Memory management in PCs: the eighteen most misunderstood acronyms. (The CPA & the Computer)by Smith, L. Murphy
Some of the most confusing computer terms relate to memory-management in personal computers. For example, similar terms are often used such as, expanded and extanded, or high and upper memory. These terms refer to different concepts, in spite of the similarity of words. The acronyms commonly used for these concepts sometimes reflect the wrong characters as a means of distinguishment. For example, the difference between expanded memory and extended memory should be highlighted by emphasizing the "PA" of exPAnded or "TE" of exTEnded--otherwise the words are identical. Yet, the literature uses "EM" (in EMS--expanded memory specification) or "XM" (in XMS--extended memory specification) to describe the two terms, respectively. The letters EM and XM are both found in both terms, and do not distinguish the two terms at all for an uninitiated newcomer.
Random Access Memory (RAM) in a PC starts with the first megabyte (1024k) of RAM divided into lower (conventional) memory, which is the base 640k found in most machines, while the remaining 384k of the first megabyte is known as High Memory. The High Memory consists of upper memory blocks (UMBs) where certain programs can be stored by using suitable memory management software. To add to user confusion, the first 64k of exTEnded memory (RAM) just above the first megabyte) is known as the High Memory Area (HMA). It is the next area above the first 1024k, and has nothing to do with the 384k High Memory previously described. Needlessly confusing? Memory addressed above the first 1024k is exTEnded memory.
For the most part, exTEnded memory can the used only for RAM disks, disk caching software, and certain Windows uses, unless it is converted to exPAnded memory. ExPAnded memory is any memory that can be mapped- into the base 640k; that is, DOS can be tricked into accepting exPAnded memory as if it were conventional 640k memory. ExTEnded memory can be converted to exPAnded memory by using memory management software. ExPAnded memory has more versatile uses than exTEnded memory, such as being usable by spreadsheet programs for loading large files into RAM all at once.
The industry-standard specifications for exPAnded memory started with the Lotus-Intel-Microsoft Expanded Memory Specification (LIM-EMS), then the Enhanced Expanded Memory Specification (EEMS) endorsed by AST Research, Quadram, and Ashton-Tate. The EMMS increased the amount of accessible exPAnded memory beyond the early 64k amount provided under LIM-EMS. Current EMS standards have unified and elaborated upon both of these two early approaches. Today, PCs can have up to 64 megabytes of RAM, and that continues to grow -- although it is now rare to find software that will access more that 16 megabytes of RAM.
Read Only Memory (ROM) is memory that is used to store information such as the system BIOS (basic input output system) that essentially runs the computer. Information in RAM is lost when the computer is turned off, while information in ROM is not lost. Early attempts to make ROM writable included PROMs (programmable ROMs) which could be programmed. Then came EPROMs (erasable PROMs) that had a little window built into the memory chip that was covered with an opaque paper sticker. If the sticker were removed, ultraviolet light would erase the information programmed into the EPROM. Next came EEPROMs (electronically erasable PROMs) which could be erased using electronic signals, rather than by physical exposure to ultraviolet light.
Since RAM tends to operate at faster speeds than ROM, a process of "shadowing" is frequently used whereby the BIOS information residing in slow ROM is copied into the faster High Memory (384k) RAM upon system start-up. The BIOS then can operate much faster, and this noticeably accelerates performance of the entire system.
System setup information for disk drives, video adapter type, etc. is stored in memory known as CMOS (Complimentary Metal Oxide Seminconductor). CMOS memory can be user programmed and reprogrammed, and will retain information as long as it is sustained by a battery. Static RAM (SRAM) is very high speed memory and is used exclusively to cache (buffer) the central processor chip. This is for purposes of reducing the number of processing wait-states to effectively zero. Because of this dedicated function, SRAM is not directly addressed by the user.
The type of exTEnded or exPAnded RAM chips used for program memory purposes are known as dynamic RAM (DRAM), and tend to operate at relatively slower speeds than SRAM. The speed at which any RAM chip operates is measure< in nanoseconds (ns) -- the smaller the number of ns, the faster the memory. DRAM can be obtained in speeds of 150, 120, 100, 80, 70 and even 60ns, while SRAM can be obtained in speeds as fast as 25ns. The speed of the RAM uses must match the speed at which the PC's main circuit board (motherboard) can operate. RAM that is too fast for the motherboard will result in both wasted capability and wasted money, because faster RAM is, of course, more costly. RAM chips come in modules knows as SIMMs (single in-line memory modules), SIPs (single in- line packages), or as individual chips.
It seems that the computer arena is one of the professions most enamored with acronyms. Some reference works cite as many as 10,000 computer-related acronyms. Often the meanings of computer-related acronyms and abbreviations are not really discernable from the immediate context in which they are found. Exhibit 1 provides a list of additional source books for more extensive details on computer related acronyms and Exhibit 2 summarizes the acronyms used in this article.
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