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<Paper uid="C86-1112">
  <Title>GENERALIZED MEMORY MANIPULATING ACTIONS FOR</Title>
  <Section position="5" start_page="474" end_page="474" type="concl">
    <SectionTitle>
5. Parsing efficiency
</SectionTitle>
    <Paragraph position="0"> A relevant claim is that the data structure and the set or actions of EXAM.-LC-ATN improve the computational efficiency of an ATN parser. This depends upon two peculiarities of the parser, the reduction in register setting and storage accesses and the limitation of backtrack. In ATN, register setting is done by searching through a register table; if the called register is already present it is set to the new declared value, while if it is not, it is initialized and filled. In EXAM-LC.ATN ordinary register setting (SETR} is substituted by an operation which stores information in an already initialized memory subspace. Thus, for an equivalent grammar, the number of storage accesses in EXAM-I,C-ATN is exactly the same of a traditional ATN, but no searching or initialization is performed. There is a similarity between a register initialization operation and the opening of a new memory subspace (PUSH}. However, this is done only for the higher level constituents, such as S, NP, PP, etc, while in an ATN new registers are activated even for the storage of terminals or features.</Paragraph>
    <Paragraph position="1"> Extra register setting, commonly used in an ATN to laterally pass parameters to different levels of computation from the current one (SENDR, LIFTR), is unnecessary in EXAM-LC-ATN.</Paragraph>
    <Paragraph position="2"> In fact. the required parameters are retrieved if and when they are needed from the level where they are needed.</Paragraph>
    <Paragraph position="3"> This guarantees efficiency in two ways. On one hand. the SENBR/LIFTR - GETR pairs are substituted by a single search through the left context Within a memory subspace the search proceeds linearly as for pattern-matching. On the other hand. the overhead due to the sending or lifting of parameterswhich may turn out to be useless at the next upper or lower level is eliminated. In conclusiom the average of storage spaces activation operations and storage accesses is in favour of EXAM-LC-ATN.</Paragraph>
    <Paragraph position="4"> Backtrack is the typical mechanism by which non-deterministic depth first parsers like ATN manage the choice of alternatives. It consists in going back to the nearest choice point and trying the following alternative It involves also a restoring of the input pointer to the element in the input string where it was before the current alternative was chosen, and reprocessing of the input from that point in the new alternative. Generally this mechanism is activated when some incoming piece of information signals that the chosen alternative is the wrong one In some cases, the information give meaningfull indications about the correct alternative analysis to be chosen. In these cases backtrack can be substituted by special techniques for modifying the output of the analysis without roprocessing the involved segment of the input string INSERT is a case of such a modification action and avoids backtrack The fiJll access to the left context allows for the introduction of other ways of limiting backtrack</Paragraph>
  </Section>
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