MADSPACK — the container

All 206 .SS, 35 .PIK and 5 .FF files have this shape:

0x00  13    "MADSPACK 2.0\x1a"
0x0d  BYTE  0
0x0e  WORD  count of entries (1-16)
0x10  16 x 10-byte entry slots:
        BYTE  type    0 stored, 1 FAB-packed
        BYTE  flag    carried; identical across one file's entries
        DWORD size    bytes after unpacking
        DWORD packed  bytes in the file
0xb0  the entries' data, back to back, in entry order
....  sometimes more bytes after the last entry (4 of the 246 files)

Evidence #

ff_open (file 0x76e50) reads the 16-byte head, checks the magic and reads count entries; ff_read (file 0x77100) takes the entries in order, treating type 0 as a stored copy and any other type as packed. Every shipped file closes on that arithmetic: 0xb0 plus the sum of packed is the file size, or the start of the trailing bytes.

Carried, not computed #

The slots past count are not zeroed in the shipped files. They hold whatever was in the packer's memory — x86 code among it — so the whole 0xb0-byte header is carried verbatim. The flag byte is not what ff_read decides anything on, and it is carried too, as are the trailing bytes.

That is what lets a rebuilt file with one edited entry differ from the original in that entry and its sizes only.

Packed entries use FAB.