Subject: [rohc] SigComp User Guide, deflate bytecode

From: Herve Siegrist <[email-masked]>
Date: Mon, 14 Mar 2005 08:48:32 +0100

I am testing my UDVM against the bytecodes provided in the
draft SigComp User Guide for the sample algorithms, and I
am getting into some trouble with the DEFLATE bytecode.

 From the assembly code, it appears that the bytecode is, as
for other bytecodes in this draft, meant to be loaded at
UDVM address 128.

However, this bytecode starts with a MULTILOAD instruction
which writes 122 words at address 64, hence writing bytes
at addresses 64 to 307 inclusive.

The bytecode for the MULTILOAD instruction however occupies
addresses from 128 to 285 inclusive.

As a result, it seems to me, unless I missed something big
(again...), that the execution of the MULTILOAD instruction
causes a decompression failure as it overwrites its own
code (and even more...). At least, that's what happened to
me the first time around...

Is there really something wrong with this bytecode?

Actually, the problem does not really seem to be with the
bytecode itself, but more with its assembly: loading the
bytecode at address 320 (destination field = 4) instead of
128 makes everything work just fine...

Hence, seems to me that replacing the "align (64)" in the
DEFLATE assembly code with a "at (320)" should resolve the
issue?

One thing I still worry about though: as the circular buffer
is right after the bytecode, would this circular buffer
overwrite an eventual static dictionary (SIP?) loaded at
address 1024 as recommended in the draft (not sure what the
circular buffer size will be and if there will be enough
room left after changing the bytecode start address)?

While I'm at it, just one more little question on that topic:
what are the extra "0000 00ff ff00" bytes added at the end of
the sample message provided in the draft? They do not seem to
relate to actual compressed data, and the sample message
decompresses just as well without them...

Herve Siegrist.

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