VisualDSP++ 4.5 Loader and Utilities Manual - Analog Devices

ADSP-2126x/2136x/2137x Processor Booting
The second 32-bit word of the compressed block header hold the size of
the compression window (takes the upper 16 bits) and padded word count
(takes the lower 16 bits). For ADSP-2126x/2136x/2137x processors, the
loader utility always rounds the byte count of the compressed stream to be
a multiple of 4. The loader utility also pads 3 bytes to the compressed
stream if the byte count of the compressed stream from the loader compression
engine is not a multiple of 4. An actual padded byte count is a
value between 0x0000 and 0x0003.
The compression window size 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.
The next 32 bits of the compressed block header holds the value of the
compressed stream byte count, excluding the byte padded.
A window size selection affects, more or less, the outcome of the data
compression. Streams in decompression windows of different sizes are, in
general, different and most likely not compatible to each other. If you are
building a custom decompression kernel, ensure the same compression
window size is used for both the loader utility and the kernel. In general, a
bigger compression window size leads to a smaller outcome stream. However,
the benefit of a big window size is marginal in some cases. An
outcome of the data compression depends on a number of factors, and a
compression window size selection is only one of them. The other important
factor is the coding structure of an input stream. A compression
window size selection can not cause a much smaller outcome stream if the
compression ability of the input stream is low.
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