Disk Copy 4.2… ah yes, the venerable Macintosh floppy disk copying software. What if you can’t run it since you are operating on a different operating system, but you want to get at the data contained? Or what if you are writing your own disk copy software and you also want to be able to write out data in that file format? No problem, I’ve found some pretty great references on the Disk Copy 4.2 file format right here.
20190926/https://en.wikipedia.org/wiki/Disk_Copy
20190926/https://wiki.68kmla.org/DiskCopy_4.2_format_specification
Also, good news, for I had not been fully aware of this, but Mini vMac also supports direct emulation on the Disk Copy 4.2 file format. The main benefit of this, beyond convenience, is that the Disk Copy 4.2 file format has provisions for storing file tags, so you get a more accurate emulation of Macintosh floppy disks in that respect.
The Disk Copy 4.2 format specification is also informative about the functioning and operation of the underlying hardware. In particular, Apple 400K/800K floppy disks have a GCR format nybble in the header of every sector. This indicates information about the particular sector formatting, which can vary from one sector to the next. The most important information encoded in the GCR format nybble is whether or not this particular sector has file tags. In Apple ProDOS formatted sectors, there will not be file tags available, but in Macintosh formatted sectors, there will be file tags available. Please note, however, that the GCR format nybble is primarily specified to indicate whether a disk has 2:1 interleave (Macintosh) or 4:1 interleave (ProDOS).
Nevertheless, I do mention this because it is consistent with my experimental observations. When I use Apple II Disk Transfer ProDOS to copy a Macintosh disk image, then read out the Macintosh disk image on a Macintosh, I notice that all the file tag data is undefined/invalid. But, after I write one Macintosh file to that disk from the Macintosh computer, then I read the disk image from the Macintosh again, I notice that there are some valid file tags on the disk, but only for the newly written data. Everything else still has invalid file tags.
Please see my newer blog article on the same subject for important additional info regarding Apple’s official deprecation of file tags in newer Macintosh systems.
Regarding file tags, the key to remember is that a disk must be specially formatted so that it can store more than 512 bytes per sector. In the case of Apple 400K/800K floppy disks, this is easy since the custom IWM chip (Integrated Woz Machine) is used for the floppy disk controller (FDC). But, PC standard MFM 720 KB or 1.44 MB floppy disks do not have the space to spare for more than 512 bytes per sector, so by definition, Macintosh filesystems on such disks will never feature file tags.
The general understanding is that with the earliest of Macintosh technology, the means to the end of implementing file tags support was relatively easy, but as newer Macintosh computers increasingly needed to use more mass market solutions shared in common with the PC world, the ability to maintain the file tags system became impractical, so Apple deprecated them altogether.
You can easily see this in the evolution of Apple’s hard disk drives. Apple’s early Hard Disk 20SC uses 532-byte sectors, reserving 20 bytes for system use. Out of these 20 bytes, 12 bytes are certainly available for use to store the file tags. It’s not until you get to the next generation, that of the Internal Hard Disk 20SC and Internal Hard Disk 40SC, where things get modernized. Zone bit recording and 512-byte sectors, no longer any space for the file tags, and the later Monster Disk Driver Technote officially marks them as obsolete. When you read blocks on these devices, the disk driver simply doesn’t write to the file tags buffers, so whatever stale or garbage data was in there previously is left unchanged as you read blocks.
Does the original Hard Disk 20 that connected through the floppy disk connector have file tags? I would guess it does due to the use of a custom controller, and chances are that the disk technology of it is rather similar to the Hard Disk 20SC. Well, the “original” Hard Disk 20SC, that is. Maybe there was more than one version of the Hard Disk 20SC, and the newer versions did not feature 532-byte sectors with room for file tags.
20SC Hard Drive Info
- 532 bytes per sector (20 used by system)
- 32 sectors per track
- 305 cylinders
- 4 heads (technically 4 surfaces and 1 head per surface)
20,769,280 formatted byte capacity
Internal Hard Disk 40SC Specification
Capacity
- Data capacity: 40MB (formatted)
- Data surfaces: 3 and 4
- Heads/Surface: 1
- Block size: 512 bytes
- Total disk blocks: 82,020
- Blocks/Track: 28, 33, and 35
Note: Drive uses zone recording, so the usual sector size/sectors/ cylinders/heads volume specification will not work.
2010 era webpages:
20190924/https://web.archive.org/web/20110516051041/http://docs.info.apple.com/article.html?artnum=1931
broken/http://docs.info.apple.com/article.html?artnum=6976&coll=ap
Unfortunately, the Internal Hard Disk 40SC Specification page was lost to time, never captured by the Wayback Machine. Fortunately, I took some steps as a forward thinker to copy out the pertinent information to my simple text-based notes, although I should have also thought in advance to save this page to the Internet Archive Wayback Machine.
AND, unfortunately, even the Wayback Machine’s archive of the page does not match the original text that copied off of an earlier version of the page. It really doesn’t look anything like the original page that I remember seeing.