 Number 198 - November 1999 |
| Thanks for the Memory | ||
| by Alan Lynn NOCCC. edbytes@aol.com - July 27, 1999 | ||
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Memory can be a complex
subject, but, like many complex things, it can be broken down into
simpler pieces. For the sake of argument, memory is going to be classed
as one of three kinds of memory.
Storage. Slower, changeable, but long lasting. Sometimes, erroneously, called permanent storage. ROM, Read Only Memory. Unchanging (mostly) not written to, you only read from it. Permanent instructions for the computer on a chip. RAM Random Access Memory. This is where the work of the computer is done. You can read and write to it. It only lasts as long as the power is on. It is called transient memory . There are those who do not classify storage as memory, yet it is measured in bytes, just like the other kinds of memory. Let us look at and define some terms. Bit. Computers are binary, there is either power or there is not. Power on is a one, power off is a zero. With this binary system, you have to count differently. 00000000=zero, 00000001=one. Easy enough, but that is as far as the first column will take you. Every time you move over a column, you double the previous column. The first column is Ones, making the second column Twos. 00000010=Two and 00000011=Three because one two equals two and two plus one equals three. The following columns are Four, Eight, Sixteen, Thirty-Two, Sixty-Four, and One-Hundred-Twenty-Eight. These are the first eight bits. Byte. Eight bits make a byte, a byte makes a character such as letters or numbers. Kilo. Metric for one thousand. Mega. Million or one thousand kilos. Giga. Billion or one thousand megs. Tera. Trillion or one thousand gigs. Peta. Quadrillion or one thousand teras. A small problem occurs; memory is measured in binary, if you keep multiplying by two you get to 1024, the closest to one thousand you can get. Therefore a kilo in memory is equal to 1024 bits. Storage started as punch cards, then paper tape. Magnetic storage started on tape, large tape for mainframes and cassette tape for home computers. Tape has a problem, it is linear, you go from one end to the other, and there is no shortcut. This means that for some things, like backup, it works but it is slow. Next came floppy disks; this was a major improvement, but they are limited in storage space and they are fragile. The first hard drives were expensive, their storage was small, about five megabytes, and physically large. This has improved; hard drives are now measured in gigabytes. Windows uses your hard drive as virtual memory, a swap file. It stores data there when it runs out of RAM. Even with the size improvements and access speed increases, hard drives are many times slower than RAM. Of course, slow memory is better than no memory. For relatively long-term storage, a hard disk is very good. However, they are mechanical devices, they do wear out, they are not permanent. ROM. Read Only Memory. The main thing stored on ROM is instructions. Your motherboard uses ROM for its BIOS (bye-ose) Basic Input Output System. The BIOS tells your computer it is a computer instead of a microwave or other high-tech gadget. It doesn't stop there; many other parts of your computer also have |
their own BIOS. Video, Sound anything that
is plug and play has BIOS. ROM is hardwired instructions; you, as a
user, cannot put anything into it. The exception is upgradable Flash
BIOS, but that is just an upgrade of the instructions.1
RAM. As you can see, the two kinds of memory we have just discussed are not what you usually mean when you talk about memory. The kind your computer can use to run programs is called RAM, Random Access Memory. RAM comes in two varieties:dynamic and static. Dynamic RAM is volatile, it has to be refreshed or recharged every few milliseconds. It is relatively simple, doesn't use much energy, is physically small per megabyte, and is cheap. This is the memory most people are thinking of when they say, My computer has 32 megabytes. DRAM, as it is called, has been improved over the last twenty tears. PCs started with 64 kilobytes of RAM. This RAM ran at over 100 nanoseconds (that is slow). About two years ago the best RAM was 72 pin EDO DRAM (Extended Data Output). If your computer is over about a year or year and a half old you could have this kind of RAM. EDO DRAM runs at 60 nanoseconds and that is as good as that kind gets. A new player is 134 pin SDRAM. Synchronized DRAM, synchronized with the CPU, (central processing unit), runs at 10 nanoseconds. Next was PC100 SDRAM running at less than 9 nanoseconds; this is needed by machines powered by Intel with Pentium II 350 Megahertz and up, also AMD, American Micro Devices, K6-2-3D-Now, 300 MHz and up. Memory requirements are up but prices are down. It costs less to buy 64 Mb of PC100 SDRAM today than it did to buy 4 Mb of EDO DRAM three years ago. Newer faster memory is on the horizon; there will be major changes over the next few years. As fast as DRAM is, the CPU is faster. By the time we had the 386 the CPU spent increasing amounts of time in wait states while the DRAM refreshed itself. Something new was needed, Static RAM. More complex, using more energy, physically larger per Byte, more expensive, but also, faster. SRAM has no refresh cycle, so small amounts placed close to the CPU could reduce the number of wait states by storing data to be used during the DRAM refresh cycle. This memory is called cache memory. The French trappers of the past would take out supplies and put them into storage areas on the outbound trip for use on the homeward trip; they called them caches. You carried less with you, yet had supplies when you needed them. This small cache built on the chip itself is called the Level 1 cache. All CPUs since the 486 have this L1 cache. If you take even more cache and place it on the motherboard, you then have a Level 2 or L2 cache. The closer the cache is the faster it can be; therefor, if L1 is good why not put the L2 on the chip also, like the Pentium Pro. This can be expensive so compromise by making a small circuit board close to the chip and placing the L2 there like the Pentium II. On the other hand, you can place an L1 and L2 on the chip and add another level on the motherboard, an L3, like the AMD K6-3. There are many ideas on how to do this best and new ways are being tried; always a balance between complexity and feasibility. The question is, it may be exotic and fast, but can we make it at a price that will sell?
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Number 198 - November 1999
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