A small bit of unusual Atari ST/Commodore Amiga history regarding 3.5" disks

Bill Loguidice's picture

Remember back in the late 1970s and early 1980s when games used to come on cassette, publishers such as Avalon Hill would pack on as many as a half dozen or more different platform versions of one game onto the front and back of one cassette? And how in the age of the 5.25" disk, the front and back of a disk were sometimes sold with, for instance, an Atari 8-bit version on one side and a Commodore 64 version on another? This became a lost art with the rise of the 3.5" disk, as there was only one side and no way to split formats; it wouldn't be until optical media rose to prominence that we would again see multiple platforms on one disc (usually Windows and Mac). Or was it a lost art on 3.5" disk? I was unaware until about a year or so ago that multiple platforms on a 3.5" was not only possible, but was actually used in a commercial product by at least one company, Rainbird, who developed a seemingly impossible dual format Atari ST/Commodore Amiga disk for their game, Starglider II. As luck would have it, I recently won a dual format Starglider II to go along with the standard, single platform releases I already have in the series. Of course, according to its Wikipedia entry, releasing Starglider II in this format made the game extremely unreliable so the technique was abandoned, but it's still of significant historical interest as far as I'm concerned. Perhaps some time after it comes I'll attempt to load it on each of the systems and see what happens!

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Mark Vergeer
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True Double sided 5.25" vs the Fake double sided 'flippy'

Sure 3.5" disks can be double sided or single sided but when two sides are used they are in the same format and the disk cannot be 'turned over'.

With 5.25" disks it was possible to create a 'flippy' which basically meant that both sides of the disk were used as a single sided disk and could in fact house completely different formats. The trick was to cut another 'write protect' notch on the opposite side of the 5.25" disks and turn it around so that the drive was 'tricked' into using the flip-side of the disk as well.

This worked with most 5.25" disks - even some white label disks worked although the magnetic material on the flip-side wasn't always up to it. Read errors were more common on the flip-side of cheap white label disks. Most big/well known brands didn't have any problems. The safest thing to do was to buy double sided 5.25" disks and use them as flippies in single sided drives.

The flippy format was very very popular with the 1541 drive in combination with the cbm 64 and the same was done on Atari 8bit computers and the 5.25" floppy drives.

Now the double sided cbm 5.25" drives for the 8bit cbm micro range can run into trouble reading the flippy format as the 1570 drive likes to use both sides of the disk and with true double sided disks the flip-side is formatted quite different from a flippy disk. Some drives have been equipped with a switch to select a flippy-compatible-floppy-bios - but those switches are only after market home-brew mods.

Definition of: flippy-floppy
A method for increasing the capacity of a single-sided 5.25" floppy disk. It was converted to double-sided use by punching a second notch in the disk so that it could be flipped over and inserted upside down. This was a fleeting and poor approach to the storage problem of the early 1980s because the disk's rotation was alternated.

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Bill Loguidice
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Flippy disks

For a while I used scissors or a whole punch to create flippy disks for my C-64, but at a computer show I ended up getting a notcher that lined up the disk perfectly and created a nice, professional square notch. Definitely better than occasionally messing it up with the scissors or hole punch.

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Mark Vergeer
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Still have that notcher!

Yeah I used to cut some holes with small nail-scissors. But early on got a notcher!

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Matt Barton
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Hole punchers?? Disk

Hole punchers?? Disk notchers??

Geez, guys, you didn't bite'em off? That's the way Real Men (TM) do it. :P

Just kidding!

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Calibrator
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Double Sided & Double Headed
Bill Loguidice wrote:

Sure, there's "double sided, double density" and what-not, but I don't believe there's a way to independently control each "side" since you're still accessing the disk the one way from the one side. I'd love to hear someone provide a true technical explanation, but I think being double sided had more to do with capacity than any ability to write to one side or the other, as writing like that was what the drive and disk just did and didn't really know anything about "sides".

Of course double sided disks used both sides of the disk - the drives for them have two read/write heads. And of course this doubled the capacity.

