User's Manual

Data compression
47
The following table illustrates this simplified operation.
Input
Byte
Current
String
Match
Build
Entry
Output Code
Value
R
R
Y
—
—
I
RI
N
RI
(R)
—
I
Y
—
—
N
IN
N
IN
(I)
—
N
Y
—
—
T
NT
N
NT
(N)
—
T
Y
—
—
I
TI
N
TI
(T)
—
I
Y
—
—
N
IN
Y
—
—
T
INT
N
INT
(IN)
—
T
Y
—
—
I
TI
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:
•
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.