That's what I meant, keep it to the critical OS items and install disks, but not user-editable or writeable disks of course.

Isn't the whole XP vitual PC thing in Win7 based on diffs against the original image? or am I misremembering that.

btw I think MESS will at least need some pre-canned 'installed' versions of things like the OS as recognized software images which can be specified / mounted. Having to install a game is one thing, having to mess around and install entire OS each time you want to try something new, especially under an emulator isn't really productive or useful.

I'm hoping the major result of these changes will be
a) better documented software
b) greatly improved usability
c) greatly improved accessibility

But you wouldn't have to install the OS each time, just once per machine you want to mess with. Given that most older machines that even had them came with blank HDDs and you had to install the OS to begin that seems quite reasonable.

I agree, at least some plain installs for significant versions of OSes, but trying to keep it at the minimum. I would say the minimum amount of pre-canned OS images so that all the software lists for a certain system can be run, preferably only one. Also things like a DOS boot disk for PC and things like that.

The problem I see here is, what size CHD do you create for each of the pre-canned OS installs?, do these files grow as the space of the CHD get's filled up or they are a certain size from the start?

I think this has to be handled with a lot of care, as already was said, software lists for certain systems can be almost infinite. In that case, I think only some minimum pre-canned software should be included and that's all.

CHDs are a fixed size and cannot grow. That's why if we supply canned OS images on HDDs at all such "boot drives" must be read-only, with the user supplying a second HDD for installing stuff.

I honestly don't see the problem with the 'diff' system MAME uses now. Nobody is going to be using the emulated OS as their primary OS, so for simple things like installing software it should be fine..

You'd be surprised how much installing people do in the real world on these things. There's a pretty large need for people to open e.g. weird old word processing files that nothing modern will touch. Anyway, I thought the canned software lists were to coexist with the existing mount methods. If you want to make it only canned software like MAME I'm out, I'll fork MESS and work on the Mac driver my way.

Was there ever any doubt?

I do give people the benefit of the doubt, even Haze. Anyway, if he wants to trap himself in .diff land it's his funeral as long as nobody removes or otherwise damages -hard1 =)