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•••••••••••••••••••••• I UNDERSTANDING • • () Memory is one of the most important parts of a personal computer but is probably the least understood. This article will describe how the PC uses memory, what types of memory are available and how to make best use of the options. By BRIAN RICHARDS* Let's start with a little history. IBM introduced the PC in 1981, with the capability of using up to 640 kilobytes (640Kb) of memory. At the time, this was 10 times the memory available in virtually all other personal computers. This was enough for most users for the following 2-3 years, during which the IBM PC developed as the number one personal computer. It also became the standard upon which many other manufacturers would base their own designs. · • Brian Richards is Marketing Manager of Hypertec Pty Ltd, 408 Victoria Road, Gladesville, NSW 2111. 4 SILICON CHIP As PC users became more numerous, the demand increased for more powerful software and with this came the demand for more memory to run this software. Also, more power was demanded by users no longer content with the processing speed of the 8088 chip. This resulted in the development of the 80286, the 80386 and the 80486 processors, each delivering more power and functionality. Added to this was the introduction of networks and the memory hungry software needed to run them, often alongside the operating system and the applications. Today, it is commonplace to buy a PC or PS/2 machine with 1Mb or 2Mb (Mb = megabytes) of memory as standard. However, many business machines are running with 8, 10, 12Mb and more, with the advent of operating systems like OS/2 and Xenix, which need large amounts of memory to load and run individual applications. DOS&memory The original and still the most popular operating system for the PC is DOS. To understand how PC memory works we need to look at how DOS uses memory. Each PC has a set amount of memory that it can address. This includes the ROM (Read Only Memory for controlling the display, the hard disc and the movement of data in and out of the computer); RAM (Random Access Memory for the software to use); and Video Memory (for controlling the display image). The system used by PC designers to address memory through DOS is shown diagramatically in Fig, 1. The total amount of memory available to the PC is 1Mb but only 640Kb is available to the operating system and application programs. This segment is known as system memory. This is normally found installed on the motherboard when you get your PC, unless you have a system with less than 640Kb. This is more common in older PCs, which often came with 256Kb or 512Kb of memory. One of the most cost-effective upgrades for the PC is to install a board which brings it up to the full 640Kb of system memory. Most of the more popular application programs now need at least 640Kb to run, shutting out machines with less memory. For the PC or XT, the Hypertec Hyper 256 and Hyper 512 are examples of boards available for just this purpose. If there is a need to add further serial or parallel ports to the machine, then both of these boards are available with the option of extra ports. If a PC AT needs to be upgraded, then the Hyper 128-16 will give an extra 128Kb of memory. All of these products come with easy to understand instructions for installation and a comprehensive manual. Expanded Memory Expanded memory came about because program developers became frustrated with the fact that DOS could only make 640Kb available to them to work in. So three major companies in the PC industry, Lotus, Intel and Microsoft, got together and developed LIM EMS (Lotus-Intel-Microsoft Expanded Memory Specification). What this in effect did was to use a 64Kb window of memory on the "free" area of addressable memory (see Fig.1) to access a much larger region of expanded memory. In EMS version 3.2, there can be up to 8Mb and in version 4.0, up to 32Mb. Now, because DOS came before EMS, it does not know it exists. So to use an expanded memory board, an expanded memory device driver is required. This is loaded from the CONFIG.SYS file during the boot process. Once the hardware and software drivers have been installed, application programs use the expanded memory by making a request to the driver, and must therefore be written specifically for EMS 3.2, 4.0 or both. The driver translates the requests into the correct commands to the EMS memory board, to move the window to the correct position in the expanded memory to retrieve the page requested STANDARD MEMORY SYSTEM BIOS VIDEO ROM EXPANDED MEMORY Fig.1: this diagram shows the system used by the PC to address memory through DOS. The first 640Kb is reserved for system memory, with the space above that used for video memory & B1OS ROM. If expanded memory is used, this is accessed via a "free" area of addressable memory (the EMS window). NORMAL MEMORY T EXTENDED MEMORY Fig.2: unlike the earlier XT machines (8086 & 8088 processors), the 80286 & 80386 processors can address up to 16MB of memory. The memory above 1Mb is called extended memory and is accessed via the "protected mode" of the processor. (see Fig.2). Each page is 16Kb in size. The window is divided into 16Kb sections, so a number of pages can be accessed at one time. EMS Version 3.2 has been out for some time now and all expanded memory boards on the market should support it. Version 4.0, which is rapidly becoming the new standard, is a radical upgrade to the capabilities of EMS. Version 4.0 does everything 3.2 does and more. The enhancements of Version 4.0 open the way to new applications, such as the multi-tasking capabilities of Microsoft Windows. Hypertec's