Main memory
A PC's third and principal level of system memory is referred to as main memory, or Random Access Memory (RAM). It is an impermanent source of data, but is the main memory area accessed by the hard disk. It acts, so to speak, as a staging post between the hard disk and the processor. The more data it is possible to have available in the RAM the faster the PC will run.
Main memory is attached to the processor via its address and data buses. Each bus consists of a number of electrical circuits or bits. The width of the address bus dictates how many different memory locations can be accessed, and the width of the data bus how much information is stored at each location. Every time a bit is added to the width of the address bus, the address range doubles. In 1985, Intel's 386 processor had a 32-bit address bus, enabling it to access up to 4GB of memory. The Pentium processor - introduced in 1993 - increased the data bus width to 64-bits, enabling it to access 8 bytes of data at a time.
Each transaction between the CPU and memory is called a bus cycle. The number of data bits a CPU is able to transfer during a single bus cycle affects a computer's performance and dictates what type of memory the computer requires. By the late 1990s, most desktop computers were using 168-pin DIMMs, which supported 64-bit data paths.
Main memory is built up using DRAM chips, short for Dynamic RAM.
Ecc memory
Unlike parity memory, which uses a single bit to provide protection to eight bits, ECC uses larger groupings. Five ECC bits are needed to protect each eight-bit word, six for 16-bit words, seven for 32-bit words and eight for 64-bit words.
Additional code is needed for ECC protection, and the firmware that generates and checks the ECC can be in the motherboard itself or built into the motherboard chipsets (most Intel chips now include ECC code). The downside is that ECC memory is relatively slow - it requires more overhead than parity memory for storing data and causes around a 3% performance loss in the memory sub-system. Generally, use of ECC memory is limited to so-called mission-critical applications and is therefore more commonly found on servers than on desktop systems.
What the firmware does when it detects an error can differ considerably. Modern systems will automatically correct single-bit errors, which account for most RAM errors, without halting the system. Many can also fix multi-bit errors on the fly or, where that's not possible, automatically reboot with the bad memory mapped out. 2470
Memory upgrades
In recent times, as well as becoming much cheaper, RAM has also become more complicated. There are currently a proliferation of different varieties, shapes and voltages. The first step in planning an upgrade is therefore to determine what memory modules are already fitted.
Often the memory module as a whole will have a part number, and the memory chips that are mounted on the module will have different part number(s). Of these, the latter is by far the more important. Memory chips tend to have 2 or 3 lines of text on them that include a part number, speed, and date code. Most part numbers start with a two or three character abbreviation that identifies the manufacturer, such as "HM" (Hitachi), "M5M" (Mitsubishi), "TMS" (Texas Instruments) or "MT" (Micron Technology). The numbers (and sometimes letters) that follow describe the memory configuration of the chip, for example "HM514400" is a 1Mx4 configuration.
After the part number, there is usually a "A", "B", "C", or "D." This is how some manufacturers the revision of the memory, with "A" being the oldest and "D" being the most recent. In many cases, there will be an additional letter that codes the package type of the memory, e.g. "HM514400AS". In this example, "S" stands for SOJ-type packaging.
Speed of the memory is an important aspect of identification. A 70ns chip may be encoded at the end of the part number, e.g. "HM514400AS7". In this case, the "7" stands for 70ns. Sometimes there is a dash before the speed marking and at other times the speed is printed on a line above or below the part number. If the speed is printed on a separate line, a dash usually precedes the speed number. For most common memory chips, speed ranges from 50ns to 200ns. The trailing zero is commonly left off, so that "-6", "-7", "-8", "-10" or "-12", represents 60ns, 70ns, 80ns, 100ns and 120ns respectively.
On most chips, there is a date code printed above or below the part number. The date code indicates when the chip was made, most typically in a year and week format (such as "9438" for the thirty-eighth week of 1994). Often, the first digit of the decade will be omitted, so that "438" may also represent the thirty-eighth week of 1994.
