Flippy Disk Confusion Between Commodore 1571 vs 1541 Drive

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JasonCA
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Flippy Disk Confusion Between Commodore 1571 vs 1541 Drive

Post by JasonCA » Sat Apr 28, 2018 9:46 am

I'm trying to grapple with what a Flippy Disk is in the view of a Commodore 1571 Drive vs a Commodore 1541 Drive where later Kryoflux would be used for preservation of disks created from a 1571 drive. Here we have two different drives where the 1571 has two heads and the 1541 has a single head.

Where's my confusion may lie is how each drive uses:
  • Write-enabled notch (Requires 1 or 2 notches? When? Why?)
  • Index Hole (Requires 1 or 2 index holes? When? Why?)
According to Wikipedia, it says "A flippy disk (sometimes known as a "flippy") is a double-sided 5¼-inch floppy disk, specially modified so that the two sides can be used independently (but not simultaneously) in single-sided drives" which can be found here: https://en.wikipedia.org/wiki/Floppy_di ... ippy_disks

Here, the single sided drive as mentioned above would be the 1541 drive. However, the 1571 has 2 heads unlike the 1541.

Question #1: Does this mean for a 1571 drive, the disk need not be flipped since the 1571 has two heads (where internally it is flipping the disk simply by using the second head)? But the concept of a Flippy Disk leaves me with another question: since the 1571 has dual heads and can write to both sides of a double sided disk without being flipped, how does one distinguish a "Flippy Disk" from a disk that was written with two heads like a 1571? Or, is there somehow no difference?

Question #2: With regard to the write enabled notch and the 1571 drive, do you need one notch to write to side A, and a second notch to write to side B? Or, would you only need one notch for a 1571 drive to be able to write to both side A and side B considering you don't have to flip the disk over?

I'm of the understanding, that for the 1541 drive you need TWO write enabled notches to write to both Side A and Side B. However, to write to side B, you must flip the disk for the 1541 since it only has one head.

But what about the index hole for Side B to be read by a 1541 vs a 1571 ...

The Index Hole:

According to Wikipedia, "For disk operating systems that do use index sync, a second index hole window has to be punched in both sides of the jacket, and for hard-sectored formats, an additional window must be punched for the sector holes". Can someone help to clarify this?

Question #3: Assuming you did flip the disk for the 1541 to read Side B, wouldn't the drive FAIL to write to the disk since there is NO secondary index hole? Otherwise, does this mean for the 1541 when reading Side A it is index synched to the index hole of the disk where for Side B it is NOT index synched? When would a second index hole have to be punched? I have seen plenty of Commode disk with simply two notches on both sides and usually only ONE index hole. If the 1541 simply ignores the index hole for Side B, then why would a 1541 drive even care about the index hole at all even for Side A? What is meant by "hard-sectored" formats?

Question #4: Does the 1571 have two index hole sensors just as it has two heads? At this point, for all I know the 1571 is index syncing on the first index hole in order to write to Side B since it can still SEE the first index hole considering it has dual heads and does not need a disk to be flipped to read/write to Side B. But then, how does the 1541 handle this with regard to the index hole it can't see when the disk is flipped? Hmmmm

Question #5: It would seem that some floppies written by professional copiers/duplicators, do NOT even have notches at all, but may have DUAL index holes and still contain data on both Side A and Side B. If a disk has two index holes, does that mean it's a Flippy Disk? Or, if it has data on Side B with one index hole what does that even mean then? A picture of a professionally written disk with NO notches yet with 2 index holes found here: https://en.wikipedia.org/wiki/Floppy_di ... y_Disk.jpg

Finally, I guess my underlying confusion is on how to determine when looking at a physical floppy disk whether it is a Flippy Disk or not and what a "Flippy Disk" even really means with regard to the required notches (1 or 2?) and index holes (1 or 2?) from the perspective of use in the 1571 and 1541 drives. If the 1571 can indeed write to both sides of a disk without flipping it, would such a disk still be considered a Flippy disk? Or the ability of the 1571 to write to Side B written in a different format? In other words, is Side B always written in the same format by a 1571 and 1541 drive or are there different formats (possibly due to how a single head drive writes to side B vs how a dual headed drive can write to side B?)

Where am I going with this? Eventually, I want to preserve these Commodore disks utilizing Kryoflux but am currently VERY confused about this whole Flippy disk concept. In reading through the forums, it seems that since the 1571 has dual heads, I would NOT need a flippy modded PC disk drive to use Kryoflux to image both side A and B of the disk as long as the disk was written by a 1571. I believe I have read that both Side A and Side B can be read in a single pass a drive that is NOT flippy modded. However, if the disk was written by say a 1541, I would need a Flippy modded PC disk in order for Kryoflux to be able to image Side B (the Flippy Side).

A lot of the disks I have were written by a 1571 drive. I've yet to understand how to determine when looking at some of these disks if some are Flippy Disk and some are simply not flippy disks but may still have data on Side B since the 1571 can magically write to Side B. How this is all done is what I'm better trying to first understand.

