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How it works: Random Access MemoryMar 08 '00 Write an essay on this topic.Random Access Memory (RAM), is probably the most misunderstood bit of hard-ware in a Personal Computer (PC). Most people want more of it in their PCs without having a thorough understanding of its benefits or limitations. They’ve all been told the more RAM their PC has, the better it will perform, especially under the Microsoft Windows 95/98 and now 2000, operating environments. Just how much RAM is enough anyway? And better still, what is RAM and how can it make your Windows 95/98/2000 PC stronger, swifter, and able to leap the memory barrier in a single byte? RAM comes in many shapes, sizes, speeds, and configurations, but the most popular is Double Inline Memory Modules (DIMMS) which are fast replacing Single Inline Memory Modules (SIMMS). SIMMS memory modules—also called chips—come in 1, 4, 8, 16, 32, and 64-megabyte denominations, while DIMMS typically come in 32, 64 and 128-megabyte configurations. Both types can be purchased over the counter in either 72 pin (SIMMS) or 128 pin (DIMMS) configurations. The modules are easy to install, and the PC will recognize them upon system startup, or after a simple BIOS refresh. Microsoft Disk Operating System (MS-DOS, and yes there are still some people out there that use it), and other DOS’s that emulate it’s functionality, break RAM up into three distinct memory regions: 1. Conventional memory (0-640 kilobytes); 2. High Memory (640k-1024meg); and Extended Memory (XMS), and in some cases Expanded Memory (EMS), which cover all memory above 1024megabytes. Each region of memory plays a role in the functionality of the PC, and each is critically important. Note: MS-DOS is an 8 bit operating system and as such has certain built-in limitations, one of which is flat memory management, while 32bit operating systems such as IBM OS/2, Linux, Microsoft Windows 95/98NT/2000, and Apple System 8, address memory in a linear fashion. This means that they can address RAM directly without limitations, so if you have 16 megabytes of memory, the operating systems sees that much and can load a program into any region of RAM. The following is a breakdown of RAM by memory region. CONVENTIONAL MEMORY: is the first 640 kilobytes of RAM and is by far the most critical in terms of program loading in the MS-DOS environment. Conventional memory is where all programs must load—including DOS itself, as well as program device drivers (real mode drivers in Windows 95/98), and Terminate Stay Resident (TSR) programs. When an icon is double clicked in Windows 3.1 or a command given at the DOS prompt, the executable file for that program is taken from the hard drive or floppy and placed into RAM where it will reside until the pro-gram is closed. If the program needs more files to run itself, such as a spell checker, these too are brought into RAM, where they will reside until no longer needed. As more and more programs are initialized conventional memory goes down, until there is not enough left to start other programs. It does not matter if you have 4, 16, or 32 megabytes of RAM all programs in the MS-DOS environment must start in the first 640k of memory. This is know in as the 640k barrier and is the exclusive province of MS-DOS and other DOS’ that emulate its functionality. In an operating environment such as Windows 3.1, where multiple programs can be opened at once, the 640k-barrier becomes a very noticeable problem. Contrary to popular belief, Windows 3.1 does not solve the 640k-barrier problem; indeed Windows 3.1 must itself load into the first 640k of memory! Windows 3.1 can only take control of the memory above 1024 megabytes. HIGH MEMORY: also know as Upper Memory, is 640k to 1024 Megabytes, which is 384k of XMS memory. This memory is reserved for those devices needed to run the PC, i.e. video cards, I/O calls, hardware interrupts etc. The entire 384k is not used however, and as much as 250k lay wasted until the release of MS-DOS version 5.0. With MS-DOS version 5.0, the portion of high memory not used by the PC could now be accessed by a device know as EMM386.EXE, an EMS memory emulator/manager that can convert XMS memory into EMS memory. Once converted a portion of MS-DOS itself loads into high memory, along with as many device drivers and TSR’s as can fit. Loading MS-DOS and the device drivers/TSR’s into high memory frees up conventional memory, and remember, the more conventional memory you have the more programs you can load. With each new MS-DOS version, memory management became easier and more efficient, recovering more high memory for use. EXPANDED MEMORY/EXTENDED MEMORY: is that portion of RAM, 1024 mega-bytes, 640k and above. Certain programs can address EMS/XMS memory directly, but most must rely on memory managers such as MS-DOS’ HIMEM.SYS (even Windows 95/98) to do the work for them. EMS memory was first on the scene and was used in the mid 1980’s by programs such as Lotus’s Lotus 123 and Aston Tate’s Dbase III. These programs had the ability the address the memory directly and used it to load very large data files. But EMS memory is limited in it’s use and can only be addressed 64k at a time, so XMS memory was born. Very few programs written today use EMS memory. XMS memory a vast improvement over EMS memory, in so far as all of XMS memory can be addressed. With the introduction of Windows 3.1, XMS memory took on a new importance, for Windows 3.1/95/98 can address XMS memory directly and takes control of all available XMS memory. Windows then uses it to store files, as a print spooler and can hand it out to programs that request it. Windows can also emulate expanded memory from extended memory if a program needs it. So how much memory is enough? Well that depends on what operating system your PC is presently running under. If you operate under the MS-DOS/Windows 3.1 combination, 8 megabytes is certainly not unreasonable. If you use Windows 95/98/2000 then the sky’s the limit since they address RAM as a whole. Since every machine is different the question of how much RAM can only answered by you and your PC.  |
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