The C64 FDC simply used a different format using DIFFERENT HARDWARE which made them incomptible. Yes, the Commodore had a drive that could read and write both formats, because BOTH FDCs were incorporated, and the hardware was flexible sufficently to write both formats. NO BURNING, idiot! Older PC FDCs had a command where one could write deleted sectors, but at some stage in "improvement" of ASICS (part of which the FDC is emulated) that command got dropped (early in 586 production, so use an original PC or 286, 386 or 486). BTW, without the "deleted sector" command, the "newer" PCs can neither read nor write them.
It seems it involves modifications... which would ofcourse be way too risky to do...
Maybe there is another way to read/write c64 disks on pc or vice versa without hardware modifications ?
Otherwise I don't understand the initial excitement about his discovery... maybe he tought it might work... he doesn't seem sure yet... (the dude from the original link in first post ;))
Me trying to make sense of this all... does this sector layout solve anything ?
No, there is not; the controller is too stupid for that. There are alternative controllers that aren't as braindead, but that's not 'without hardware modifications'.
But what made you think this was the case here? Why would a C64 need to read a PC floppy, or vice versa? The beauty of this trick is that it's not required at all; a PC will 'see' a floppy with a whole lot of bad tracks which it will just ignore, and the C64 will do just the same.
It's a very clever trick, and elegant in a strange way. But I don't think you'll be able to understand that.
It's not about sectors, dimwit, it's about tracks.
A C64 disk and a 360K PC disk are both 40 tracks, but that's where the similarity ends. The whole reason this hack can exist is that a PC stores volume information on track 0, including the file allocation table and directory entries and what not, whereas the C64 does this on track 18. What happens is that the PC will read it's info from track 0, and will see that several tracks, including track 18, are 'bad', and it will just avoid them, not realising that they really contain a C64 filesystem. The C64, on the other hand, will start reading info from track 18, and will notice that track 0 (which is just a 'normal' data track as far as a C64 is concerned) is not available for storage, and will happily avoid reading it.
So there you have it: A disk which seems readable on both a C64 as well as a PC. Of course, this will only work as long as both sides don't try to access tracks 'of the other side', which would typically happen when you try to copy a disk.
The comments clearly describes how it works, but I think you're just too stoned to recognise that. Lay down those drugs.
(To alt.c64: where I write "C64 disk" and "C64", I obviously mean "1541 disk" and "1541" respectively, but I didn't want to complicate matters. Skybuck won't be able to understand it as it is)
The 1541 might have been the version that had both controllers. It has been tooo many years and i never had a Commodore computer, only supported data transfer from one to a PC.
Apple, especially in the earlier daze, used spiral tracks which would be impossible to read with modern hardware/software. I am guessing that Commodore used spiral tracks as well.
I can understand his excitement. Those kind of format is hard to make. One would need to "hand-craft" it (track by track and sector by sector) prior making a specialized software for it.
I don't know the details about C64 drive, but PC drive is very much capable of producing any kind of formats as long as they support the track density of the floppies which I suspect the same for C64 drive.
It's simply because floppy drives can be controlled almost in every aspect which allows one to restructure the data on each track - making it possible to do real low level formatting and restructuring the sector layout and other finer details. It's not even close when compared with so-called harddisk drive low level format because it has far more greater control of the floppy drive hardware. CD-ROM writer drives (for writing only) has a little more control than harddisk drives.
Some parameters in track & sector structures are:
- Track & sector gaps
- Track & Sector synchronizations
- Sector IDs
- Sector Addresses (CHRNs)
- Sector CRC
Some sector IDs:
- Hidden Sector
- Unknown Sector
- Deleted Data Address Mark
- Bad CRC
- No Data Address Mark
- Long Gap
- Duplicate CHRN Record
- Short Data
- LA (unknown to me)
The above parameters are not just for low level formatting, but also for general reading and writing. MS-DOS keeps a number of supported IBM-PC disk formats and use it each time a new floppy is accessed.
I once tried to read my Atari 800XL single-sided, single-density (35 tracks/inch) 5.25 inch floppies (mine is equipped with an external floppy drive) just out of curiosity and the drive really can read it. The hard part at first was determining the Atari 800XL disk format which I'm completely blind at that time. But thanks to a utility called Rescue (from the same maker of Trans Copy), I can find out about it and make a little utility to make a raw disk to image file copier for the Atari floppies so that I can inspect the sector data using a hex viewer. Of course, it just read and copy them sector by sector without knowing what file-system it uses.
These information can be found in the IBM PC/XT Technical Reference book. It's complete details of IBM PC/XT hardware including IBM PS/2 (in later revision). You'll be very lucky to get one of these since it's very old and hard to find. Other very useful source is the Ralf Brown Interrupt List which also contains a reference for hardware ports (plus other references). In this case, it's the floppy disk controller ports and the BIOS disk interrupts.
I don't remember about the tracks... I don't think they were spirals, but Commodore used variable motor speed so that the linear bit density (rate) was constant. I know.... I designed the motor-speed controller, back when Commodore had a design center in Mesa, AZ.
...Jim Thompson
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| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
On the websites in the comments section some write that C64 had variable track or sector lengths or something like that.
If true then pc floppy drives would need to be able to do that to be able to make such a floppy which some say the pc floppy drives/controllers cannot do...
Otherwise it can only be done in partial steps to get the full floppy:
The PC makes the PC section on the floppy. The C64 makes the C64 section on the floppy.
So then it would require at least two computers to make the floppy ! ;)
No, they didn't. They used more sectors per track as the tracks got longer. The 1541 used the same ALPS or Newtronics 180 kB single sided drives that were used in early IBM clones. The 1571 used the same ALPS or Newtronics 360 kB double sided drives that were used in later IBM clones. Basically, they bought the drive mechanism without OEM electronics. Both ran at a fixed 300 RPM. I repaired hundreds of the drives when I was involved in Orlando's 'Central Florida Commodore User's Group.'
formatting link
has an archive of the schematics of a lot of the 8 bit Commodore products.
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The more sectors per track would increase the data rate if the RPM's were keep constant. This could cause timing problems. Varying the speed would allow this to occur and not effect the signals to the controller or other timings.
I know I designed a variable speed motor controller for Commodore, as well as a 5:1 tuning range PLL. But that group dissolved into the Bay Area and I never kept track of what they were doing.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
That may be, but the Commodore drives were fixed speed at 300 RPM. They changed the divider from the system clock for different groups of tracks.
formatting link
is a schematic of the drive mechanism. It only has a motor on/off control line, as you can plainly see.
formatting link
and
formatting link
are the full schematics of the drive's electronics. Show me where it changes the speed, track to track. Look at UC2, & UE6 in the upper right side of the first half of the schematic.
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Consultants, in particular, are always kept in the dark. MANY of my projects I have no idea what the end product does. In the few times I am able to see the whole enchilada I am often able to make gross system simplifications.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
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