Someone please help set me straight if possible!

brightcaster
Posts: 131
Joined: Fri Nov 08, 2013 10:48 pm

Re: Flippy Disk Confusion Between Commodore 1571 vs 1541 Drive

Post by brightcaster » Sat Apr 28, 2018 3:05 pm

Answer #1: The 1571 can't read the second (flippy) side of a 1541 without flippying the disk. (See further down why this is the case). There's a flag in the 1571 BAM at Byte 3 indicating double or single side format.

Answer #2: You only need one notch for a 1571 as long as you don‘t flip the disk

Answer #3: No, the 1541 doesn‘t have an index-sensor at all! It dosen't write index-alligned regardles of any index hole..

Answer #4: The index hole sensor of the 1571 is only connected to the mfm controller and not used in gcr mode.

Answer #5: you don‘t need any index hole to read/write with a 1541 and a 1571 (in gcr mode)

You may find a lot of more information at wikipedia under „commodore 1541“ and „commodore 1571“. Also keep in mind that by flippying a disk you write the data in reverse to the disk side-b. So you can't read a 1541 written flippy disk b-side without flippying it in a 1571! The second head would see the data in reverse otherwise... (dtc turns the tracks of side-b by software in flippy mode.)

You're right in assuming that you don't need a flippy modded drive to dump both sides of a 1571 formatted double sided disk.


David

SomeGuy
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Re: Flippy Disk Confusion Between Commodore 1571 vs 1541 Drive

Post by SomeGuy » Sat Apr 28, 2018 6:37 pm

There is still the question of how to tell the difference between the two. The best way would be to examine the tracks on the second side and see if they are recorded forwards or backwards. But I'm not sure precisely how to do that with the Kryoflux.

Just glancing at the KF manual, all of the listed Commodore formats are single sided only?

brightcaster
Posts: 131
Joined: Fri Nov 08, 2013 10:48 pm

Re: Flippy Disk Confusion Between Commodore 1571 vs 1541 Drive

Post by brightcaster » Sun Apr 29, 2018 8:00 am

As I said in answer #1 there is byte 3 in the BAM (side 0) with a flag indicating single-sided 1541 mode or double -sided 1571 mode!

As far as I remember the byte contains 00 for 1541 and 80 for 1571 format. But I'm not shure now, after all theese years....

David

JasonCA
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Joined: Tue Oct 11, 2011 3:37 am
Location: California

Re: Flippy Disk Confusion Between Commodore 1571 vs 1541 Drive

Post by JasonCA » Mon Apr 30, 2018 6:18 am

brightcaster wrote:
Sat Apr 28, 2018 3:05 pm
Answer #1: The 1571 can't read the second (flippy) side of a 1541 without flippying the disk. (See further down why this is the case). There's a flag in the 1571 BAM at Byte 3 indicating double or single side format.
Very good! I looked into this. For those who have forgotten, it is Track 18, Sector 0, Byte 3.
brightcaster wrote:
Sat Apr 28, 2018 3:05 pm
Answer #2: You only need one notch for a 1571 as long as you don‘t flip the disk
So when Byte 3 indicated double sided, then the format is 1571 and only one notch is needed.
brightcaster wrote:
Sat Apr 28, 2018 3:05 pm
Answer #3: No, the 1541 doesn‘t have an index-sensor at all! It dosen't write index-alligned regardles of any index hole..
No index sensor at all! So it seems the index sensor is most relevent then to the PC drives (Newtronics, Teac, Panasonic, ...etc) when attempting to preserve the Flippy disks made by a 1541. I suppose this means that the index sensor holes could be taped over and the 1541 would be happy as can be.

How is this to be viewed in terms of professionally copied flippy disks versus user copied disks? Does this mean to preserve user written disks made by a 1541 drive, I need not require a drive aligned to the index sensor? However in order to copy a professional flippy disk the index sensor comes into play and the drive must be aligned? I'm trying to therefore understand why this index hole sensor is so relevant for preservation purposes.
brightcaster wrote:
Sat Apr 28, 2018 3:05 pm
Answer #4: The index hole sensor of the 1571 is only connected to the mfm controller and not used in gcr mode.
OK, no index hole sensor for the 1571 either.
brightcaster wrote:
Sat Apr 28, 2018 3:05 pm
Answer #5: you don‘t need any index hole to read/write with a 1541 and a 1571 (in gcr mode)
Yes, so really these 1541 and 1571 are really never using the index holes for reading and writing. But, then again why is this index hole so relevant?
brightcaster wrote:
Sat Apr 28, 2018 3:05 pm
You may find a lot of more information at wikipedia under „commodore 1541“ and „commodore 1571“. Also keep in mind that by flippying a disk you write the data in reverse to the disk side-b. So you can't read a 1541 written flippy disk b-side without flippying it in a 1571! The second head would see the data in reverse otherwise... (dtc turns the tracks of side-b by software in flippy mode.)
Will do!
brightcaster wrote:
Sat Apr 28, 2018 3:05 pm
You're right in assuming that you don't need a flippy modded drive to dump both sides of a 1571 formatted double sided disk.

David
My understand then is that if Byte 3 says it's double sided (1571 double sided format), then I can simply dump the floppy in a single path all without a modified drive.

