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The Great RAM Scam Of 1994 The 9-bit wide SIMMs in your computer may not be 9-bit devices at all. Your PC could be headed for a fall because of a new cost-cutting trend in the Asian market. We take a look at the possible consequences & what you can do to guard against it. By DARREN YATES Picture this: you’re sitting at your PC and working away feverishly. All of a sudden, for no explained reason, your PC crashes and you’ve lost the last half hour’s work. Believe it or not, this is becoming a more common event than most people re­ alise but more often than not it is blamed on the old dreaded “power glitch”. While the adoption of the IBM standard has ensured that software designed to run on the PC will run on most “compatible” machines, it seems the same cannot be said for the hardware side of things. As the number of manufacturers climbing onto the PC bandwagon appears to be forever increasing, so too are the chanc­es of hardware clashes and conflicts. And we’re not only talking about add-on boards here. In the last few weeks, we’ve found an alarming trend in the one area you would have thought was considered safe against the ever-vigilant eye of the penny-pinchers – the RAM modules. We recently received information from a couple of readers, David Eather and Pat Andersen from the Queensland University of Technology, about a new RAM scam: some 1Mb and 4Mb 9-bit 3-chip single in-line memory modules (SIMMs) are being supplied with only 8-bit wide RAM with the socalled parity bit RAM being replaced with a cheaper parity generator chip instead. To understand the consequences of this fully, let’s look at the basics of a RAM module. In the IBM PC, memory is organised into rows of eight bits, called bytes, into which information is stored. To their credit, the designers of the IBM PC incorporated parity error detection. Parity error detection goes back a long way and was first used in computers during the 1950s. There are two different parity error detection systems: odd and even. SIMM modules with the bogus parity chip are readily identified at present because they have two surface mount resistors on the chip-side of the board. These are not present on the “real” modules but it is ex­pected that they will disappear eventually. To make matters worse, the parity chips are labelled in such a way that they can easily be mistaken for 1Mb chips. 10  Silicon Chip Both add a single bit to an 8-bit data word and its value is determined by the number of ‘1’ digits in the data word. That extra bit is referred to as the parity bit. In an odd parity system, as used in the IBM PC, the parity bit is assigned a value of one or zero so that the total number of ‘1’ digits in the transmitted word is odd. For example, if an eight bit data word 01011010 is to be transmitted, the parity bit becomes 1, to give five 1s in the 9-bit transmitted word 101011010. Now if a 1-bit error in any digit position occurs in the storage (writing) or retrieval (reading) process, the actual parity of the received data word will not agree with the parity bit. Hence the error can be detected. However, there is no way of knowing which bit is wrong. Furthermore, if there are an even number of 1-bit errors in a single data word, the parity of the received data word will not change and the errors will not be detected. So parity only provides a limited degree of error detec­tion. Be that as it may, it is better than no error detection at all. And when you have a SIMM with a parity bit generator instead of genuine parity bits, you do indeed have no error detection at all. Bogus SIMMs have no parity bit What is happening now in a few Asian manufacturing houses is that this parity bit RAM is being replaced by what they call a parity generator. This chip looks at the 8-bit data words stored in memory and generates the parity bit itself. So instead of the computer receiving what it thinks is 8-bit data words together with the parity bits stored in RAM, the parity generator IC feeds it a parity based on what it sees in the RAM. So even if the stored data in the RAM is wrong, the corresponding parity bit received by the computer is correct and no bit errors are detected. It simply boils down to the fact that with these SIMM modules, no parity checking is being done at all and the data, warts and all, is being processed as normal. This is basically a scam - people think they are getting 9-bit wide RAM with error checking when in fact they are being sold 8-bit wide RAM with no error checking. Time delay errors However, there is something potentially more dangerous in this bogus system of generating the parity bit and that concerns the time delay. When parity is retrieved from memory, it is available at the same time as the byte of information required so there is no time delay. With parity generation, as is the case with these new SIMMs, there is an inevitable time delay between the byte of information appearing and the parity bit being produced as the parity generator chip does its calculation. Our information is that this delay could be anywhere bet­ween 7ns and 30ns. Now while that might not sound like much of a delay, most memory today runs at 60ns. A 30ns delay constitutes half a clock cycle on these SIMM modules and this could cause severe timing problems within the computer. There are many functions being performed on memory in just one clock cycle. Things such as refreshing memory and multiplex­ing of address lines so that the correct byte can be found are all performed within a clock period. To now have a parity bit arriving up to 30ns late could easily prove disastrous, particu­larly if the time “window” for obtaining the value of the parity bit has come and gone. Can it really be true When we first heard of this, we thought it too fantastic to be true, even though our correspondents David Eather and Pat Andersen had provided us with a sample bogus SIMM. To get confir­mation, we called RAM suppliers Pelham Pty Ltd (who, by the way, do not supply these bogus SIMMs) to check the story and they con­firmed it to be true. Apparently it is widespread. So why do the manufacturers do it? Well, surprise, sur­prise, there are big savings to be had. Based on the information from Pelham, it seems that there is a $5 saving in production costs by replacing the parity bit RAM with a parity generator chip. Now that may not seem like much but it gets better (or worse, depending on how you look at it). For a 4Mb SIMM, the saving increases to $19. For an 8Mb (72-pin) SIMM, it’s $27 and this increases to a whopping $137 for a 32Mb 72-pin SIMM! Yet only a tiny portion of these savings is passed on to the consumer who remains “in the dark”. When you consider the huge quantities of SIMMs produced, it adds up to millions of dollars. SATELLITE SUPPLIES Aussat systems from under $850 SATELLITE RECEIVERS FROM .$280 LNB’s Ku FROM ..............................$229 LNB’s C FROM .................................$330 FEEDHORNS Ku BAND FROM ......$45 FEEDHORNS C.BAND FROM .........$95 DISHES 60m to 3.7m FROM ...........$130 What can you do? If you’ve bought a new PC or upgraded your current PC in the last couple of years, then chances are you’ve bought some SIMMs along the way. So how can you check to see if your have the ridgy-didge item? At present, those SIMM modules with the bogus parity chip have two surface mount resistors on the chipside of the board. These are not present on the “real” modules, however, it is ex­pected that they will disappear eventually. What makes it even worse is that the parity chips are labelled in such a way that they can easily be mistaken for 1Mb chips. The sample that we have, as you can see from the photo­ graph, has the parity chip labelled as BP41C1000A-6. Now the “1000” code is commonly used to designate a 1Mb x 1-bit wide RAM chip. So the use of this code for the parity chip is clearly meant to deceive the purchaser. The less scrupulous resellers are supplying these SIMMs in place of the proper item. However, we have been assured by Pelham that they only stock the genuine 9-bit wide SIMMs. The basic lesson here is be careful if you come across cheap SIMMs. Chances are, they could be dodgy. And when you are buying a new system, it would be wise to specify SIMMs with SC genuine parity bits! LOTS OF OTHER ITEMS FROM COAXIAL CABLE, DECODERS, ANGLE METERS, IN-LINE COAX AMPS, PAY-TV DECODER FOR JAPANESE, NTSC TO PAL TRANSCODERS, E-PAL DECODERS, PLUS MANY MORE For a free catalogue, fill in & mail or fax this coupon. ✍     Please send me a free catalog on your satellite systems. Name:____________________________ Street:____________________________ Suburb:_________________________ P/code________Phone_____________ L&M Satellite Supplies 33-35 Wickham Rd, Moorabin 3189 Ph (03) 553 1763; Fax (03) 532 2957 December 1994  11