A LowPower CAM Design for LZ Data Compression

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A Low-Power CAM Designfor LZ Data Compression

• Kun-Jin Lin and Cheng-Wen Wu, IEEE Trans. On
  computers, Vol. 49, No. 10, Oct. 2000.
• Presenter Ming-Hsien Tsai

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Outline

• Introduction
• CAM Structure
• Removal of Redundant Comparisons
• Conditional Comparison Mechanism
• Redundancy Analysis
• Conclusion

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Outline

• Introduction
• CAM Structure
• Removal of Redundant Comparisons
• Conditional Comparison Mechanism
• Redundancy Analysis
• Conclusion

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Introduction

• CAM (Content Addressable Memory) has been
  considered the fastest architecture among all
  proposed hardware solutions for searching for a
  given string , as required in LZ77.
• LZ77 is the lossless data compression algorithm

5
Outline

• Introduction
• CAM Structure
• Removal of Redundant Comparisons
• Conditional Comparison Mechanism
• Redundancy Analysis
• Conclusion

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CAM Structure
Traditional SRAM Cell
Cross-coupled XOR Gate
Pull-down Transistor
Fig. 1. A typical CAM cell
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CAM Structure
1
1
0
1
0
off
on
High-impedance
off
0
Fig. 1. A typical CAM cell
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CAM Structure
1
1
0
0
1
off
on
low
on
1
Fig. 1. A typical CAM cell
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Outline

• Introduction
• CAM Structure
• Removal Of Redundant Comparisons
• Conditional Comparison Mechanism
• Redundancy Analysis
• Conclusion

10
Outline

• Introduction
• CAM Structure
• Removal Of Redundant Comparisons
• Conditional Comparison Mechanism
• Redundancy Analysis
• Conclusion

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Conditional Comparison Mechanism
The masking of unnecessary comparison results on
the match nodes
Only match node is in the high-impedance when the
word identical to input symbols
Fig. 4. A typical match logic
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Conditional Comparison Mechanism
1
0
0
0
When all the words do not match
0
0
0
1
If all words match
Fig. 4. A typical match logic
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Conditional Comparison Mechanism
Difference between CCM and typical match logic
Fig. 5. Conditional comparison mechanism
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Conditional Comparison Mechanism
The static current is totally blocked when mi
is low
1
0
0
0
1
When all the words do not match
1
0
OFF
0
1
0
If all words match
Fig. 5. Conditional comparison mechanism
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Outline

• Introduction
• CAM Structure
• Removal Of Redundant Comparisons
• Conditional Comparison Mechanism
• Redundancy Analysis
• Conclusion

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Redundancy Analysis

• Consider a typical data compressor using a CAM of
  2,048 8-bit words as the buffer
• Assume that the input symbols are randomly
  distributed. On average, only 2048 x1/256
• 8 words can match any input symbol.
• There are 2048 8 2,040 words that will
  require the power.

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Redundancy Analysis
TABLE 1. Experimental Results on the Files from
the Calgary Corpus
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Outline

• Introduction
• CAM Structure
• Removal Of Redundant Comparisons
• Conditional Comparison Mechanism
• Redundancy Analysis
• Conclusion

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Conclusion

• We showed that the redundant comparisons in the
  compression process can be removed by turning off
  the power supply to those words that do not need
  to participate, saving about 80 percent of the
  power consumption of the comparison mechanism as
  compared with the conventional CAM.

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END