Obviously, if there aren't any empty sockets, existing memory modules will have to be replaced with ones of greater capacity. The best way to ensure compatibility is to add more memory of the same specification, so new memory should match the old on the following points:
Physical format: It's preferable to stick to the same module format. This format also determines how many modules should be fit: a 486-based PC accepts 72-pin SIMMs individually, while 30-pin SIMMs must be installed in sets of four. A Pentium will accept DIMMs individually, but 72-pin SIMMs have to be fit in sets of two. When installing a set of SIMMs its important to ensure they're all alike and have the same capacity.
Parity or non-parity: Parity memory has 3,6,9,12 or 18 chips on each SIMM, non-parity has 2,4,8 or 16. The existing RAM should be used as a guide - fitting parity RAM to a board that doesn't use it may result in it grinding to a halt, while adding non-parity to parity RAM is likely to render parity checking inoperative.
Number of chips: A few really fussy motherboards prefer a particular number of chips per SIMM. They are mainly 486 boards and they might not be happy with a mix of three-chip 30-pin SIMMs with nine-chip 30-pin SIMMs, or a mix of "single-sided" 72-pin SIMMs with "double-sided" 72-pin SIMMs.
Voltage: Most memory these days is 5V. If the motherboard manual claims it also supports 3.3V memory, the jumper setting should be checked. If 3.3V is installed, any upgrade should also use 3.3V.
Type: The three main types of memory are: standard page mode (aka fast page mode or FPM), extended data out (EDO) and synchronous (SDRAM). A system will normally report (at bootup or in the BIOS) if it is using EDO or SDRAM - if it doesn't the chances are that it's FPM. It's safest not to mix memory types, although there are often occasions where this is possible.
Speed: The speed is usually the last two digits of the chip part number, often 60ns or 70ns. The exception is SDRAM where the speed is measured in megahertz corresponding to the maximum host bus speed it can cope with. It is advisable to stick to the same speed as fitted.
Matching the installed memory on all six points practically guarantees compatibility. It is still advisable, however, to check the motherboard manual on allowed DRAM configurations and there are three things to look out for. Before installing really big SIMMs (32MB and upwards) it is important to check that the motherboard supports them. For motherboards with a lot of SIMM sockets (over eight 30-pin or four 72-pin) it's necessary to check there aren't any restrictions on using too many double-sided SIMMs. Last, if the motherboard manual mentions memory interleaving, extra care must be taken in checking the intended configuration as boards with interleaved memory are usually more fussy.
There may be occasions when it is not possible to keep the memory consistent and different modules to those already installed have to be fit. First, there may be no free sockets of the occupied kind but only ones of a different type (for instance boards with both 30-pin and 72-pin SIMM sockets, or Triton VX boards with both SIMMs and DIMMs). Here, it is important to consult the motherboard manual, as any board mixing different socket-types is non-standard and likely to have strange restrictions on socket population. For instance, Triton VX boards with both SIMMs and DIMMs often require that a SIMM bank be vacated, or be populated with single-sided SIMMs only in order to be able to use the DIMM socket.
Alternatively, the motherboard may support memory of a higher performance than already fitted. Some chipsets will happily mix memory technologies. Triton VX- and HX-based boards allow different banks to be populated with different memory types (FPM, EDO or SDRAM) and run each bank independently at its optimum speed. Triton FX allows a mix of FPM and EDO within the same bank, but both will perform like FPM. However, with less well-known chipsets, mixing RAM types is a dangerous ploy and should be avoided. 6370
Communication Systems and Information Theory
1. Communication Theory. Communication theory deals primarily with systems for transmitting information or data form one point to another. A rather general block diagram for visualizing the behavior of such systems is given in Fig. 4.1. The source output might represent, for example, a voice waveform, a sequence of binary digits form a magnetic tape, the output of a set of sensors in a space probe, a sensory input to a biological organism, or a target in a radar system. The channel might represent, for example, a telephone line, a high frequency radio link, a space communication link, a storage medium, or a biological organism (for the case where the source output is a sensory input to that organism). The channel is usually subjected to various types of noise disturbances, which on a telephone line, for example, might take the form of a time-varying frequency response, crosstalk from other lines, thermal noise, and impulsive switching noise. The encoder in Fig. 4.1. represents any processing of the source output performed prior to transmission. The processing might include, for example, any combination of modulation, data reduction, and insertion of redundancy to combat the channel noise. The decoder represents the processing of the channel output with the objective of producing at the destination an acceptable replica of (or response to) the source output.
