Dictionary - Seagate STD124000N Product Manual

Data Compression

Dictionary

Product Manual
The following table illustrates this simplified operation.
Input
Current
Byte String
R R
I RI
— I
N IN
— N
T NT
— T
I TI
— I
N IN
T INT
— T
I TI
N TIN
— N
The dictionary is built and contained logically in external RAM and is not output as a
distinct item. Rather, the decompressor recreates the dictionary to recreate the
original data.
The dictionary allows up to 4,096 entries with each entry made up of:
z
The unique string found in the data stream
z
The codeword for that string
Codewords represent strings of up to 128 characters and are formed by adding a
new character to an existing codeword. These codewords range from 9 through 12
bits in size and are assigned a number in the range 0 through 4,095.
These codewords are either control flags, encoded bytes or dictionary codes. The
following points explain these three types of codewords.
z
Control Flags, codewords 0 through 7: These control flags are reserved
codewords that flag specific conditions as follows:
0 Dictionary frozen
1 Dictionary reset
2 Increment codeword size
3 End of record (EOR)
4–7 Reserved
z
Encoded bytes, codewords 8 through 263: These encoded bytes represent
single bytes of the input data stream and contain the values 0 through 255.
Build Output Code
Match
Entry Value
Y — —
N RI (R)
Y — —
N IN (I)
Y — —
N NT (N)
Y — —
N TI (T)
Y — —
Y — —
N INT (IN)
Y — —
Y — —
N TIN (TI)
Y — —
Chapter 7
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