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Understanding emor By Paul Lynch Confused about computer memory? Learn how the memory in your PC is organised & what the various terms mean. SK ME THE years my parents were born and I will consult my memory and tell you. If I have the same information in my computer, I won 't consult its memory I'll look on the hard disc. The stuff there is called "storage ". It's not memory. "Memory" is one of the most misleading words in the computer field. Understanding the difference between memory and storage will make things much clearer for computer users. Failing to understand it causes simple errors which are hard to correct. Here's a simple example: a colleague some years ago bought a type-font A program which allowed for varying sizes of cache. The faster the cache, the faster the font operation. Reasoning that he had a 40Mb hard disc, he allotted an 8Mb (eight megabyte) cache. His system, of course, fell over. According to his computer, all he really had was 2Mb - in RAM (random access memory). He now has a modest 192Kb cache and things work well. The original PC Would that all memory difficulties be sq simple to explain and understand. Of course, it isn't so, for two main reasons. One is that, in 1992, we're still trying to push back the This EEMS RAM card accepts the DIP-style RAM chips. A total of 2Mb can be installed on this full length card. (Electronic Solutions, PC Marketplace). 6 SILICON CHIP envelope created for the original IBM PC more than 10 years ago - a machine without hard discs, a machine with an 8-bit bus, a machine designed with the idea that it might help do things like balance the family cheque book. The other reason is that software developers .h ave run amok designing applications, operation systems and the like, with no common, comprehensive standard for memory usage. So instead of an orderly traffic-like flow of instructions and data through such memory as your computer may hold, you've got something closer to the 9am rush on the first day of the post-Christmas department store sales. Several different applications in your computer can demand the same particular item - and demand it simultaneously. Sometimes there's a winner. Often, everybody loses - the systeip. hangs, or even crashes. This may cost you important data. It certainly isn't what you want. Almost certainly within the next two years, you'll be able to buy PCs which don 't have this problem - or at least, don't have it nearly as often. While they will look like today's PCs, they will -have memory organised differently in what a number of US and Most high density memory expansion boards use at least one VLSI (very large scale integration) chip to take care of memory refreshes and memory formatting. This 8Mb board uses SIMMs memory cards and is designed for the 286 bus. It features an EEPROM (electrically erasable programmable read only memory) to store the memory configuration and will operate as conventional, extended or expanded memory. (Pelham Pty Ltd). European manufacturers have agreed to call the Advanced Computing Environment (ACE). ACE machines will be fast and highly-optioned. Sadly, even before the first ones hit the market, it's obvious that the companies which have set up the standard have ever-soslightly different views of exactly how it should work. So the millennium is not quite as close as advertisers might be suggesting later this year. Let's have a look at the memory in today's PC and examine why it's as it is. today the lineal descendants of that first DOS are still used on all PC clones today: today's machines are much faster and more complex than the original PC and DOS has had to become far more complex with them. DOS originally addressed only 1Mb -- 2Mb EXTENOED MEMORY The DOS environment All PCs are designed to operate in an environment known as DOS. There are many proprietary DOS packages, Microsoft's MS-DOS being the bestknown. IBM's is called PC-DOS and another leading brand is Digit al Research's DR DOS. All arise from a few weeks' work by a young Bill Gates back in the 1970s. Gates was asked by IBM to figure out an environment for a machine that IBM never expected to be such a big seller and so he designed an operating system for the size and capacity of the first machine IBM had in mind. It's a tribute to human ingenuity that ,- 1Mb HIGH MEMORY USUALLY HAS BIDS AT TOP, VIOEO DISPLAY AT BOTTOM 640Kb VIDEO DISPLAY ,_ AVAILABLE RAM :=~ PERATING SYSTEM Fig.1: how memory is configured on most PCs with 2 megabytes of RAM. All or part of the extended memory shown could be configured as expanded memory. of memory, which was the amount in the original PC chip, the 8088. It seemed generous, fabulously generous, at the time and in terms of the simple software available at that time, it was. The 8088 chip stored its own BIOS (basic input output system) at the "top" of that 1Mb. IBM decided that the video memory - the stuff that lets you see on your monitor what's going on in the CPU and your applications - would start at hex A000, an "address" located 640Kb from the start of the chip's memory. The space below the 640Kb was allocated to CPU random access memory (which is what most people mean when they talk about "memory"). DOS itself needs chip space, and grabs the lowest part of that 640Kb of memory for itself. The memory above the 640Kb limit was called the Upper Memory (UM) by IBM. No matter how much memory you have in your PC today, the space between 640Kb and 1Mb is still called upper memory. It totals 384Kb. This was sensational 10 years ago and many thought it offered more than mere humanity could ever desire. Software complexity They did not reckon on the increasing complexity of software. Here's an example of that: one of the first DOS packages was the word processor WordStar which sold on one 360Kb floppy disc. The last version I bought, some five years ago, occupied 19. It did a great deal more than the first APRIL 1992 7 This 6Mb RAM card comes with drivers that support both EMS & LIM 4.0 operation. Also included is software to support a printer spooler & RAM disc. The RAM disc can be configured to any size and will speed up any application where frequently used files can be stored in RAM. (Rod Irving Electronics). This 30-pin SIMMs (single-in-line memory module) card has a capacity of9 x 1Mb Fujitsu MB81C1000 