Thanks for helping to clarifying brightcaster :D

JasonCA
Posts: 32
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Location: California

Re: Flippy Disk Confusion Between Commodore 1571 vs 1541 Drive

Post by JasonCA » Tue May 01, 2018 5:43 am

Thanks for your input SomeGuy! :)
SomeGuy wrote:
Sat Apr 28, 2018 6:37 pm
There is still the question of how to tell the difference between the two. The best way would be to examine the tracks on the second side and see if they are recorded forwards or backwards. But I'm not sure precisely how to do that with the Kryoflux.
I think the biggest problem I've had was not clearly distinguishing that a Flippy disk and a disk formatted for Dual Sides (where Byte 3 says Dual Sides) are two separate things. As it seems, a Flippy modded drive is not needed for a dual sided D71 formatted floppy. I think that's confusing because on some floppies I have have 2 notches. I think others perhaps have tried to distinguish this for me, but I didn't seem to understand the difference. However, for disks that really flipped in the drive, require a Flippy modded drive where it can then be copied in a single pass.

It makes me wonder what happens if, for some reason, Byte 3 became corrupted! What then? Hmmm. Is the disk simply not readable on the backside? Perhaps someone may have done so to HIDE data where it could then be retrieved by changing Byte 3? Just a curiosity I guess. ;)
SomeGuy wrote:
Sat Apr 28, 2018 6:37 pm
Just glancing at the KF manual, all of the listed Commodore formats are single sided only?
There are some commands for dual sides. Such as "Panasonic dual sided dump". However, this would be for a Flippy dual side dump and not a D71 formatted (I guess I can call it this) dual sided dump.

SomeGuy
Posts: 158
Joined: Wed Feb 18, 2015 8:18 pm

Re: Flippy Disk Confusion Between Commodore 1571 vs 1541 Drive

Post by SomeGuy » Tue May 01, 2018 2:06 pm

Exactly, someone could format a disk as double sided then erroneously flip it over and write a single sided image on the back - of course that would overwrite data but might leave you scratching your head unless you looked at the direction of the individual tracks.

I would expect the 1571 format to be listed as "DS" in the manual. Using a flippy command would result in two disk images, one for each side - that would not be useful for dumping a proper double sided image. I don't have any c64 disks, so I don't know how that works.

BTW, I would suggest archiving all of them as a preservation stream dump including the extra tracks provided by a flippy modded drive. This would let you sort things out and decode later, regardless if a disk is double sided or flippy.

A few other things in case you don't already know:

The big issue with flippy disks is that the tracks on each side have the same physical location. Normal double sided drives instead have a slight track offset on the second side. This prevents the magnetic heads in a double sided drive from needing to be exactly right on top of each other. This means a flippy disk will have several tracks in a location that a normal double sided drive can not access - hence why a special modded drive is needed.

The index hole is important to the Kryoflux. When reading, it uses that to measure timing so it knows where everything really is. There is also the issue that 1.2mb 5.25" drives usually will refuse to operate if they can not find an index after the disk is inserted.

So to read a "flippy" disk with a Kryoflux one must A: have a specially modded floppy drive or B: have a second index opening in the disk's jacket and flip the disk for each side. Neither are needed for a normal double sided format.

brightcaster
Posts: 131
Joined: Fri Nov 08, 2013 10:48 pm

Re: Flippy Disk Confusion Between Commodore 1571 vs 1541 Drive

Post by brightcaster » Tue May 01, 2018 4:12 pm

The side two of a 1571 double mode disk would only give errors when read with flippy commands as the data have been written the oposite direction. Maybe that's the most definite way to determine if it has been written double sided or flippy mode.

David

rcade
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Re: Flippy Disk Confusion Between Commodore 1571 vs 1541 Drive

Post by rcade » Fri May 04, 2018 2:42 am

Actual 1571-mode disks are very rare. Even most 128 software was only distributed in single-sided format because you couldn't assume a 128 owner used a 1571. I think I've only see one or two actual double-sided 1571 mode disks.
-
Pete Rittwage
C64 Preservation Project
http://c64preservation.com

JasonCA
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Location: California

Re: Flippy Disk Confusion Between Commodore 1571 vs 1541 Drive

Post by JasonCA » Mon May 14, 2018 9:08 pm

brightcaster wrote:
Tue May 01, 2018 4:12 pm
The side two of a 1571 double mode disk would only give errors when read with flippy commands as the data have been written the oposite direction. Maybe that's the most definite way to determine if it has been written double sided or flippy mode.
This doesn't seem to be the case. When reading a C128 Double Sided disk in Flippy mode, the backside in my testing shows up as this:

Code: Select all

10.1[18]: CBM DOS: <unformatted>
In my testing, it seems if you try to dump a C128 Double Sided disk with the Flippy commands, the back side of the C128 disk will come up as unformatted. On the flip side (no pun intended ;) ), if you try to dump a C64 Flippy disk with the C128 dump commands, the backside of the Flippy disk will come up unformatted as well.

Because of this, I don't know how else to determine before 'trying' both ways to dump a disk (other then looking at the Byte 3 of the BAM) to determine if a disk is a C64 flippy disk or a C128 dual side disk? :(

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