Content addressable memory

Consider the basic probkun of how information is distributed throughout a system, and the manner in which it is retrieved. We. know that in a conventional computer, for example, information is stored in random-access memory (RAM). This means that the memory address of where the information actually exists and the information itself are uncorrelated. In order to retrieve the information, one must know its address exactly, as even the slightest error renders that information effectively unretrievable. In particular, it is in general impossible to retrieve RAM data if armed only with a partial knowledge of its address. In contrast, associative memories (sometimes also called content-addressable memories), much like the form of memory believed to be used by human brains, are such that they can be completely retrieved even when searched for with partial information,... 

The situation is somewhat reminiscent of a content-addressable memory [72], in which partial information is converted by the brain to recover the complete information. Such content-addressable memories [72], as well as the energy landscape [73] suitable for prebiotic evolution [74], have been modeled through spin glasses [75]. The energy landscape of spin glasses is also characterized by diversity and stability arising from randomness and frustration, which is quite distinct from the the physical mechanisms of short tubes in the marginally compact phase. 

PC s extensive network of associative connections, taken with the distributed nature of afferent inputs to PC from the bulb has been modeled recently by Haberly, Bower and others as a distributed association network that is hypothesized to function as a content addressable memory for spatially distributed odor patterns. 

Microelectronics Electrochemical phenomena are essential in the manufacture of electronic and photonic systems as well as responsible for the quality and reliability of such systems. Applications and research are outlined in areas that include manufacture of microcircuits, interconnecting networks, lightwave communication devices, parallel processors, content-addressable memories, and nerve-electronic interfaces. 

Hopfield networks [18] are able to store patterns as point attractors in n dimensional binary space and recall them in response to partial or degraded versions of stored patterns. For this reason, they are known as content addressable memories where each memory is a point attractor for nearby, similar patterns. Traditionally, known patterns are loaded directly into the network (see the learning rule 10 below), but in this paper we investigate the use of a Hopfield network to discover point attractors by sampling from a fitness function. A Hopfield network is a neural network consisting of n simple connected processing units. The values the units take are represented by a vector, u ... 

Fig. 1.19 Demonstration of content addressable memory (CAM), (a) Global sum system realized by using an optical nano-fountain with three input-ports, (b) Broadcast system realized by using three nanophotonic switches...

Some special memory structures do not follow the general accessing scheme of using an address. Two of the most frequently used are content addressable memory (CAM), and first-in-first-out (FIFO) memory. Another type of memory device, which accepts multiple addresses and produces several results at different ports, is called multiport memory. There is also a type of memory that can be written in parallel, but is read serially. It is referred to as video RAM or VDRAM since they are used primarily in graphic display applications. We will discuss these in more detail later. 

The trend in memory devices is toward larger, faster and better-performance products. There is a complementary trend toward the development of special purpose memory devices. Several types of special purpose RAMs are offered for particular applications such as content addressable memory for cache memory, line buffers (FIFOs) for office automation machines, frame buffers for TV and broadcast equipment, and graphics buffers for computers. 

A special type of memory called content addressable memory (CAM) or associative memory is used in many applications such as cache memory and associative processor. A CAM stores a data item consisting... 

This is the heart of the microcomputer. The CPU or microprocessor examines the contents of memory and interprets them as instructions or data. The way in which the processor interprets the numbers stored in memory depends on the design of the chip, that is, on the manufacturer. This is the machine code and the microprocessor s repertoire of arithmetic and logic functions is called its instruction set. The CPU is connected to the memory by two sets of wires called the data bus and the address bus. The data bus is used to transfer data to and from the memory. The address bus is used to identify that part of memory with which the processor wishes to communicate. It is important that the processor and the memory act in a synchronized manner. All microcomputers contain a crystal-controlled oscillator which acts like a metronome to which all actions are synchronized. This oscillator is sometimes referred to as the clock. 

Computers contain read-only memory whose contents are permanent (i.e., can only be read and not written to by the user) along with random access memory that can both be read from and written to by the user. The basic computing unit is a bit (b), which stands for binary digit 8 bits comprise a byte (B). Table 3.5 illustrates calculation of computer memory bytes, i.e., the number of locations that can be addressed. 

After the image has been successfully captured, the contents of the buffer memory may be mapped onto the address space of the Unibus of a PDP11/04 minicomputer for quantitive analysis, via a group of programs (SIM-1) (7). Approaches to analysis available to us include least squares fitting (5), eigen analysis (3). and... 

In most utilities that scan memory to find its contents, you will see memory addresses listed as hex addresses. The table above will be valuable when you re determining where a particular program or driver is resident in memory. 

The second level of operator maintenance is through a software module called DGNSTC. The diagnostic module operates in coniunction with front panel LED s and toggle switches. DGNSTC interprets the 16 panel switches as a memory address and outputs the contents of that memory address on the LED s. Peripheral device addresses (considered memory addresses by the hardware) can be viewed as well. If a byte address is selected, it is displayed in the low byte of the LET) word. Switches requesting a display of a non-existent memory location cause all ones to appear on the LED lights. 

The index register originated with the inclusion of an auxiliary storage tube in the miniature machine constructed at the University of Manchester 28). This tube, designated the B tube since there was already an A-tube accumulator and a C-tube control, was added to perform the address modification required for iterative computations. A bit in each instruction was set to one or zero to indicate whether or not the contents of the B tube were to be added to that instruction address prior to execution. The instruction address in memory remained unchanged. 

Next the multichannel analyzer must add one count to memory location Nc. This is carried out during the memory cycle time [Fig. 4.28(e)]. The memory location whose address or channel number is Nc is identified and its present contents are read. If memory location Nc presently contains m counts, the number m + 1 is written back in. At the end of the memory cycle the multichannel analyzer is free to process another amplifier pulse. If a pulse is already present above the lower-level discriminator level at the linear gate input, the linear gate remains closed until the pulse drops below the discriminator threshold. At this point the linear gate opens and the multichannel analyzer is ready to process the next amplifier pulse that exceeds the lower-level discriminator threshold. The measurement process is repeated on a pulse-by-pulse basis to build up the energy spectrum histogram in memory. 

RAM stands for random-access memory. It is really read-write memory because ROM is also random access in the sense that given a random address the corresponding entry is read. RAM can be categorized by content duration. A static RAM s contents is always retained, as long as power is appHed. A DRAM, on the other hand, needs to be refreshed every few milliseconds. Most RAMs by themselves are volatile, which means that without the power supply their content will be lost. All of the ROMs mentioned in the previous section are nonvolatile. RAM can be made nonvolatile by using a backup battery. 

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