surface-mount chips. Eight of these cards can be fitted to the BOCA 8Mb card shown on the previous page. (Pacific Microelectronics). version and it made major demands on my hardware to do it. But even this 1: 19 ratio doesn't give you the full picture of how software is pushing the hardware envelope. For presentation-quality word processing, I now use a Windows-based package and its demands are such that no original PC can run it. I need a higher-level CPU chip - 80286 or better - and I need more memory than that original 1Mb. The new minimum is 2Mb of RAM. And remember, word processing is one of the simplest data processing applications. Now the 80286 chip is much faster than the original 8086s and 8088s. The 80386s and i80486s are faster again. But all these younger chips are backward compatible with the original PC chips. So they all have the original 1Mb limit of chip memory and the original 640Kb limit on RAM. Expanded memory So a new form of memory had to be 8 SILICON CHIP designed for those programs that need more RAM than 640Kb. Leading software (yes, software) companies got together with the chip-maker Intel and agreed on EMS - the Expanded Memory Specification. Its first version allowed software to run above the 640Kb barrier, in upper memory spaces unused by video, BIOS and accessory cards. Version 4, released in 1987, allows you to run up to 32Mb of expanded memory. This was quite an advance on the original 1Mb but it's only a small portion of the amount of memory you can address on the modern PC if you have a lot of money and the burning desire to spend it. You can address up to four gigabytes of memory from an 80386 chip. I can't imagine that anybody would, but then, the IBM engineers couldn't imagine that anybody would want more than 640Kb of RAM not so long ago. Most PC users know of one more type of memory - extended. This is not a particular memory type. It's simply any and all memory in the computer above the original 1Mb that the CPU can address. Expanded memory is usually fitted to the computer in expansion boards. It can be converted by software from extended memory. Microsoft Windows allows this on 80386 and i80486 machines and other applications allow it as well. Software juggling of this type speeds up the operation of most applications and makes others possible. If you are working in graphic-intensive applications, you want expanded memory and software juggling can be the cheapest way of getting it. It can also create clashes with other applications or your normal mode of operation. The memory you've software-configured as expanded cannot be used at all on an 80286 or 80386 machine running Windows in protected mode. You can see that a certain amount of confusion about memory arises from the language designed to describe machines now almost obsolete. If you lash out to buy an i80486 with 16Mb of RAM, the "upper memory" finishes 15Mb below the "top" of the actual memory present. And by software juggling, you can have either no expanded memory in the machine, or megabytes of expanded memory, whenever you wish it. You may find this linguistic eccentricity irritating and confusing, but it arises from honourable motives. The backward compatibility of the 86 series means that no favourite old software has to be discarded as you move up the speed slope within the family. Shadow RAM There are other features of memory that the original IBM team probably never considered. Take shadow RAM, for example. Much of upper memory holds Read Only Memory files. You can read them but you can't change them. ROM is slow and if you're using software that calls on your BIOS a lot, this can slow things down. So with some of the newer chips, you can "shadow" (or imitate) your ROM with part of your much faster RAM. This gives you faster operation, at a price: the part of upper memory you're using for shadow RAM is no longer available for applications to use for RAM. So it may be to your benefit to use shadow RAM or it may be to your disadvantage. Only you can tell by trial and error. Another piece of software juggling is the RAM disc. You've just read how RAM can pretend to be ROM ..It can also pretend to be a hard disc. If you set up a "RAM disc", you have fooled DOS into believing it's your hard disc. You have mock tracks and sectors and DOS can call on it just as it does on your hard disc. But much, much faster. When calling out information from the RAM disc, DOS has no need to move read/write heads, no need to wait while the right hard disc sector rotates under the heads, and so on. RAM discs are usually not large. They measure in the kilobytes or less. If you lose power while your RAM disc is open, you lose everything in it. And there's a certain amount of housekeeping. The most recent versions of DOS offer RAM discs as part of the package. Disc caches More versatile for most purposes is the software attribute called a "disc cache". There's a DOS disc cache and a Windows disc cache and you can go to the computer shop and buy a specialised utility that's nothing but a cache. The better ones work out what you want your computer to do and change around to help you do it. Key in a print instruction, for example, and part of the disc cache switches to enlarge your print spooler. Most hard disc users certainly have provisions for a disc cache in their software, whether they use it or not. Using it reduces wear and tear on your disc and speeds up many of your operations. It can cause trouble in a number of ways - especially if you plan to compress or unfragment your hard disc using Norton Utilities, PCTools or some similar package. All such packages advise you to disable your disc cache before disc compression. If you don't do this, you run the risk of trashing many of the files on your disc. The easiest way to disable it is to put a REM notation at the start of the line establishing the RAM disc in your CONFIG.SYS file. I've not addressed the issue of memory clashes in this piece. It merits a fresh start with a clear mind. What they are, how you recognise them, and how you resolve them, will be the subject of a future article. SC *** SAVE ON OUR APRIL SP COMPUTER CONNECTOR & CABLE SPECIALS Quality connectors, leads and adaptors at great prices including DB9/25 and IDC connectors, ZIF SQCkets and many popular memory ICs. 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