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by thought_alarm 655 days ago
It's all just an evolution of their existing disk drive technology that started well before the last-minute decision to go with the Sony 3.5" drive.

The variable drive speed comes of the development of the "Twiggy" drive, which was an 850 kB 5.25 disk format originally intended for the Apple III in 1980 but never worked reliably.

BTW, the Atari ST uses the same floppy disk format as the IBM PC, 360 kB per side.

The Amiga uses a variable drive speed like the Mac, but they eke out extra capacity by eliminating sectors. This allows an extra 512 bytes per track, but the trade off is that the disk controller can only read or write an entire track at a time, rather than individual sectors.

An infamous Apple II copy protection scheme used the same trick to expand 5.25 disk capacity from 16 sectors to 18 sectors (512 bytes per track).
4 comments
rasz 655 days ago
>Amiga uses a variable drive speed

Amiga uses standard PC drives with slight tweaked pinout https://linuxjedi.co.uk/2020/12/05/converting-a-pc-floppy-dr...

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s800 655 days ago
The Amiga is fixed RPM or CAV, not CLV like the og Mac. With one exception- later models could halve the RPM to read/write HD floppies (1.44MB PC or 1.76MB Amiga).
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tom_ 655 days ago
360 KB/side was indeed the default for the Atari ST, but there were numerous tools (I think Fastcopy III was the one I usually used) to format with more sectors per track, and 10 sectors/track (so 400 KB/side) was the standard recommendation if you just wanted more data per disk and no hassle. More than 80 tracks was also an option, and 81 or 82 tracks was apparently also reliable. That never sat right with me though, so I didn't do it.

(18 sectors per track with 256 byte sectors is also possible with the 1770 series. This was one of the disk format options on the BBC Micro. Definitely not written a track at a time! There just wasn't the memory for that.)

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ForOldHack 655 days ago
You only read the data sheets. The twiggy drives had 4 heads to cut latency and had two access windows one in the back like all 5.25 and one in the front for the extra heads. The Lisa used this too.
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