Hyperam 286 and Hyperam MC 32/ 16 Expanded/Extended memory boards offer Version 4.0 support. How much memory It is also important to be clear about how much memory you will need. If the multi-tasking capabilities of Version 4.0 are to be used, then it is advisable to specify at least 2Mb. However, if EMS is to be used only to allow larger spreadsheets, it should be remembered spreadsheets can only use about 1Mb of expanded memory. This is because most spreadsheet applications store information in system memory about the information stored in expanded memory. Typically, a spreadsheet runs out of system memory when about 1Mb of expanded memory has been used. This is, of course, just a rule of thumb. This principle applies to all application programs that use expanded memory, not just spreadsheets. However, it is also possible to use expanded memory for utilities such as RAM discs and print spooling. A RAM disc will improve the performance of disc intensive processes, while a print spooler can significantly reduce the amount of machine time you waste while waiting for a print job to finish. Hypertec boards are supplied with some of these utilities as standard. Extended memory boards Extended memory is memory in the address space above 1Mb, the maximum addressable by the PC. The 80286 processor on the AT bus can address 16Mb of memory. The region between 1Mb and 16Mb is called extended memory, and is accessed via the "protected mode"of the 80286 processor. The 80286 can run in two modes: real and protected. In real mode it behaves like a faster 8088, while in protected mode it allows access to all the extra facilities of the 80286, including the 16Mb of memory. The 80386 also has protected and real modes, as well as a third mode called virtual, in which it operates like a number of 8088 processors, so it can run a number of different programs simultaneously. Extended memory cannot be used in a PC or any maDECEMBER 1990 5 machine. The only difference is that when installed in a 286 machine, it can be configured as extended memory as well as system or expanded memory. A multi-function version is also available with one serial and parallel port. 80386 (32-hit) machines AT-style machines with an 80386 or 80486 processor sometimes have a proprietary 32-bit slot for add-in memory boards. The advantage of this is higher performance for the memory; the disadvantage is that the addin memory boards are typically supplied by the machine manufacturer, at substantial -extra cost. It is worth remembering that 16-bit memory can still be used in the AT bus, at greatly reduced cost, and the performance difference may not be noticeable. MCA machines The Hyperam MC 32/16 can be used to provide extended memory in virtually any MCA (Micro Channel Architecture) machine and can act either as a 32-hit or 16-hit board. It can he supplied as a multifunction version and offers EMS Version 4.0 support. chine with an 8088 or 8086 processor. No device driver No device driver is needed for, extended memory as it is assumed that programs needing this amount of space will be able to control it themselves. There are, however, special programs such as IBM's VDISK driver, which provides an extended memory RAM disc. VDISK puts the processor into real mode and returns control to DOS. Extended memory becomes particularly important with the new generation of operating systems, such as OS/ 2 and Xenix. OS/2 Standard Edition requires at least 1Mb of extended memory to be able to boot and, for satisfactory performance, at least 4Mb is likely to be needed. A rule of thumb for estimating the memory needed to run Xenix is 1Mb to start plus 512Kb 6 SILICON CHIP for each user on the system. As with OS/2, the more extended memory available, the faster the system will run. The right memory board The PC (ie, a machine using the original PC bus) can only use system or expanded memory. Expanding the system memory to the 640Kb limit is the first step in upgrading, as mentioned previously. If expanded memory is required above the system memory, then a board such as the Hyperam 286 is the answer. The Hyperam 286 is a 16-bit board but has been designed so that it also runs in 8-bit PC slots. It supplies 2, 4 or 8Mb of memory which can be used iri PCs as system or expanded memory. As a 16-bit board, Hyperam 286, may also be used to offer the same memory upgrades to a 286 or AT-bus Roughly the same considerations apply to the PS/2 range as for the AT. A PS/2 MCA (Micro Channel Architecture) machine has at least 1Mb already installed, which supplies 640Kb of system memory and 384Kb of extended memory. Some of these machines allow you to place more memory on the motherboard but this will not necessarily provide expanded memory capability. A more flexible and cost effective solution may be a memory board and in the case of MCA machines, there are 32-bit boards available from third party manufacturers. The Hyperam MC 32/16 is one such board and it may be used in any MCA machine. It acts as a 32-bit board in the Models 70-386, 70-486, P70 and 80, while also acting as a 16-bit board in the Models 50, 50z, 60 and 65SX. It can come in a multifunction version and offers EMS Version 4.0 support. So, before purchasing a memory upgrade for your particular machine, think carefully about what you are currently using it for and what you may wish to use it for in the future . This much determined, use the guidelines above to ascertain how much and what type of memory you require. Remember, applications are getting more and more memory hungry with each new release. You are probably going to be using your PC for some years yet, so a little forethought should ensure fewer problems ih the long term.