That '8E54OS_.PGM' is 4 times larger than the Flash NOR boot prom you have illustrated in the schematic. Could there be 4 of those flash IC in the design? Are you sure that that file isn't for the bootprom, but for the Wav-roms? No software, just audio samples?
That particular "PGM" file, version 1.60 of the Motif XS OS, is 4 times larger than any other PGM file I have:
v1.12 of the Motif XS,
v1.06 of the S90XS, and
v1.50 of the Motif XF (a newer but largely similar keyboard to the XS architecturally). I haven't a clue what accounts for this difference. To my memory, no update ever added new audio to the wave rom. I suppose it's possible that Yamaha replaced the wave rom in order to fix some sort of problem with the existing data, but never publicized the problem or solution?
There are two additional 64MB flash IC on the board, each dedicated to wave rom. There is also a small 1MB NOR flash which is dedicated to the Firewire interface, but given that the
Firewire firmware update is delivered in a separate file and installed from a computer connected via firewire cable (totally different method from the PGM installation), I don't think this particular bit is touched by the PGM file
I can be certain that the S90XS v1.06 PGM file contains an update for its bootloader; this file successfully loaded into the Motif XS and got far enough to break it (see attached photo), whereas the Motif XF file gave an error regarding a missing "update.sh" file after unpacking the PGM but before starting the actual update process. Yes, I got extremely foolhardy with investigating the behavior of these files...
In any case, even if that file contains more than just OS data, I am certain that the OS data is in there as well given the features that it and the bundled 1.56 update provide. I additionally identified that the onboard Samba server was updated from version 3.0.10 to 3.0.24 between versions 1.12 and 1.60 of the XS. So, there's definitely a Samba server somewhere in that particular '8E54OS_.PGM' file. I can't be certain that it includes the data for the bootloader.
If the analysis regarding encryption is correct, then at least some of the data in the file is both encrypted and compressed because the boot sequence described in the service manual includes a step labeled "Decompress Linux kernel on NOR flash to DDR SDRAM."