixel without any interruption – even if the system is busily updating the same areas of RAM containing this data. If you are upgrading a VGA card that has VRAM, check the user manual for the VRAM chips you need for your board as there are many different configurations on the market. If you have lost the manual, check the other chips nearby as the empty RAM upgrade sockets on the VGA card will usually be in parallel with the existing chips. Apart from VGA cards, VRAM is also often used for image capture, in high speed printers and for signal processing. VRAM WRAM, SGRAM, MDRAM Ordinary RAM doesn’t allow reads and writes to be carried out simultaneously. However, VRAM or video RAM does, as it is “dual ported”, which Often, the newest types of video RAM are “vamped up” ver­sions of the latest DRAM, sometimes with some extra features thrown in. Usually, 42  Silicon Chip it’s just the latest DRAM but made dual-ported and given a fancy new acronym. For example, WRAM (or “Win-dowed RAM”) is simply dual ported RAM for use on VGA cards. It’s based on EDO type RAM, which means that it offers better performance than older types based on FP (fast page) DRAM. MDRAM is Multi-Bank Dynamic RAM. SGRAM (Synchronous Graphic RAM) is single (not dual-ported) SDRAM that is designed for use on VGA cards. Many SGRAM chips include on-chip functions to speed up line drawing operations! Parity In addition to handling data bits, some DRAM modules use an extra bit of memory to check for errors. This type of memory is known as parity RAM. Most companies now ship PCs with­out parity RAM but many older PCs can and do use parity memory. Once it was unusual to find a PC without parity RAM. Then, a few years ago and not long after SIMMs were introduced, some manufacturers found that they could shave costs by replacing the parity chip on the SIMM with another chip known as a “parity generator”. They could then sell this RAM cheaper than genuine parity RAM or for the same price but at a greater profit. With genuine parity RAM, the system detected any memory errors and warned if there was a problem. Conversely, if a parity generator was used, a parity check always returned an OK result, even if it wasn’t. This meant that the user wouldn’t be warned of any problems until much further down the track, when the problems had become much worse and affected the PC quite dramatically. These days, memory is considered very reliable and virtual­ly all systems ship with non-parity RAM. On some motherboards, there is provision in the system BIOS to enable or disable parity checking and it’s simply a matter of selecting the correct option to suit the type of RAM installed. DIMMs & SIMMs Two other terms that you will commonly hear in relation to memory are “DIMMs” and “SIMMs”. These terms simply refer to the type of module that the memory is mounted on (the packaging, if you like). SIMM stands for “single inline memory module”, while DIMM stands for “dual inline memory module”. In plain English, a SIMM has a single row of pins and a DIMM has two rows of pins. The buzzword used to be “DIP”, which is an acronym for “dual inline package”. This type of memory (now obsolete) looked just like conventional ICs and the chips were slotted en masse into rows of sockets on the motherboard, sometimes occupying up to a third of the total area. Unfortunately, the DIP sockets sometimes became dusty and corroded over time and the overall reliability suffered because of the sheer number of RAM ICs requiring sockets. SIMMs come in two basic sizes: 30pin and 72-pin. Both types are keyed with a notch at one end, so that they can only be installed the right way around on the motherboard. The 72pin SIMMs are used in more recent equipment but there are still plenty of older 286/386 PCs and 486 PCs that accept 30-pin SIMMs. Most 30-pin SIMMs used for PCs had 9 bits (8 data bits plus 1 bit for parity). 72-pin SIMMs are perhaps the most common RAM package used today but are now being superseded by DIMM SDRAM. DIMMs Featuring 64-bit data, the 168-pin DIMM package has been used for some time in the Apple Mac and is currently the standard type of RAM for most new PCs. Since the popular SDRAM (synchronous DRAM) usually comes in a DIMM package, the terms DIMM and SDRAM are often used inter­ changeably. A “DIMM” is merely the JEDEC standard package, just like 30 or 72-pin SIMMs, and can come fitted with FP (fast page), EDO (extended data out) or SDRAM chips. Common DRAM types Although SIMMs and now DIMMs have made it easier and cheap­er to upgrade RAM than ever before, you do have to choose what type of DIMM or SIMM memory you need. And that depends on your motherboard. Let’s take a look at the three basic types: (1) FP or “Fast Page” RAM: part of the delay in getting data out of RAM is simply getting the addresses loaded into the RAM to begin with. This lead to the development of FP DRAM which provides faster reads from the same “row” (or page) than conven­ tional DRAM. Many older PCs use FP DRAM but later, faster PCs will experience RAM access bottlenecks because the RAM is unable to keep up. (2) EDO or “Extended Data Out” RAM: faster than FP DRAM, 72-pin EDO SIMMs are currently the most common type of DRAM used in Pentium family motherboards. Although EDO DRAM outperforms FP DRAM, your system needs to “know” how to “talk” to it – ie, both the motherboard and the system BIOS must be designed for the job. Unfortunately, most 486 PC users must upgrade using FP DRAM since using EDO would cause timing problems. On the other hand, if you’ve upgraded a Pentium processor or if you’re doing a little overclocking to tweak it up a notch, you should consider using SDRAM since higher clock speeds could run EDO DRAM close to (or beyond) its speed limits. (3) SDRAM or “Synchronous DRAM”: SDRAM is the latest and greatest but it’s not for everyone. If you already have EDO RAM and don’t have stability problems, stick with it. On the other hand EDO RAM might not be too stable at bus speeds of 83MHz, while SDRAM with the correct speed rating can run at up to 100MHz or more. Although the most common DIMMs used in PCs are 3.3V unbuf­ fered SDRAM types, Pentium II users should check when buying DIMM RAM as they will likely need it with a “Serial Presence Detect” EEPROM. Ordinary DIMM memory might not be suitable in such ma­chines. Which type of memory? So which type of RAM should you buy if you want to add extra memory