.ros appers to be used for Marvell-powered switches. So far it has been spotted in some Dell and Netgear switches
Header 
Edit
The offset and length is sometimes bigendian, sometimes little-endian, probably depending on CPU-type of the device.
| Offset | Comment |
|---|---|
| 00-0Fh | Devicename? |
| 10-1Fh | Type of compression? |
| 20-2Fh | ??? |
| Filemetadata, in 0x20 blocks | |
| 30-3Fh | ASCII filename |
| 40-43h | Offset in file, excluding first 0x20 bytes |
| 44-47h |
Length of file |
| 47-4Fh | ??? |
Compression Edit
Some of the files are uncompressed, some are compressed with LZMA (recognizable by 5D 00 00 80 header), some are compressed with LHA, some are ZIP-ish, etc. Interestingly, all parts are mostly decompressable by standard tools (lharc, zip, lzma, etc.)
I have not yet been able to replace a LHA-part yet, succesfully. Probably there is a minor difference between the format the bootloader expects and what 'LHa for UNIX V 1.14i' generates. If I don't use any parameters it complains about the compression method (lh7) used. If I use compression method lh5, I get the following error on my PowerConnect 3324:
Autoboot in 2 seconds - press RETURN or Esc. to abort and enter prom. Preparing to decompress... Decompressing SW from image-1 There's an extended compress header. There's an extended compress header.
