Word, computer memory

The number of additional Fourier spectral components to recover is the option of the researcher. The number of iterations to execute with the most general computer program written is also the option of the researcher. A tolerance is presently used to determine the number of iterations performed. However, it is found in practice that only 5 or 10 iterations yield sufficiently accurate results for nearly all experimental data of interest. With the presently used computer program, restoration is to the spatial function, and the improved spatial function and the improved values of the coefficients are both generated with each iteration. If the improved Fourier spectrum is not desired, then additional computational time could be saved by neither reading nor writing the Fourier coefficients. When M data points are treated, the computer memory requirements are seldom more than 1M words. If it is not necessary to determine the extended Fourier spectrum, then more than 5M words are seldom needed in computer memory. 

Computer Hardware. The use of a computer for data acquisition and control is mandatory because of the clock frequency used by the Reticon array (20 kHz) and the volume of data that it is capable of outputting. A PDP-8e computer made by Digital Equipment Corporation, with 12 K words of memory was employed. 

The hardest task involved in adding memory to a PC is buying the correct RAM for your computer. Memory technology has been changing at a rapid pace to match the changes in CPU and architecture. There are numerous types of RAM available, and most are not interchangeable. In other words, you must buy the correct type of memory and all of your memory must be of the same type. 

Perhaps the most dramatic changes in the MCSCF method, and indeed in electronic structure methods in general, will result from the new computer architectures that are becoming available to computational chemists. Not only the supercomputers with about 10 words of memory and 10 arithmetic operations per second capability, but also the massively parallel computers with hundreds or thousands of computational units operating simultaneously may come to play important roles in modern computational chemistry. 

A distance geometry program requires at least 2N N = number of atoms) words of memory, which until recently restricted the method s applicability to relatively small problems. Modern workstations speed and memory now easily satisfy the computational demands of distance geometry for structures up to about 2000 atoms. Several programs are now available that run on machines from small workstations to supercomputers. A.io-is... 

Fig. 1.2. The minicomputer revolution in computational chemistry began as machines like this Datacraft 6024/4 at the University of CaUfomia (1973) came into use. This was a 24-bit machine (48 bits per word in extended precision). Note the two rows of switches. These could be used to directly set the bit strings for one word of memory at a time, a low-level feature that at least proved helpful for certain crashes.

This is the place where the computer stores (1) the instructions of the program it executes, and (2) the values of the initial data, intermediate data, and final results from computations. The smallest unit of storage is the bit, but the memory is organized in terms of words. Thus 4K words memory is composed of 4096 16-bit words (for 16-bit word computers). Each memory word is characterized by a unique address and during the execution of a program the CPU keeps track of the memory... 

The cycle time of a computer is the time required by the CPU to read the content of one word from memory and restore its content. According to the value of the cycle time, we can distinguish the following types of RAM ... 

Even when the NMR spectrometer has been adjusted for optimum resolution, the spectrum may still have a poor resolution if enough computer memory was not allocated for the purpose, i.e., it will have poor digital resolution. A large allocation of computer memory (i.e., a greater number of words of data storage) requires a corresponding increase in acquisition time. The maximum acquisition time, in seconds, required after each pulse is given by ... 

Whether or not we worry how computer scientists represent a negative number by using a word of memory composed of ones and zeroes is largely irrelevant. It is important, however, that we imderstand that each unique voltage in the allowed range will in turn be mapped into some unique combination of ones and zeroes in the output. 

Word. A portion oF computer memory devoted to the storage of one number. A word will normally consist of four or eight bytes (one byte is eight bits, or eight 1 s or Os). 

A binary tree is particularly useful because it is easy to represent in computer memory. To begin with, the data words (the contents stored at each node in the structure) can be distinguished from successor links by a one-bit flag. Another one-bit flag indicates a left/right characteristic of the two pointers. Thus, at a cost of very little dedicated storage, basic information that is to be kept can be stored with the same memory word field assignments. The binary tree version of Fig. 2a is Fig. 2b. 

Here is a distinct approach, promising some practical improvement. Suppose that we seek the product Ab of a 1000 X lOOOmatrix A by a vector b, both filled with zeros and ones and use a computer with 100 bit precision. Each entry of Ab is at most 1000 and occupies almost 10 bits, so that 10 entries of Ab can be easily computed and stored by using one arithmetic operation and a single word of memory, thus almost by 10 times improving the classical approach. The algorithm can be extended to matrices and vectors whose entries are bounded by integers [see Pan (1984) Bini and Pan (1994)]. 

Year Computer Memory Words Direct storage Program language... 

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