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Writing Mac 800kB disks

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hjalfi

Experienced Member
Joined
Feb 11, 2017
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Zürich, Switzerland
People might be interested to know that I've just merged in successful write support for those horrible 800kB disks to FluxEngine, my USB FDD interface project (tl;dr: it's essentially a very cheap and open source DIY Kryoflux or SuperCardPlus replacement). I can download a System 6 DiskCopy 4.2 image from MacintoshGarden, write it to a floppy, and then boot from it.

If anyone's got one of the 800kB-only machines like a Mac Plus, I'd be really interested to know if it works there. I have very limited hardware for testing (one beaten-up PowerBook 150).

http://cowlark.com/fluxengine/
 
hjalfi said:
People might be interested to know that I've just merged in successful write support for those horrible 800kB disks to FluxEngine, my USB FDD interface project (tl;dr: it's essentially a very cheap and open source DIY Kryoflux or SuperCardPlus replacement). I can download a System 6 DiskCopy 4.2 image from MacintoshGarden, write it to a floppy, and then boot from it.

If anyone's got one of the 800kB-only machines like a Mac Plus, I'd be really interested to know if it works there. I have very limited hardware for testing (one beaten-up PowerBook 150).

http://cowlark.com/fluxengine/
Click to expand...

Are you asking for someone to volunteer receiving a floppy disk from you and testing it on their machine to ensure it works ?
 
hjalfi said:
I can download a System 6 DiskCopy 4.2 image from MacintoshGarden, write it to a floppy, and then boot from it.
Click to expand...
Just for the record: I can write 800k DiskCopy images to a 1.44MB disk using WinImage and a USB floppy drive and my Mac Classic will boot from them, too. Doesn't say much about whether or not a Mac with a real 800k drive can boot from them. Don't own one for testing, sadly.
 
Yeah, you can put a 800kB filesystem image on a 1440kB MFM disk and it'll work just fine, if you have a Mac which can read them. Of course, if you do that, then you don't get to hear the lovely tunes as the machine boots and the CLV drive spins up and down.
 
Timo W. said:
Just for the record: I can write 800k DiskCopy images to a 1.44MB disk using WinImage and a USB floppy drive and my Mac Classic will boot from them, too. Doesn't say much about whether or not a Mac with a real 800k drive can boot from them.
Click to expand...
I wouldn't count on it. You did the floppy equivalent of partitioning a hard disk, and leaving some of the total capacity unused.
 
I wasn't aware WinImage would do that, but either way that is the same as converting to a 1.44mb MFM formatted disk, even if it does not resize to add the extra space. Unless I am mistaken, That trick would only work with HFS volumes, not MFS disks.

So the resulting disks written by Winimage/USB drives will NOT be readable on any Macintosh that has only a 400k/800k floppy drive.

Real 400k/800k Macintosh disks are very tricky because they use both CGR encoding and variable bitrates on different tracks.

IBM PC style floppy controllers can not read/write this format at all. Flux-level devices that read/write this format using a PC style floppy drive must change the bitrate going to/from the drive since IBM PC style drives always spin at the same speed. (On a Macintosh, the bitrate at the controller is always the same, but the disk spins at different speeds)

Note that not all standard PC 1.44mb floppy drives can deal with GCR/variable bit rates. Electronic filters designed to remove random noise from MFM signals will treat GCR/VBR as "noise" and filter it out. I've found that typically Teac and Toshiba 1.44mb drives are usually the best.
 
Most Mac Performas can handle both the 800K (some, the 400K) GCR floppies, as well as the 1.44M MFM floppies. Not so the early 68K Macs.

An IBM PC with a Deluxe Option Board installed can read and write the GCR floppies and comes with utilities to understand the file system.
 
So the mac classics and the following similar models
with 1.44MB drives weren't backwards compatible with the 800k disks?
 
DFinnigan said:
I wouldn't count on it. You did the floppy equivalent of partitioning a hard disk, and leaving some of the total capacity unused.
Click to expand...
I'm not counting on it. I was just adding to hjalfi's post that it does not take much to write a 800k image back to disk in a way a Mac with a SuperDrive can read.
 
maxtherabbit said:
So the mac classics and the following similar models
with 1.44MB drives weren't backwards compatible with the 800k disks?
Click to expand...


"superdrives" are backwards compatible. you can write 400 and 800k floppies at least up through the powerbook G3's
I do that all the time, and you can hear the motor changing speed to do it.

drives are physically not backwards compatible. i forget if the change happened with the cost reduction, but at some point
the physical interface changed and putting a newer drive with an older machine will cause it to constantly try to eject

the earlier big change was the original 400k drives require a pwm signal from the mac to set motor speed. that was
eliminated on the 800k

i have some documentation on the drives under http://bitsavers.org/pdf/apple/disk/sony

superdrive retrofit kits were available for some machines to give them mfm support, which involved replacing the IWM with a SWIM and
changing out the drives

getting gcr read working reliably on fixed rpm drives is going to be difficult. the read channel filtering needs to be capable of a much wider
variation in pulse rates than normal. you are also going to need to deal with write precompensation on the inner tracks even more than
normal because of the lack of variable rotational speed
 
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My recollection (however faulty) is that the DOB (I'd have to check the docs with mine) came with a couple of extra pages that detailed the 1.44M drives that have been found to be compatible with the aforementioned DOB and 800/400K disks.
 
