Module Compression

When the module Initialisation Code offset at module+4 has bit31 set, it indicates the module is compressed. In this case:

Does the rest of that word still hold the Initialisation offset?
Is all the header replaced by the uncompressed data?

These do not seem to be covered by the Relocatable Modules File Format wiki page.

I was surprised to notice this myself for the first time just the other day. When did compressed modules become a thing?

RISC OS 3? The code responsible (…kernel.s.unsqueeze) is dated “15-Feb-90”.

After a bit more digging round in my memory, I can answer myown questions!

Does the rest of that word still hold the Initialisation offset?

No. It holds the length of the compressed module.

Is all the header replaced by the uncompressed data?

Yes. Everything, including the header, is replaced.
The header in the compressed version (title, help, etc) is just to aid identification while compressed.

I discovered this after remembering two utilities in the DDE.Apps folder – !ModSqz and !UnModSqz

IIRC back in the Acorn era, !Squeeze was part of the C toolkit, but ModSqueeze was only available on request / via developer support. I remember receiving it from Sue Wall, I believe.

I remember it was a life saver (well, not literally) when we did SecureSockets module, and needed it to fit on the floppy disk!