The 3 1/2" drives are often called "semi-intelligent" and though they have a much higher capacity than, say, the older - so-called "intelligent" - Commodore 1541 or Atari 1050 drives, they are less capable: They can't format a whole disk on their own for example. They need either a driver program (a "fashion" that really started with the 16 bit computers like the Macintosh, the Amiga, the ST and - in limits - with the IBM PC, which was definitely behind back then and still used a "DOS") or "low-level programming", which was often the method of disk copy programs.
Several disk copiers for the Amiga trash the OS anyway to get the maximum amount of free RAM (to minimize disk swapping with a single drive) - they also access the drive controllers registers directly (either because the OS was shut down or for speed reasons).
When programs do the latter they have to specify which "drive head" they want to use - often named "head 0" and "head 1" - to control which side they want to access.
Wikipedia has a good article on floppy disks, btw:
http://en.wikipedia.org/wiki/Floppy_disk

When more and more RAM was available and hard disks became the norm nobody trashed the OS anymore (after all: a reboot was necessary) and the only programs to access the floppy controller were either the DOS or the drivers for the host OS themselves - or diagnostic programs which needed to get "direct" results from the hardware and not "watered down" error messages from the OS.

Mark Vergeer wrote:

This worked with most 5.25" disks - even some white label disks worked although the magnetic material on the flip-side wasn't always up to it. Read errors were more common on the flip-side of cheap white label disks. Most big/well known brands didn't have any problems. The safest thing to do was to buy double sided 5.25" disks and use them as flippies in single sided drives.

I never had problems with the "backside" of my 5 1/4" floppies when I still used them regularly with my Atari 1050 drive (low density 128 KB per side). Nowadays I only use 5 1/4" occasionally with my Apple II's and haven't had defects either (140 K drive).

There are some interesting texts floating around that more or less say that the "back side is worse"-mantra is, well, only a sales tactic to sell "certified double-sided" disks (ever seen a single-sided disk?).
The reason stated for the second side being as good as the first side was that the manufacturers (I don't talk about third-grade Hong Kong backyard producers here) coated the disk material on both side evenly - not knowing which side will finally end up being the "top" side in the disk jacket. Even the jacket has two holes for the disk heads to save them a penny...

take care,
Calibrator

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Bill Loguidice
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More disk talk
Calibrator wrote:

Of course double sided disks used both sides of the disk - the drives for them have two read/write heads. And of course this doubled the capacity.

Understood, but I'm trying to clarify if the sides can be individually written to. For instance an Amiga on 0 and an Atari ST on 1. From what I understand, you can't, so the two sides are only functionally one side since any capacity is tied to the one computer.

Calibrator wrote:

There are some interesting texts floating around that more or less say that the "back side is worse"-mantra is, well, only a sales tactic to sell "certified double-sided" disks (ever seen a single-sided disk?).
The reason stated for the second side being as good as the first side was that the manufacturers (I don't talk about third-grade Hong Kong backyard producers here) coated the disk material on both side evenly - not knowing which side will finally end up being the "top" side in the disk jacket. Even the jacket has two holes for the disk heads to save them a penny...

take care,
Calibrator

I have to agree that it's probably more urban legend than anything. I can't recall ever having a problem, and it didn't matter the quality of the disk either.

Vintage Games book!
Xbox 360: billlog | Wii: 1345 2773 2048 1586 | PS3: ArmchairArcade
Bill Loguidice, Managing Director | Armchair Arcade, Inc.

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Calibrator
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Hybrid formats, cont'd
Bill Loguidice wrote:

Understood, but I'm trying to clarify if the sides can be individually written to. For instance an Amiga on 0 and an Atari ST on 1. From what I understand, you can't, so the two sides are only functionally one side since any capacity is tied to the one computer.

