Short answer: that’s just names, it does not matter. Yamaha can change everything they want and which is within the capability of the hardware with such an update.

Longer answer: the Montage is what you would call an embedded system. That means it is seen as an integral combination of hardware and software where one part is meaningless without the other. In such system the software can often be updated only as one block which is then called the firmware.

The firmware comprises usually different parts, the hardware drivers, potentially some operating system (in the classical sense), the application software, DSP programs, data, ... For the user that’s one opaque block. You get everything with the „OS“ update.

It should not be important if there is a ROM for this question. Usually there is a so called boot loader which allows to load the real software from Flash. The boot loader may or may not be in ROM. But this can be in the order of a few kBytes, it does almost nothing and should never need an update. Everything else is most probably in Flash.

Note: it might be that the waveforms coming with the Montage are actually in a ROM. My understanding is that the OS update is too small to contain them all. Also I believe no update so far added waveforms. But I don’t really know that.

The OS and firmware is not permanent. Hardly anything is. True ROM means you can "burn" the hardware once to "fuse" the part and then the contents of the ROM cannot be changed. This was true of very old hardware - so manufacturers had to be sure to get it right or have higher warranty costs or face other customer satisfaction issues. Later became the advent of electrically erasable parts which were then able to be re-programmed after an erase (EEPROM). There's some stages between the two (ultra violet erasable parts, etc) - but the point remains that the evolution of XXXROM (meaning things that end with ROM) include parts that can be re-programmed even in the field by customers.

Montage has such a system. You'll see loose language of "ROM" to describe the libraries sometimes, or "ROM" for the firmware. This is an incomplete description as the technology is not ROM -- you can erase both of these areas and start over with new content.

What exactly is the underlying "fabric" of the non-volatile memory which hosts the firmware is of no real consequence to you - so the level of specificity is sufficient just to know it's erasable and reprogram-able.

This addresses your "cannot be undone" comment - which is off-center.

Firmware, broadly, is code and data that a manufacturer controls and is programmed into non-volatile memory storage parts (hardware) in electronic devices. Sometimes firmware is "write once" and cannot be modified by a user. Sometimes users are given the opportunity to update firmware themselves using a variety of methods ("over the air", USB sticks, floppy disks, utilities downloaded from the net, etc). Montage allows for users to update the firmware.

The firmware released by Yamaha contains several "regions". Once is the operating system. The operating system is based on Unix and comprises of all the code and data in order to make Montage function. There is also a region of data which contain the presets (waveforms, samples, performance parameters, arpeggios, etc). When you install a different OS like Windows 10 - it comes stock with certain background images. Although images by themselves are not an operating system - since they are bundled with the OS (Windows) - they become referred to as part of the Windows OS. Likewise, the preset data is part of the Montage OS.

"Firmware" and "operating system" are not universally interchangeable. However, in Montage's case - they are synonymous.

Note that there is other "firmware" inside Montage that may not be user serviceable. The touch screen itself likely has a programmable part that helps it operate and was programmed by the touchscreen manufacturer (not Yamaha). Even though this is "firmware" -- it and other pieces of firmware that really do exist inside of Montage can be ignored from a user perspective since there's no way to mix up a bunch of parts you can never touch with the portion of Montage that you CAN update when a new operating system version is released. Although "firmware" is ambiguous to the engineering team (since there are lots of different firmware images they can touch) -- the user only has one firmware component they can update.

The "Library" part of Montage looks and smells just like firmware. Libraries use the same hardware components as the OS firmware does to store its image. Libraries use the same sort of programming mechanism to update the Libraries as the OS firmware does. Libraries are not "firmware" because the contents are controlled by the user (not Yamaha, the manufacturer). Users can pick from a vast menu of things to stick in Libraries or leave them empty. Libraries are a file-system that allow for reprogramming but must be entirely erased (each Library slot) in order to change the contents of any given Library slot. This aligns with a "flash" type underlying fabric -- but remember we're not really concerned with the technical details of what to call this memory.

Jason, do you actually know that the Montage uses something based on Unix? Was there a confirmation about that? I know that the Kronos uses Linux, for the Montage I did not hear anything so far.

Yes.

http://download.yamaha.com/sourcecodes/synth/

This was not my original tip-off, but it provides the confirmation from an official source.

You can see use of an RTOS or somewhat off-the-shelf OS has been the backbone of Yamaha synths predating Montage back several generations.

When I saw one approach pitted against the other claiming one manufacturer uses a "PC" while another one doesn't -- I drew lines that the distinction is not as stark as was being painted. Everyone generally uses the same kind of approach - as letting someone else deal with the underlying operating system and using standards gets you TTM where rebuilding your own glue around the bare metal either gets you stuck in time or causes lots of re-do every generation.

Thanks for the info, good to know. And yes, nowadays using Linux makes a lot of sense.

Note that the search term "Hackintage" does not currently bring up any search results. Someone has to coin it first.

@ Jason
Thank you for your answer.
You always amaze me at the amount of information. You keep in your head.
Sorta reminds me of myself. 36-38 years ago. When I knew the every workings of IBM 370 computers.
And many other things. Now I try to. Avoid any information. That is not relevant to my life.
I only want to write/play music and enjoy my Family. These last years I have left.