Browse Prior Art Database

Small Priority Encoder for Content Addressable Memory

IP.com Disclosure Number: IPCOM000117992D
Original Publication Date: 1996-Aug-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 2 page(s) / 59K

Publishing Venue

IBM

Related People

Niijima, H: AUTHOR [+2]

Abstract

Disclosed is a method for reducing the area size of the priority encoder in a Content Addressable Memory (CAM). The reduction is achieved by designing the CAM so that every CAM sub-block uses the same column priority encoder in common.

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This is the abbreviated version, containing approximately 75% of the total text.

Small Priority Encoder for Content Addressable Memory

      Disclosed is a method for reducing the area size of the
priority encoder in a Content Addressable Memory (CAM).  The
reduction is achieved by designing the CAM so that every CAM
sub-block uses the same  column priority encoder in common.

      The Figure shows a diagram of the CAM using the disclosed
design.  Every CAM sub-block comprises both a CAM array and an
OR&XFER circuit.  A total of n-1 CAM sub-blocks, a column priority
encoder and a  row priority encoder are arranged as shown in the
Figure.  When there is  a "match" in a CAM sub-block, the local match
line (Mx) in the sub-block  is connected to the global match line
(MATCHx) through the XFER gate. If  there are "matches" in multiple
CAM sub-blocks, the Mx that is in the sub-block with the lowest
row-address (the rightmost sub-block in the figure) is selected and
connected to MATCHx.

The CAM works as follows:
  1.  A down-pulse is input at CL1 to reset the output signals of
       the dynamic OR circuits (R0 - Rn-1) and the select signal
       (X0 - Xn-1).
  2.  The select signals X0 -Xn-1, which are fed back to XFER
       circuits, remain at 1 when all the OR signals from lower
       addresses are 0 (i.e., Xa = 1 if R0 - Ra-1 are 0).  This
       also implies that Xa = 1 if there is no "match" within
       from the CAM sub-blocks 0 to a-1.
  3.  All the local match lines in the sub-block wher...