I perfectly understood you the first time and I had already begun a lenghty reply but it wasn't finished before I had to go to work ;-)

What you are implying is that the disk format is tied to the host DOS or disk controllers. Of course the hardware is a "must" as one has to work within the limits of it (though much can be achieved when being creative).
The former isn't a must, though, as many games (and other apps) for different systems brought their *own* mini-DOS for different reasons (increased speed and/or capacity, copy-protection). Of course those disks aren't natively readable by the "normal" (D)OS of the respective machine.
If you have a machine like the Apple II, for example, you wouldn't really be surprised to be able to collect disk images with 30+ different disk operating systems...

Bonus question: What do the programs using those hybrid formats have in common?
Are all of those applications/games of the self-booting kind with their own "DOS" or are they usable from the host OS (as normal disks - albeit perhaps with reduced capacity)?

If they are self-booting they could *very well* be organized in the fashion you describe above:
One surface for Amiga, the other for ST (for example) - each with about 360-400 KB. A separate "DOS" can control how the disk drive is controlled (adapted to each system of course) and the only thing needed would be a "boot sector" for each system that points to the respective start sector (and drive head).
Different boot sectors on the same disk are of course the hard part of the theory!
(This is the part where my knowledge really comes to a halt as I always used native systems to stay as compatible as possible - with future storage systems etc.)

Another variant could be employed if both systems have flexible enough disk controllers that could read the disk encoding formats (MFM and/or GCR).
This method would give the application access to the whole disk capacity!

This was actually possible and may have been used for some games as I can give you a real world example: The Amiga disk controller inside the "Paula"-chip (in practically every Amiga) is able to read and write 720KB MS-DOS format disks!

There actually was a IBM PC emulator for the Amiga platform (a very lowly 8088-compatible-emulation, text-only) that could read original DS-DD-disks (but of course not DS-HD "High Density" 1,44 MB) with a standard Amiga drive.
Very funny to tinker around with the text-mode DOS programs of the time but not really good enough to live with. IMHO it was probably made to show off the power of the Amiga as the PC couldn't even dream to emulate a lowly Amiga 500.

take care,
Calibrator

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Catatonic
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Joined: 05/20/2006
Disk sides
Bill Loguidice wrote:

I'm trying to clarify if the sides can be individually written to.

I wondered the same thing myself. I do know that in a hard drive, all surfaces are written to at the same time, because that is obviously much faster. (e.g. when you write 8 bits to the drive, each bit ends up on a different surface if you have four 2-sided platters in the drive) I do not know if floppies do that - i.e. when you write 8 bits, 4 go on one side and 4 go on the other side.

Mark Vergeer
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Nothing is absolute....
Calibrator wrote:

There are some interesting texts floating around that more or less say that the "back side is worse"-mantra is, well, only a sales tactic to sell "certified double-sided" disks (ever seen a single-sided disk?).
The reason stated for the second side being as good as the first side was that the manufacturers (I don't talk about third-grade Hong Kong backyard producers here) coated the disk material on both side evenly - not knowing which side will finally end up being the "top" side in the disk jacket. Even the jacket has two holes for the disk heads to save them a penny...
take care, Calibrator

Perhaps the situation in the Netherlands was again rather unique. Most 5.25" disks on offer were what I call 'white label' which actually meant very low quality super cheap - probably of unknown Asian origins. Branded 5.25" disks (pretty rare in 80's Netherlands) - even single sided disks - Nashua, TDK, Philips or whatever where of a higher grade which meant that even the back sides of single disks could be used without any problems.
The white label disks I am talking about are of such a low quality that even the 'right side' often was rubbish.

In the US and Germany the use of floppy drives for home computers was much more common than in other European countries. As 5.25" disks where relatively expensive quite a few countries including the Netherlands and the UK especially relied on the compact cassette format. And 5.25" disk-use was only started relatively late in the cbm/c64 life span.

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Mark Vergeer
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150K Atari vs 170K CBM

The difference between storing 150K Atari vs 170K CBM per disk-side could make a slight difference in reliability - also the way the disks are 'coded' is different which could also cause a slight difference in reliability when you compare 1050 and 1541 flippy disk operation.

Xbox 360: Lactobacillus P | Wii: 8151 3435 8469 3138
Armchair arcade Editor | Pixellator | www.markvergeer.nl

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