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Checking Least Recently Used Code for Validity

IP.com Disclosure Number: IPCOM000122662D
Original Publication Date: 1991-Dec-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 2 page(s) / 37K

Publishing Venue

IBM

Related People

Doettling, G: AUTHOR [+2]

Abstract

A 6-bit code is used to indicate the order in which the columns of a four-column memory have been used. The 6 bits of the code allow it to take 64 possible values. However, of these, only 4! = 24 are relevant, the other 40 values indicating errors.

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Checking Least Recently Used Code for Validity

      A 6-bit code is used to indicate the order in which the
columns of a four-column memory have been used.  The 6 bits of the
code allow it to take 64 possible values.  However, of these, only 4!
= 24 are relevant, the other 40 values indicating errors.

      The bits of the code are set as follows:
      Bit 0 = 1 - column 1 was less recently used than column 0
      Bit 1 = 1 - column 2 was less recently used than column 0
      Bit 2 = 1 - column 3 was less recently used than column 0
      Bit 3 = 1 - column 2 was less recently used than column 1
      Bit 4 = 1 - column 3 was less recently used than column 1
      Bit 5 = 1 - column 3 was less recently used than column 2

      A circuit for use with this code is depicted in the figure.
The column of the memory least recently used (LRU) is indicated by
one of the outputs L0-L3 being 1 and the column of the memory most
recently used is indicated by one of the outputs M0-M3 being 1.  One
of the outputs L0-L3 and one of the outputs M0-M3 must be one,
otherwise an error indication will be given.