Seagate STD2401LW-S Instrukcja Obsługi Pobierz pdf (Strona 54)

Data compression

The following table illustrates this simplified operation.

Input

Byte

Current

String

Match

Build

Entry

Output Code

Value

RR Y — —

IRI NRI(R)

—I Y — —

NIN N IN(I)

—N Y ——

TNT N NT(N)

—T Y ——

ITI N TI(T)

—I Y ——

NIN Y ——

T INT N INT (IN)

—T Y ——

ITI Y ——

N TIN N TIN (TI)

—N Y ——

Dictionary

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:

•

The unique string found in the data stream

•

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.

•

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

•

Encoded bytes, codewords 8 through 263: These encoded bytes represent

single bytes of the input data stream and contain the values 0 through 255.

1 2 ... 49 50 51 52 53 54 55 56 57 58

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