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A Low-Power CAM Design
for LZ Data Compression
Kun-Jin Lin and Cheng-Wen Wu, IEEE Trans. On computers, Vol. 49, No. 10,
Oct. 2000.
Presenter: Ming-Hsien Tsai
Outline
1.
2.
3.
Introduction
CAM Structure
Removal of Redundant Comparisons
► Conditional Comparison Mechanism
► Redundancy Analysis
4.
Conclusion
Outline
1.
2.
3.
Introduction
CAM Structure
Removal of Redundant Comparisons
► Conditional Comparison Mechanism
► Redundancy Analysis
4.
Conclusion
Introduction
1.
2.
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
Outline
1.
2.
3.
Introduction
CAM Structure
Removal of Redundant Comparisons
► Conditional Comparison Mechanism
► Redundancy Analysis
4.
Conclusion
CAM Structure
Traditional SRAM Cell
Pull-down Transistor
Cross-coupled XOR Gate
Fig. 1. A typical CAM cell
CAM Structure
“1”
“1”
“0”
off
“0”
“1”
“0”
on
off
Fig. 1. A typical CAM cell
High-impedance
CAM Structure
“1”
“0”
“0”
off
“1”
“1”
“1”
on
on
Fig. 1. A typical CAM cell
low
Outline
1.
2.
3.
Introduction
CAM Structure
Removal Of Redundant Comparisons
► Conditional Comparison Mechanism
► Redundancy Analysis
4.
Conclusion
Outline
1.
2.
3.
Introduction
CAM Structure
Removal Of Redundant Comparisons
► Conditional Comparison Mechanism
► Redundancy Analysis
4.
Conclusion
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
Conditional Comparison Mechanism
“1”
“0”
“0”
“0”
When all the words do not match
“0”
“1”
If all words match
Fig. 4. A typical match logic
“0”
“0”
Conditional Comparison Mechanism
Difference between CCM and typical match logic
Fig. 5. Conditional comparison mechanism
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”
“1”
“0”
If all words match
Fig. 5. Conditional comparison mechanism
“0”
Outline
1.
2.
3.
Introduction
CAM Structure
Removal Of Redundant Comparisons
► Conditional Comparison Mechanism
► Redundancy Analysis
4.
Conclusion
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.
Redundancy Analysis
TABLE 1. Experimental Results on the Files from the Calgary Corpus
Outline
1.
2.
3.
Introduction
CAM Structure
Removal Of Redundant Comparisons
► Conditional Comparison Mechanism
► Redundancy Analysis
4.
Conclusion
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.
END