2. Information Theory. In the early 1940's a mathematical theory, for dealing with the more fundamental aspects of communication systems, was developed. The distinguishing characteristics of this theory are, first, a great emphasis on probability theory and, second, a primary concern with the encoder and decoder, both in terms of their functional roles and in terms of the existence (or nonexistence) of encoders and decoders that achieve a given level of performance. In the past 20 years, information theory has been made more precise, has been extended, and brought to the point where it is being applied in practical communication systems. As in any mathematical theory, the theory deals only with mathematical models and not with physical sources and physical channels. One would think, therefore, that the appropriate way to begin the development of the theory would be with a discussion of how to construct appropriate mathematical models for physical sources and channels. This, however, is not the way that theories are constructed, primarily because physical reality is rarely simple enough to be precisely modeled by mathematically tractable models. The procedure will be rather to start by studying the simplest classes of mathematical models of sources and channels, using the insight and the results gained to study progressively more complicated classes of models. Naturally, the choice of classes of models to study will be influenced and motivated by the more important aspects of real sources and channels, but the view of what aspects are
important will be modified by the theoretical results. Finally, after understanding the theory, it can be found to be useful in the study of real communication systems in two ways. First, it will provide a framework within which to construct detailed models of real sources and channels. Second, and more important, the relationship established by the theory provide an indication of the types of tradeoffs that exist in constructing encoders and decoders for given systems. While the above comments can be applied to almost any mathematical theory, they are particularly necessary here because quite an extensive theory must be developed before the more important implications for the design of communication systems will become apparent.
3. The source and Channel Encoders. In order to further simplify the study of source models and channel models, it is helpful to partly isolate the effect of the source in a communication system from that of the channel. This can be done by breaking the encoder and decoder of Fig. 4.1. each into two parts as shown in Fig. 4.2. The purpose of the source encoder is to represent the source output by a sequence of binary digits and one of the major questions of concern is to determine how many binary digits per unit time are required to represents the output of any given source model.
The purpose of the channel encoder and decoder is to allow the binary data sequences to be reliably reproduced at the output of the channel decoder, and one of course, whether restricting the encoder and decoder to the form of Fig. 4.2. imposes any fundamental limitations on the performance of the communication system. One of the most important results of the theory, however, is that under very broad conditions no such limitations are imposed (this does not say, however, that encoder and decoder of the form in Fig. 4.2. is always the most economical way to achieve a given performance). From a practical standpoint, the splitting of encoder and decoder is particularly convenient since it makes the design of the channel encoder and decoder virtually independent of the source encoder and decoder, using binary data as an interface. This, of course, facilitates the use of different sources on the same channel.
4. Conclusion. Much of modern communication theory stems from the works of communication systems and also the desirability of modeling both signal and noise as random processes. Wiener was interested in finding the best linear filter to separate the signal from additive noise with a prescribed delay and his work had an important influence on subsequent research in modulation theory. Also Wiener's interest in reception with negative delay (that, is, prediction) along with Kolmogorov's work on prediction if the absence of noise have had an important impact on control theory. Similarly, Kotelnikov was interested in the detection and estimation of signals at the receiver.
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Arithmetically each level of human generation contributes to 6.72 generations of Computer upgrading. But, this effect could pay back to the human society only by tuning the corporate infrastructure to utilise these computer innovations optimally. The future computer electronics works towards drastic cost reduction and process speed optimization. The Fifth generation architecture with its innovative techniques will be tuned to accept keyed, voice, picture inputs and process towards decision and action guidelines, using a knowledge based management and problem solving and inference modules. The machine is also expected to become more intelligent with the passage of time. The proposed Fifth Generation organisational structure is hence designed with the maintenance of 1990s hierarchical and machine interfaces, circularly integrated policy, management work flow, intensified problem-solution/ knowledge/intelligence orientation, integrated human-machine interface and mutual training setup. Based on a survey of Hardware and Software development in USA and Japan by the author in 1984. Published in Indian Management, Journal of the All India Management Association, New Delhi. India. June 1985, Pages 19 to 28.The paper is supported with 4 Schematic Diagrams and a Post-Publication Appraisal.
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