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PC Historian Finds Puzzling Game Diskette Image
This past weekend, Trixter — a self-proclaimed IBM PC historian — picked up some old software for his archive. What he didn't count on was a couple of additional Avantage titles that had never been released into the wild. If this weren't enough of a find, one of these titles provided Trixter with an interesting puzzle: the diskette for Mental Blocks is apparently hand-formatted to work on both C64 and IBM (on a single side, not the "flippy disks" of old). Quite an interesting little piece of history.
(wow...my first slashdot post in like 5+ years...something I actually can know stuff about! LOL)
I wanted to email Trixter this but couldn't find a contact email.
It's been now about 25 years but I still have parts of the C64 ROM's memorized. There was a time that I knew pretty much what every byte in the 64k(*) of memory was for cold without needing a reference manual. Having said that:
This wouldn't have been all that hard to do by somebody who had intimiate knowledge of *both* IBM and C64 formats I'd imagine. First, I doubt it was done 'by hand' as in a manual sector by sector copy. A program would have been written, using a slave-master 2 drive config, to stream from the source drive to the dest. drive using a list ot sectors/tracks and/or using a simple formula to calc where the tracks should go. You simply would pick areas on the C64 side that you would want reserved for the IBM side and vica versa. Knowing both IBM and C64 MFM structures would allow you to pick "safe" areas for both formats.
Oh, and the directory structure of the C64 did indeed live on track 18. All the other data blocks where chained out as a linked list from the entry in this track.
All that would have been really needed is:
#1) Format the disk for IBM and use whatever areas you need via a streamed block by block copy from Src to Dst.
#2) Noting which tracks are "safe" to use on the C64, simply write a program to format track by track and write the C64 data, streaming again.
Ingenious, but really not that hard at all...
(*) Well, more like ~80k with the shadow RAM near the top of the 64k range...
Ted
The short-lived, dual-format ST/Amiga Format magazine from the late 1980s also had an appropriately dual-format cover-disk - somehow combining the apparently wildly-incompatible ST and Amiga floppy disk formats.
I've no idea how it was done (although the fact that many STs had single-sided floppy drives may have had something to do with it) - and while it could have been extremely useful to publish games in such a manner at the time, I don't know that was ever done either.
I get the impression that there was a lot of deep magic involved in these enhanced disk formats, copy protection systems and so on. I'm sure the name Rob Northen appeared on the front of a later ST Format cover disk - as the supplier of the fancy files-limited-to-particular-sides-of-disk format used to not deprive single-sided drive owners the contents of the entire double-sided disk...
Tedious Bloggy Stuff - hooray?
This is a cool hack. From what it looks like, this is possible because DOS put the boot sector and the root directory in the beginning of the disk, whereas the C64 made the sane choice of putting it in the middle (think about it, this minimizes seek times). Now the directory (or, more precisely, the File Allocation Table (=FAT)) contains information on so-called bad blocks, i.e. blocks that the OS shouldn't write to because they were known to be bad. If you label the blocks that you put the C64 data into as bad blocks, then DOS is not going to overwrite the C64 data. Now you do the same in the C64 FS and bang -- double OS format created. And it's read/write!
I wonder if someone managed to format a disk such that one was also able to share the data space between the different OSs?
It's also shockingly cool because my understanding of C64 vs. IBM formatting indicates that the read/write method is entirely different between the two, making it physically impossible for one machine to run emulation to extract info from a drive of the other.
The trick is that, if you limit each OS to half of the disk, you can do this. Each OS only uses its half and doesn't try to read or understand the other's.
IBM-standard floppies put the master directory information on the first tracks on the disk. Commodore floppies put this information on track 18 of 35, halfway in. (Fun note: you could actually run out of directory space if you put a bunch of small files on the disk and filled up track 18. There were utilities that would extend the directory links to track 19 in this case.)
So tracks 1-17 were the IBM part, and 18-35 were the C-64 part. No shared data. I think Commodore floppies only stored 110 K of data.