STANDARDS IBM-Compatibles are not always as "compatible" as you might hope. Within the IBM-compatible field there are various configurations of video display, memory and peripherals (add-ons). By knowing about the standards and variations, you can knowledgeably purchase software and hardware which will work with your system. RAM RAM stands for Random Access Memory. This is in the core of your computer and is used for temporary storage of data and program instructions as your computer is working. RAM is measured in bytes. A byte is an individual cell of 8 switches which can be set to on or off. This equals 256 possibilities, so any given byte can hold a value of 0 to 255. Characters are stored in bytes. For instance, the letter A is stored in a byte as the number 65. A practical way to understand this is to think in terms of a letter. If you write a letter that has 1500 characters in it, including letters, punctuation and spaces, it will require 1500 bytes of RAM to hold the letter as you are working on it. You need additional RAM to hold the word processing program you are using to write the letter. Large quantities of bytes are measured in kilobytes, 1024 bytes (k), and megabytes, 1024 kilobytes (mb). The RAM is divided into three categories within modern computers. The base RAM can contain 640k (655,360 bytes) and no more. Computers of a few years ago had only 640k total RAM. Beyond the 640k base RAM, most computers now have EMS (Expanded Memory) of 384 kilobytes, making a total of one megabyte, and beyond that there may be a megabyte or more of XMS (Extended Memory). Due to idiosyncracies in the original design of IBM-compatible computers, and in order to maintain compatibility with older software, the way these three areas of RAM are used is limited. Most actual processing must occur in the base RAM (640k). Programs which use more than the base RAM must shuffle their additional information in and out of the base to the EMS or XMS as needed. Occasionally this can mean that although you might have 6 megabytes free, your RAM is still all used up. Some programs will access only the base RAM, and it is possible to pile up several such programs, and fill up the base memory. Frequent offenders are TSR and shell programs. TSR stands for Terminate, Stay Resident. These programs are loaded into RAM when first started, but remain invisible until needed. A typical TSR would be a program to remind you that your mother's birthday is tomorrow. Shells are used to replace the user interface to DOS with something more intuitive. Shells list all the programs that you can run with a nice looking menu. When you choose to run a program from a menu, the shell is still in RAM. Other programs manage in additional RAM very nicely. With these, you can do neat tricks like storing several programs in RAM at one time for quick switching between programs, and you can transfer large blocks of data between these programs. Some programs which are large, or work with large amounts of data (graphics programs, especially) are much faster if they can work entirely within RAM. Otherwise, they would have to juggle data back and forth from the hard disk as needed, and this is much slower than shuffling it around in RAM. RAM is not permanently attached to the motherboard, so you can purchase more RAM if you wish. RAM comes in sets of chips of varying sizes, but there are a limited number of slots in the motherboard. If you plug a set of 256k RAM chips into the slots, the motherboard may become full with a total of 2k installed. On the other hand, you can purchase a set of 4mb chips to fill the slots and the same motherboard will hold 32mb of RAM. You cannot mix chips of different sizes, but you do not have to fill every slot. Therefore, when you purchase RAM, it is best to purchase the largest size chips you can, since the next time you upgrade, you might have to discard a series of smaller chips entirely when adding more RAM if the small ones fill all the available slots. VIDEO SYSTEM Most modern IBM-compatibles come with a VGA or better video system. Here are the most common types: MDA - Monochrome text. Monochrome means 2 colors, black and white, or often the actual colors are black and green or amber. This can display text very clearly in one font only, and has no graphics capability. Hercules - Sometimes also called monochrome graphics. This combines the MDA capability with monochrome graphics. The resolution of Hercules is 720 x 348. This means that up to 720 individual dots can be displayed from left to right, and 348 vertically. Dots are referred to as "pixels." All graphics and text are composed of specific arrangements of pixels. CGA - This is now regarded as primitive, but it was state of the art in 1983. CGA can display text in 16 colors, and has two graphics modes. 640 x 200 pixels in 2 colors (monochrome graphics) or 320 x 200 in 4 colors. Even the text mode uses the 640 x 200 pixel resolution which results in rather coarse text. With low quality monitors on some CGA systems, it is difficult to see the difference between an 8, a B and a 0. Many notebook computers use CGA, but because of the square, well-defined pixels they display, text is easy to read. EGA - This is a vast improvement over CGA. EGA has several video modes, plus it can perfectly emulate all CGA modes and Hercules. The best EGA mode is 640 x 350 in 16 colors. VGA - This is even better than EGA. It can emulate CGA, EGA and usually Hercules. It's best video mode is 640 x 480 in 16 colors. It also has a 320 x 200 256-color video mode which is frequently used for games and nearly photographic-quality graphics, but because of this mode's coarse resolution, it does not display small text well. Standard VGA equipment can also display XGA modes. These are not commonly used because programming for them is not well documented, but some games and graphics programs use them. The best XGA mode is 320 x 480 in 256 colors. Super-VGA - This can do everything that VGA does, but goes even further. The best common Super-VGA mode is 1024 x 768 in 256 colors, but this varies with brand and model of video card and monitor. Other modes are 640 x 400, 640 x 480, and 800 x 600 in 16 and 256 colors. Some Super-VGA cards also support more than 256 colors. Super-VGA systems come with varying amounts of video RAM. This is a separate set of RAM from the main computer RAM. The video modes require varying amounts of RAM. In 256-color modes, one byte is required for every pixel shown on the screen. In 800 x 600 mode, for instance, 480,000 bytes of video RAM are required. Some software likes to have two "video pages" available, requiring even more video RAM. With video pages, one page is shown on the screen, but there can also be one or more complete screenfuls of invisible information. Movable graphics, animations and special effects are created by making changes on an invisible page, then switching it with the visible page. The minimum amount of video RAM on a Super-VGA card is 256k. With this, not all Super-VGA modes are possible. The largest common amount is 1mb. Upgrading the amount of RAM on a video card is not usually as easy as purchasing one with all the RAM you want in the first place. DISK DRIVES Hard disks can be any size, as long as there is enough room for the software and data you want to keep instantly available. If your hard disk becomes full, then you can store additional information on floppies. A few years ago, a 5mb hard disk was common, and 20mb was a large model. No matter how big your hard disk is, you will probably fill it within a year and wish you had something bigger. Floppy disks come in two common sizes, 5.25" and 3.5". The 3.5" disks are fully encased in inflexible plastic, and are becoming more and more popular. 5.25" disks require greater care in handling and may become obsolete in the near future. Both sizes come in two flavors, "double-density" and "high-density." The high density disks hold more information, and the high density drives can still read data from the older "double-density" standard. 5.25" double-density disks hold 360k of data and 5.25" high-density disks hold 1.2mb. In the 3.5" disks, double-density hold 720k and high-density hold 1.44mb. At the current time, almost all CD-ROM disks and drives are interchangeable. A CD-ROM disk holds around 650mb, but cannot be erased or changed. This is called "read-only." MODEMS With a modem, you can communicate with other computers via telephone. Modems are measured by the speed at which they can send data. This measurement is called baud, or BPS, and roughly represents how many bits (1/8 of a byte) can be sent per second. Common modem speeds are 300,1200,2400,9600 and 14,400 bps. The faster, the better. _____________________________________________ end of file.