Main memory

MP2 correlation energy calculations may increase the computational lime because a tw o-electron integral Iran sfonnalion from atomic orbitals (.40 s) to molecular orbitals (MO s) is ret]uired. HyperClicrn rnayalso need additional main memory arul/orcxtra disk space to store the two-eleetron integrals of the MO s. 

HyperChem supports MP2 (second order Mdllcr-l Icsset) correlation energy calcu latiou s u sin g any available basis set. lu order to save main memory and disk space, the HyperChem MP2 electron correlation calculation normally uses a so called frozen-core approximation, i.e. th e in n er sh el I (core) orbitals are omitted. A sett in g in CHHM.IX I allows excitation s from th e core orbitals to be include if necessary (melted core). Only the single poin t calcula-tion is available for this option. 

With the current impressive CPU and main memory capacity of relatively in expen sive desktop PC s, non-direct SCFaft tnilto calculations involving 300-400 basis function scan be practical. However, to run til esc kin ds of calcti latiori, 20 GBytes of li ard disk space rn ight be needed. Such big disk space is unlikely to be available on desktop PCs.. A direct SCb calculation can elim inate th e n eed for large disk storage. 

HyperChem can perform quantum mechanics MO calculations on molecules containing 100 or more atoms. There is no restriction on the number of atoms, but larger structures may require excessive computing times and computer main memory. 

For fast access to the two-electron integrals, a four-dimensional array might be straightforward. The four indices of the four dimensional array correspond to the four basis function indices, p, v, X, and a, respectively. However, the four dimensional array may take a huge main memory or computer disk space even for a medium-size molecule. Therefore, this may not be practical. 

Because of the use of two double-precision words for each integral, HyperChem needs, for example, about 44 MBytes of computer main memory and/or disk space to store the electron repulsion integrals for benzene with a double-zeta 6-3IG basis set. 

For data storage two types of memory are distinguished (/) main memory with moderate capacity (currentiy 4 x 10 bit in each element) and extremely short access time (<10 s), and (2) mass memory with very high capacity (>10 bit in each element) and moderate access time (>10 s). 

Main memories almost exclusively consist of semiconductors on a siUcon basis in complementary metal oxide semiconductor technology (CMOS). The most important types are the pure read only memory (ROM) and the write/read memory (RAM = random access memory), which is available as S-RAM (static RAM) or as D-RAM (dynamic RAM). 

Polymers are only marginally important in main memories of semiconductor technology, except for polymeric resist films used for chip production. For optical mass memories, however, they are important or even indispensable, being used as substrate material (in WORM, EOD) or for both substrate material and the memory layer (in CD-ROM). Peripheral uses of polymers in the manufacturing process of optical storage media are, eg, as binder for dye-in-polymer layers or as surfacing layers, protective overcoatings, uv-resist films, photopolymerization lacquers for repHcation, etc. 

On a vector computer having vector registers that hold 64 floating-point numbers, this loop would be processed 64 elements at a time. The first 64 elements of Y would be fetched from memory and stored in a vector register. Each iteration of the loop is independent of the previous iteration, so this loop can be fliUy pipelined, with successive iterations started every clock cycle. Once the pipeline is filled, the result, X, will be produced one element per clock cycle and will be stored in another vector register. The results in the vector register will then be stored back into main memory or used as input to a subsequent vector operation. 

Banked Memory. Another characteristic of many vector supercomputers is banked memory. The main memory is usually divided into a small number of electronically separate banks. A given memory bank can absorb or supply operands at a much slower rate than the rate at which the central processing unit (CPU) can produce or use data. If the data can be spread across multiple memory banks, the effective memory bandwidth, or rate at which memory can absorb or supply data, is increased. For example, if a single memory bank can supply one operand every 16 clock cycles, then 16 memory banks would enable the entire memory subsystem to deflver one operand per clock cycle, assuming that the data come sequentially from different memory banks. 

Other Performance Considerations. Even if a program allows main memory to supply operands at peak rate, it may not be fast enough to keep the CPU operating at its peak rate. Consider the general SAXPY... 

Because of the relative slowness of main memory (compared with the CPU), most computers have a much smaller, but much faster cache memory subsystem that augments main memory. The size of the cache memory and the extent to which a program can utilize the cache can be critical deterrninants of performance. Again, there are some common optimization techniques designed to maximize cache utilization. 

Supercomputers from vendors such as Cray, NEC, and Eujitsu typically consist of between one and eight processors in a shared memory architecture. Peak vector speeds of over 1 GELOP (1000 MELOPS) per processor are now available. Main memories of 1 gigabyte (1000 megabytes) and more are also available. If multiple processors can be tied together to simultaneously work on one problem, substantially greater peak speeds are available. This situation will be further examined in the section on parallel computers. 

Linkers and loaders—Linkers resolve references between program units and allow access to system libraries loaders place code into the main memory locations from which it will be executed. 

The computational equipment was a 3.06 GHz Xeon machine with 2 GB of main memory and Linux operating system. All tests were limited to five million of visited nodes and this limit was reached in all test runs. The configuration of the waiting list W of TAopt was depth-first search combined with cost minimization. In order to reduce the search effort, the search space was reduced by reduction techniques described in Panek et al. [22] with small modifications. Results of test runs with different initial quantities for So are shown in Table 10.2. The results show the computation times, the numbers of nodes visited to find the solution,... 

There are two types of memory used in computers. The main memory is based on integrated circuit chips and all parts can be accessed with great rapidity and with equal ease. In mainframe computers this is known as the main store whilst in smaller computers, including microcomputers, it is called the random access memory (RAM). Some parts of RAM may be reserved for the storage of programs or data which are to be protected from change or accidental erasure. Such a reserved area of memory is called read only memory (ROM). ROM chips, sometimes called firmware, are often used in integrators and microcomputers dedicated to particular tasks. The... 

The purpose of interfacing instruments with computers is to enable raw analytical data to be collected as it is produced, then processed, stored and displayed or printed out. This may be accomplished as it is gathered, i.e. in real-time, or at some later time, i.e. post-run. Complete chromatograms or spectra can easily be stored in the main memory or RAM or transferred to disk. The immense storage capacity of mainframe computers can be used to provide large libraries of data (data banks) for future reference. 

These drawings first and foremost represent states. The same kind of drawing can be used to represent specific implementations of a state. For example, we could decide that each link represents a row in a relation in a relational database or they might be pointers in main memory or they might be rows in a chart at the hotel s front desk. But at the start of a design the most useful way is to say that we don t care yet we re interested in describing the states and not the detail of how they re implemented. We will make the less important representation decisions in due course as the design proceeds. 

Equally, the action could be an abstraction of a dialog with a software system. In that case, because of the Golden Rule of 00 design (that we base the design on a domain model), we can use the same picture to denote objects (whether in a database or main memory) that the system uses to represent the real world. The interesting actions are then the interactions between the system and the rest of the world They update the system s knowledge of what is going on in the world, as represented in the attributes. 

For example, C++ works with an OO model in main memory but leaves persistent data up to you you can t send a message to an object in filestore. If you can secure a good OO database you re in luck but otherwise, typically you re stuck with plain old files or a relational database and must to think how to encode the objects. Your class-layer design should initially defer the question of how objects are distributed between hosts and media. 

Components often use persistent storage although objects in an 00 programming language always have local state, they typically work only within main memory, and persistence is dealt with separately. 

Which storage medium and platform an object is in (database, serialized in file, main memory, on a listing, in a bar code, on a magnetic stripe card, and so on)... 

---