Format 720kb IBM floppies to 880kb Amiga format?

[spirantho]

The unformatted capacity is actually 1MB.
The reason it's only 720KB is because the floppy controller in an IBM PC is actually an NEC PD765A - or at least compatible. Even on the southbridge chips of modern PCs, it's basically an adaptation of that same chip which was used so much in the early 80's (Spectrums, Amstrads etc.).
The PD765A isn't a very advanced chip - it requires certain sync signals and certain gaps between the sectors. Paula, on the other hand, is much more versatile, so the Amiga has the same MFM system, but with more efficient usage of the space on the disk. The WD1770 like in the BBC B and Atari ST is slightly better than the PD765, but not much, and is pretty much compatible, which is why STs can read PC disks easily, but not the other way round (and why the ST usually only has 360KB/720KB disks too).

So the reason that there's so much space wasted is simply because IBM used a generic controller chip in the original IBM PC, and it never changed (even to this day), whereas more advanced machines like the Amiga had much lower level access, and could do clever things like change the length of the dead space between sectors, which were unchangeable on the 765.

Incidentally, Microsoft did use a different system (Distribution Media Format - DMF) to get more information on floppy disks. They did this by changing the filesystem (limiting the amount of files available), and also by changing the sectors per track. I can only imagine that the 765A is capable of *reading* with a shorter inter-sector gap than it is _writing_ (which would make sense as it's just waiting for a sync signal - it would only be writing where it would be fixed how much data it was writing).

I intend to add support for DMF at some point in the Catweasel drivers....

[spirantho]

You are, of course, correct. It's been a while since I looked at Amiga encoded disks.

I guess using the 765a makes the software a lot simpler (it just requests data at a particular location) but means it's much more limited because of it.
The Amiga I believe is basically just Paula chucking out bits which then have to get decoded, isn't it? I.e. great for copy protection, not so great for ease of use (hence Kickstart routines).

The usual way for copy protection on the 8-bits - and I'd guess the PC too - was to have illegal sector numbers in the sector headers. So the first track may have sectors 1,2,3,33,4,5,6,7,8,9 or something, which of course the DOS would think was an error.
Equally, nasty copy protections could do things like have sectors 1,2,3,1,2,3,4 on the disk which could never be written normally, but you could read all the sectors and check the existence of two different sector 1s, two sector 2s, that sort of thing.
Is this OT now? Sorry.

Edit: If you need to run old PC games, get a bridgeboard! You need a big box Amiga though (unless you're happy with CGA 8088/286 games )

[spirantho]

Don't use the original PC disks though! Use blank disks. That way you can run both and see just how much better the Miggy was in the 80's/early 90's.
If you want cheap blank disks just get them used on eBay. You can still use most of them. Heck, if you're in the UK, I have tons you can have some of for postage.

[spirantho]

I made a little YouTube video a little while back called "how to clean floppy disks". Might be worth a try.

Many apparently bad disks actually are just dirty and work fine when you've removed the dirt. Be aware though that if they do start working, make a backup immediately! It may have been oxidisation that broke the disk so you may have lost some of the surface.

[spirantho]

Not if he wants to use Amiga formatted floppies, no.

[spirantho]

Just to be clear: A USB device will not work at all. The controller is completely wrong for Amiga formatted disks.
You need a specialised floppy controller like a  Catweasel or Kryoflux, or you might get lucky with a PC's controller using the ADFread hack.