Format 720kb IBM floppies to 880kb Amiga format?

[Blinx123]

The severe lack of Amiga tech (which also includes original blank Amiga floppies) in my area had me thinking.

Are those 720kb IBM double-density floppies physically any different from 880kb Amiga double density floppies? Or is this simply a matter of formating these? e. g. could I take a 720kb IBM floppy, format it in CrossDos and write data from an 880kb ADF to it?

[Ral-Clan]

No, they are no different.  Just make sure you are using true 720K (DD) disks and not High Density disks.

[Blinx123]

No, they are no different.  Just make sure you are using true 720K (DD) disks and not High Density disks.

Nice. Thank you very much.

What would be the actual size of a 720kb/880kb floppy then? 960kb?

Seems kind of unreasonable of IBM to waste such a large amount of memory. Is there any particular reason why they did it (copy protection/performance/etc)?

[Lord Aga]

DD floppies are 1MB, HD 2MB (in raw capacity, theoretically so to say). Different computers have different efficiency when formatting.
PCs format them at 720KB. Amiga  at 880KB.
I'm not quite sure what are you trying to do ?
If you insert a DD floppy in your Amiga, and format it, it will have 880KB free space. And you can write ADFs on it.

[Blinx123]

DD floppies are 1MB, HD 2MB. Different computers have different efficiency when formatting.
PCs format them at 720KB. Amiga  at 880KB.
I'm not quite sure what are you trying to do ?
If you insert a DD floppy in your Amiga, and format it, it will have 880KB free space. And you can write ADFs on it.

I have a whole bunch of big-box IBM/Tandy games that I'd like to convert to their respective Amiga releases.

Since x86 distanced itself so much from it's own past, I'm not feeling the vibe while playing these games on my PC*


*An Intel i5 2500k rig with a 2GB GTX 680, 16GB RAM a 1TB hardisc and a Blu-Ray drive running those games in Dosbox and outputting them to a 47" TV might sound awesome, but there's nothing classic about it.

[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....

[commodorejohn]

Since x86 distanced itself so much from it's own past, I'm not feeling the vibe while playing these games on my PC*


*An Intel i5 2500k rig with a 2GB GTX 680, 16GB RAM a 1TB hardisc and a Blu-Ray drive running those games in Dosbox and outputting them to a 47" TV might sound awesome, but there's nothing classic about it.

That's why you grab a cheap 386/486 and throw MS-DOS 6 on there

[mongo]

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.

The Amiga doesn't need any space between sectors because it can only read or write a whole track at a time. The controller chip in the PC is actually far more advanced than the rather brain dead controller in the Amiga.

[Blinx123]

That's why you grab a cheap 386/486 and throw MS-DOS 6 on there

True.

But that's kinda silly.

A) MS-DOS, while certainly better than Windows Vista, is still nowhere near as good as Amiga OS 3.1

B) An Amiga is such a classic because it never had a true successor. A 386/486 is just another PC and therefore really pales in comparison.

[psxphill]

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.

Basically PC's can write individual sectors, which means they need a sync marker and a gap between each sector. Because of drive speed tolerances they were quite pessimistic when choosing the gap size. When writing it has to read until it finds the correct sector header and then switch to writing.
 
The Amiga can only write full tracks, because there is only one sync and one gap per track. It can't do sector writing because it has no controller. When writing PC format disks it has to write a full track at a time.
 
So the Amiga has more space, but it's slower to read and write.

[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 )

[Bamiga2002]

Use on PC side to format floppy:
"FORMAT A: /T:80 /N:9"

[Lord Aga]

I have a whole bunch of big-box IBM/Tandy games that I'd like to convert to their respective Amiga releases.

That's a good plan
Just go for it. There are no PC and Amiga disks. Just DD floppies. Put them in your Amiga and format them at 880K. A good recipe to enjoy the holidays

[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.

[psxphill]

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 PC was built from off the shelf parts and the upd765a was basically all that was available, you can make an argument for sector gaps or a track gap but what was available to them had sector gaps so it wasn't really a choice.
 
The Amiga had a better price/performance ratio because everything as custom and having a track gap is easier from the hardware point of view, so it was cheaper for them to design and build. The greater storage space was a side effect of that, I doubt they ever considered doing it any other way.
 
trackdisk.device would have existed no matter how the hardware worked though. Even on the PC you wouldn't talk directly to the floppy controller. The only real difference is that the Amiga generally uses the same code for booting from a floppy and once the operating system is loaded. While on the PC you have separate device drivers. Both ways have their merits.