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DATA COMPRESSION
Table 7-3 shows the dictionary based on Tables 7-1 and 7-2.
Table 7-3. Dictionary
Codeword
Byte Value
Code Value
(Pointer)
(AI)
(A)
(IN)
N
(I)
(Nl)
T
(N)
(TI)
(1)
(INl)
T
(IN)
(TIN)
N
(TI)
7.3 AHA3101 Data Compression Chip
The AHA3l0l data compression chip used in the Python DDS-DC drives provides the basis for an effective
data compression system using the DCLZ algorithm. Using this chip implements hardware data compression
in the drive itself.
Like other component devices used in the state-of-the-art Python drives, the AHA3l0l uses the low power
CMOS technology. The device is packaged in a lOO-pin plastic quad flat pack (PQFP).
The AHA3l0l contains compression and decompression circuitry, which forms the core of the data
compression system by implementing the DCLZ algorithm. This core is called the DCLZ machine. In
addition to the DCLZ machine, this data compression chip provides support circuitry in the form of four
main interfaces:
o
The processor interface allows a microprocessor to access information inside the chip and in the
SRAM buffer.
o
The Port A and Port B DMA interfaces are used to transfer uncompressed and compressed data
through the chip.
o
The external static RAM (SRAM) interface allows high speed access to the external SRAM, which is
used to store DCLZ algorithm information and to buffer data.
These interfaces offer a means of tuning parameters provided in the DCLZ algorithm. The major parameters
are
o
Dictionary size selection
o
Logical record control
o
Parameters to use in evaluating the current compression ratio
7-7
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