4 ms·
I think this is a case where a picture is worth a thousand words. This excellent article "Visualizing Commodore 1541 Disk Contents" [1] by Michael Steil about
by retrac 3y ago
I think this is a case where a picture is worth a thousand words. This excellent article "Visualizing Commodore 1541 Disk Contents" [1] by Michael Steil about the Commodore 64 disk format, includes visualizations of the magnetic flux as stored on disk.
This bit is particularly relevant: https://www.pagetable.com/docs/visualize_1541/sector.png https://www.pagetable.com/docs/visualize_1541/sector.png See the solid bit at the end of the sector, just before the next header? You could squeeze a few more bytes in there, but if the drive motor is just slightly too fast, it'll overwrite the next sector. That's why there's a gap, tolerance for timing variation.
Most floppy drive technologies wrote blindly, guessing where they were on the disk based on timing estimates since the controller last saw a sector header. This is also why disks needed to be "formatted". Not just in the sense of writing the file system data structures, but writing out all the sector headers. This had to be done all at once with the same drive, due to those small timing variations.
[1] https://www.pagetable.com/?p=1070 https://www.pagetable.com/?p=1070
- examiga500 3y agoAmiga had floppy drives that could read/write 1.76MB on HD disks and 880K on SD floppys. I think this was possible because they could control the speed.
- basementcat 3y agoMac drives had finer control over the motor RPM. Amiga drives read/wrote a track at a time and had no sector gaps. https://porterolsen.wordpress.com/2016/06/15/accessing-mac-formatted-floppy-disks-without-the-kryoflux/ https://porterolsen.wordpress.com/2016/06/15/accessing-mac-f... https://c65gs.blogspot.com/2023/10/reading-amiga-disks-in-mega65.html?m=1#:~:text=The%20following%20are%20details%20about,No%20gaps%20written%20between%20sectors https://c65gs.blogspot.com/2023/10/reading-amiga-disks-in-me....
- rasz 3y agoThe only HD drive ever available for Amiga was sold with some 4000 units - a modded Chinon FZ357A spinning at half rpm because nobody at commodore knew how to update PLL circuit in Denise. 1.76MB capacity was reached by not using standard PC format. Microsoft itself was shipping software on ordinary PC floppies formatted for 1.68MB https://en.wikipedia.org/wiki/Distribution_Media_Format https://en.wikipedia.org/wiki/Distribution_Media_Format
- Cockbrand 3y agoMy A3000 has an HD floppy drive. I was surprised to find that out, as I hadn't been aware that Amigas with these drives existed.
- snvzz 3y agoNot Denise, Paula. Denise is the video chip. Paula handles interrupts, audio, the floppy data signal (control logic signals are in CIA) as well as the uart.
- bcrl 3y agoPaula being the only chip that had absolutely zero changes from the Amiga 1000 to the 4000. That fact alone is a sad example of Commodore's mismanagement of the platform. Well, except for the CIA which traces its lineage even further back to the C64.
- snvzz 3y agoTry Commodore The Inside Story (book) for more depressing knowledge. Management just didn't let their engineers/architects get things done.
- snvzz 3y agoThis was due to different track format. Specifically, AmigaOS's floppies write one sector after another, with a single gap in the entire track. Whereas the IBM PC format has a gap in each sector. This is because RAM was more expensive back then, thus holding an entire track in memory would have an associated cost.
- skissane 3y ago> Whereas the IBM PC format has a gap in each sector. This is because RAM was more expensive back then, thus holding an entire track in memory would have an associated cost. A major factor - I'd argue even more significant than cost of memory - was that the IBM PC used an off-the-shelf floppy disk controller chip (NEC uPDC765A), which was hardwired to support the industry-standard IBM floppy track format (which had evolved from IBM's 3740 mainframe data entry system, introduced in 1973), and didn't support the Amiga's custom track format. Whereas, the Amiga could do this because it didn't actually have a floppy disk controller-the functionality of controlling floppy disks was in part contained in their custom ASICs, and in part implemented in software by the CPU. Unlike the Amiga, the original IBM PC eschewed proprietary ASICs in favour of off-the-shelf chips, in order to minimise time-to-market.
- snvzz 3y agoTrue, they use these pre-existing standard controllers. But these standard controllers and the track format were indeed designed in that way due to the ram limitation. Ironically, it is possible to read arbitrary formats with the debug read track operation. Yet, it needs to find one triple sync word somewhere in the track; a controller limitation. Unfortunately, the Amiga standard track format didn't account for that, and uses the same sync word (the 4489 one) but double. It could have been designed to use a different sync word, and include a triple 4489 at the track start, but they didn't think about it at the time. Some tricks bit-banging the controller allow for writing arbitrary tracks. It is also possible to read arbitrary tracks, if there's two floppy drives and a standard ibm pc formatted disk is present in the other one, by switching the drive after the controller has started reading.
- JNRowe 3y agoThanks to the pluggable device layer, you could push that to 984kB or 1968kB with diskspare¹. The incredible thing, IMO, is that the implementation is only 5kB(seven times smaller than the formatted English docs). ¹ https://aminet.net/package/disk/misc/Diskspr3 https://aminet.net/package/disk/misc/Diskspr3
- brk 3y agoI recall it was also not unheard of to have floppy drives that could be incompatible with each other. A drive that was a tad slow might format a disk that would work for itself, and other drive, but that disk might not work in a drive that was a tad too fast (and vice-versa). This wasn't common, just frequently enough to occur so occasionally as to always be baffling, particularly in an office with lots of PCs. Then there were things like Spiradisc (https://en.wikipedia.org/wiki/Spiradisc https://en.wikipedia.org/wiki/Spiradisc), which created incompatibilities by design.
- jacquesm 3y agoThis happened with tape drives too. Head alignment or track alignment a little bit off and you'd start to lose the high pretty quickly.
- shard 3y agoI remember having to realign my Commodore 1541 floppy drive, which involved a lot of banging sounds if I recall.
- 15457345234 3y agoAt high school (in the 1.44mb / w95 era) I encountered a floppy disk that had literally two different filesystems on it depending on what machine you read it in. Not entirely sure how that worked. One machine showed one filesystem, the rest showed another. I guess the alignment on that one particular drive must have been ridiculously far off baseline.
- ggeorgovassilis 3y agoI remember that, too. Got such a floppy from a computer magazine. I think one was for the PC and then other for Amiga or Atari.
- sonicanatidae 3y agoI have a love/hate relationship with the 1541s. Just the absolute worst drives ever released to the public, but it was that or tapes and while most will agree that the 1541 was a slow FDD, waiting 30+ mins to load a tape (don't forget to flip it) was much worse. Then copy protection companies decided that the very best way to CP software was to create unreadable/unwritable sectors and let the drive slam the head against the arrester trying to read it, because they didn't have to replace the drive. That was a you problem.