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Browse Prior Art Database

Interdigitation of Misses on a Double Word Basis

IP.com Disclosure Number: IPCOM000107347D
Original Publication Date: 1992-Feb-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 2 page(s) / 79K

Publishing Venue

IBM

Related People

Emma, P: AUTHOR [+6]

Abstract

The essential flaw with prefetching of I-LINES is that the change in I-LOCALITY is accompanied by a change in D-LOCALITY. This is manifest in I-MISSES and D-MISSES interdigitation, and the manner in which the combined miss, the I-Miss and the D-Miss can be handled as a unit should be considered.

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This is the abbreviated version, containing approximately 52% of the total text.

Interdigitation of Misses on a Double Word Basis

       The essential flaw with prefetching of I-LINES is that
the change in I-LOCALITY is accompanied by a change in D-LOCALITY.
This is manifest in I-MISSES and D-MISSES interdigitation, and the
manner in which the combined miss, the I-Miss and the D-Miss can be
handled as a unit should be considered.

      The nature of the requirements of the processor for the data
that is associated with a miss is different for I and D-Misses.  The
performance impact of providing less than the entire line or at least
the upstream portion of the line in the case of an I-Miss is severe
as the locality within the instruction stream is more sequentially
oriented than for data misses.  The essential point is that the
double word (DW) datum with the miss address within the line is a
limited communication between processor and memory that has an
immediate impact.  To reassert the point, unlike I-MISSES with
inherent sequentiality of flow, a single DW at the D-MISS address,
satisfies an AGI that allows the DECODER to proceed meaningfully.

      The use of an I-SHADOW and the use of some IxD prefetching, as
described in U.S. Patent 4,807,110, is one approach.  Another
approach that is more direct uses a table created for this purpose,
the so- called OHT (OPERAND HISTORY TABLE).  The OHT contains the
Operand DW address associated with the I-LINE that is being
prefetched.  Such a TABLE, generated based on historical occurrence,
can be an adjunct to the I-SHADOW, that is an address maintained
within the I-SHADOW.  The I-SHADOW employs a directory larger than
the one needed to support the access to a cache as a means of
trapping the changes of locality in terms of a chain of lines that
are to be prefetched based on MRU changes within the cache.

      Through the use of the OHT extension of the I-Shadow we have
the situation where at the time of the I-Miss we have a contendi...