to your PC’s motherboard? The obvious answer is check the manual. The same goes if you wish to add memory Table 3: Number Of SIMMs CPU DRAM Type No. Per Bank 386S X 30-pin SIMM 2 386D X 30-pin SIMM 4 486 30-pin SIMM 4 486 72-pin SIMM 1 Pentium 72-pin SIMM 2 Pentium 168-pin DIMM 1 to a video card (it really pays to store those manuals in a safe place). If you’ve lost the motherboard manual, Table 2 will provide a useful guide as to what type of RAM should work in most situa­tions. You should also check the RAM that’s already fitted to the machine for further clues. You can usually identify the speed of the DRAM fitted to your motherboard by looking at the labelling on the individual memory chips. Usually, you will see a number such as -60 or -70 after the main type number and this gives the speed of the RAM in nanoseconds; ie, 60ns and 70ns respectively for the examples just quoted. Sometimes, however, you have to multiply the number shown by 10ns; eg, if you just see -6, the speed of the DRAM is 6 x 10 = 60ns. The above applies to both 30 and 72-pin SIMM modules and to the older DIP RAM ICs. SDRAM is also labelled with a rating related to the access speed, either in x1ns or x10ns. For example a -1 would mean 10ns. . Installing more RAM In 486 or earlier systems, you will often have to go to CMOS, select standard setup, then exit and save to have the system recognise the extra RAM you have added to the motherboard. In later 486 and Pentium machines, simply adding the memory is enough – the amount of RAM is checked each time the machine is booted up. The amount of RAM you should add to an existing system depends on the applications you wish to run. If you are running a 486 with 4Mb or 8Mb of memory and would like to run Windows 95, then you should definitely go to 16Mb or, even better, 32Mb of RAM. At this level, an upgrade in the RAM department is also usually more cost effective than a CPU upgrade. Of course, a new computer with a Pentium processor and lots of RAM is better again but if your budget won’t stretch that far, a simple RAM upgrade to your existing system can be very worthwhile. RAM is now quite cheap, so there’s no reason to run a system that’s crippled by lack of memory. Table 3 is a general guide to the type of DRAM used in various computers and shows the number of modules needed in each memory “bank”. Some August 1998  43 The system RAM is plugged into sockets on your motherboard, as shown here. Be careful when handling the memory modules, as they are easily damaged by static electricity. older motherboards will require you to fit the same size RAM modules to all banks, while others might allow you to have different size modules. You must, however, fill all the slots in a bank. It’s generally OK to mix parity and non-parity RAM in the same bank but if you do this, be sure to disable parity checking in the system BIOS. Be careful here – many older 486 and earlier machines have no option to disable parity, so you have no choice but to use parity RAM in these systems. As before, the table is a guide only – you should consult the manual for the correct memory configuration for your particu­lar motherboard. That’s because the requirements can vary from one motherboard to another. For example, in PCs running a fast CPU, extra RAM might be needed in a bank to allow “inter­leaving” to increase the effective RAM access speed. If upgrading VGA card RAM, again check the user manual for the type of RAM chips required. If you have lost the book, check the RAM chips that are already on the card. These should be quite easy to find as they will be in parallel with the empty RAM upgrade sockets. Troubleshooting If adding extra memory and other hardware, try to do things one step at a time. Don’t go adding a new VGA 44  Silicon Chip card, a faster CPU and extra RAM all at once! Also, try to use the same type of RAM in all the memory banks; eg all FP or all EDO. Many motherboard chipsets do not allow FP and EDO RAM to be mixed in the same bank, although some might allow EDO in one bank and FP in another. If you start getting errors, try going back to using the same type of SIMMs in all banks. Similarly, if you have a motherboard that takes both DIMM and SIMM memory modules, don’t be surprised if you can’t use them together. Your motherboard manual will usually advise you of any such limitations. Now let’s take a look at some of the more common problems that are encountered with memory upgrades. (1). Parity problems: most new motherboards will not even have a parity option, as many chipsets no longer have this feature. On the other hand, a parity error can occur in an older PC if you mix parity and non-parity SIMMs. On some systems, you will not be able to get into the CMOS setup after installing non-parity RAM into a system that’s set for parity checking (ie, it creates a catch-22 situation by not letting you into the CMOS setup to turn parity checking off). The trick here is to get into the CMOS setup and disable parity checking before the non-parity RAM is installed. It’s generally not too hard to distinguish parity RAM from non-parity RAM. Non-parity SIMMs will usually be fitted with 2, 4 or 8 DIP chips, while parity memory will instead have 3, 5 or 9 chips. (2). System fails to recognise extra RAM: it’s impossible to install SIMM or DIMM RAM modules back to front. Both 30 and 72-pin SIMMs are keyed with a small notch at one end, while DIMMs are keyed by having asymmetrical plug-in connectors. A common “fault” is that the extra RAM is not recognised because the modules are not fully clipped in. If you strike problems, check that the SIMM or DIMM modules are correctly seated in their connectors and that they are retained by the clips. (3). System recognises some extra RAM but not all of it: it is usually quite OK to upgrade a system with different brands of RAM and even use RAM with different speeds, as long as you remember not to mix EDO and FP RAM in this way. Your system will recognise the lowest common denominator though, so mixing a 4Mb SIMM with an 8Mb SIMM in one bank will result in only 4Mb being recognised. Mixing RAM modules with different speeds, say 60ns and 70ns, is not the evil thing some people make it out to be – just don’t try running it all at 60ns! In normal use, you can only expect the system to work reliably at the slower speed (ie, 70ns), so be sure to select this memory speed in the SC CMOS setup.