A 1.44mb floppy splits the recorded area into two sectors and writes the data to only half of the sector, and more data to the other half.

A 800kb disk media is physically different than a 1.44mb disk media. If you record an 800kb disk onto a 1.44mb disk on an 800kb drive, it might work just fine on a 1.44mb drive as a 800kb disk, unless that same 1.44mb disk had prior data on it.

Taking a 1.44mb disk and formatting it on an 800kb disk drive results in half of the data staying behind on the disk. The 800kb drive doesn’t even see it. But a 1.44mb drive does.

A 1.44mb SuperDrive writes data to 800kb disks differently than any other 1.44mb PC or USB disk drive. It has backwards compatibility built in. The 800kb disks and drives are and never were compatible with IBM PCs. Because the drive and media are not cross compatible.

Now that we have all of that, it is not possible to write a 800kb disk with a non-Apple SuperDrive or Apple 800kb disk drive. All other drives do not have the proper hardware to write it.

This is partly the reason why an 800kb disk drive could never read any PC formatted disks. The 720kb disk is not compatible at all.
 
Chuck(G) said:
I guess I should dump my Deluxe Option Boards (see PDF page 40 et seq.) and my Catweasel, then.

I wonder what I've been doing all these years... :)
Click to expand...

I’m guessing this board then simulates or integrates some functions of the WIM chip as well as allowing for the variable density recording that 400/800 kB disks require?

I had no idea such boards existed. However, without said board, would not my statement of not being able to write a Mac 400/800kb disk on a standard PC disk drive still be valid?
 
I have a boxed DOB. It has a leaflet noting what others have said here.... that TEAC 3.5" drives are preferred for working with 800k GCR floppies. The bigger problem I had is the tools weren't all that aware of 800k ProDOS formatted disks as used on the Apple IIgs which were commonly stored in Shrink-It SDK archives.

I keep a bridge Mac around just for writing 400/800k disks. It gets much less use these days because SD/USB card storage on the Apple II is a thing now (CFFA3000 and FloppyEMU among other tools). Even if you are stuck with just floppy drives, bootstrapping a boot disk-less Apple II is WAY easier these days with ADTPro. Heck you can even netboot a IIgs if you need to. Pre-Superdrive Macintosh is an annoying platform since one can't kick start the machine at all without something like the FloppyEMU or an existing boot disk.
 
Mr Fahrenheit said:
I had no idea such boards existed. However, without said board, would not my statement of not being able to write a Mac 400/800kb disk on a standard PC disk drive still be valid?
Click to expand...

Yes, but the issue is the controller, not the drive... at least with a couple asterisks:

Asterisk the First: Not that it makes a material difference since the building the hardware to do it isn't a "problem", but it's mildly interesting trivia that a controller that can read/write the Mac disks in a standard drive actually has to work in a fundamentally different way than the Mac's controller; the Apple drive/controller combination uses a fixed data rate but changes the rotation of the speed of the drive to do a varying number of sectors per track. To replicate this with a fixed-speed drive you need a variable speed *controller*.

Asterisk the Second: Some people using Kryoflux-style controllers claim to have had problems reading Mac disks in at least *some* PC drives. The theory that keeps getting thrown out is the possibility that some drives have "filters" in them that get confused by the varying data rate. No comment as to whether this is actually a thing that applies to *any* controller meant to read Mac disks in PC drives (DOB, etc) or some kind of phantom problem the Kryoflux people keep running into that's actually due to media problems or something.
 
Mr Fahrenheit said:
I’m guessing this board then simulates or integrates some functions of the WIM chip as well as allowing for the variable density recording that 400/800 kB disks require?

I had no idea such boards existed. However, without said board, would not my statement of not being able to write a Mac 400/800kb disk on a standard PC disk drive still be valid?
Click to expand...

I would have stated that "the NEC µPD FDC controller (and its derivatives) cannot write or read non-FM or non-MFM floppies". The DOB interposes in the normal FDC to floppy path, like other of the Central Point option board family. It's been around since at least the early 1990s and its assumed function was handling copy-protected floppies; that the DOB could handle Mac, Apple II, Amiga and C64 floppies was a happy side benefit. I still occasionally use one when I want quick verification of a mystery disk. But the DOB is not the only PC board that can handle Mac GCR disks; when you have a sampling board such as the DOB or Catweasel, it's mostly a matter of software. But it isn't just Mac GCR floppies that the NEC 765 doesn't handle--it's basically unable to handle anything that doesn't conform to IBM 3740 or System/3 floppy encoding--not even M²FM as used on several early systems, such as Intel MDS 800.

Central Point abandoned the Option Boards after the acquisition by Symantec, ostensibly not wanting to be association with the software piracy trend condemned by the SPA.

After all, it's all flux reversals in the end... :)
 
I've definitely written 400K and 800K mac floppies with a kryoflux and a standard PC drive. Sometimes they are a little flakey, but usually I can get something to work.

There is a tool HERE a file called macintosh.zip that someone wrote that will turn diskcopy 4.2 images into kryoflux raw files.
 
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