VisualDSP++ 4.5 Loader and Utilities Manual - Analog Devices

Blackfin Processor Booting
The first 16 bits of the compressed block header hold the padded byte
count of the compressed stream. The loader utility always pads the byte
count if the resulting compressed stream from the loader compression
engine is an odd number. The loader utility rounds up the byte count of
the compressed stream to be a next higher even number. This 16-bit value
is either 0x0000 or 0x0001.
The second 16 bits of the compressed block header hold the size of the
compression window, used by the loader compression engine. The value
range is 8–15 bits, with the default value of 9 bits. The compression window
size specifies to the compression engine a number of bytes taken from
the window during the compression. The window size is the 2’s exponential
value.
As mentioned before, the compression/decompression mechanism for
Blackfin processors utilizes the open-source lossless data-compression
library zLib1. The zLib1 deflate algorithm, in turn, is a combination of a
variation of Huffman coding and LZ77 compression algorithms.
LZ77 compression works by finding sequences of data that are repeated
within a sliding window. As expected, with a larger sliding window, the
compression algorithm is able to find more repeating sequences of data,
resulting in higher compression ratios. However, technical limitations of
the zLib1 decompression algorithm dictate that the window size of the
decompressor must be the same as the window size of the compressor. For
a more detailed technical explanation of the compression/decompression
implementation on a Blackfin processor, refer to the readme.txt file in
…\Blackfin\ldr\zlib\src subdirectory of the VisualDSP++ 4.5 installation
directory.
In the Blackfin implementation, the decompressor is part of the decompression
initialization files (see “Decompression Initialization Files” on
page 2-60). These files are built with a default decompressor window size
of 9 bits (512 bytes). Thus, if the user chooses a non-default sliding window
size for the compressor by sliding the Compression Window Size
slider bar in the Compression tab (under Load in the Project Options
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