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Old 18 December 2005, 00:57   #17
Not a Rebel anymore
Phantasm's Avatar
Join Date: Apr 2005
Location: UK
Age: 46
Posts: 274
Mac floppies use GCR but not alot else as far as I know.

They also use constant angular velocity drives instead of constant linear velocity (or is it the other way around - i forget).

PC and Atari ST used MFM encoding but used the index flag rather than an mfm sync value unless im mistaken.

I've never actually tried to read / write a gcr track, but if memory serves there is a flag somewhere in one of the registers that selects between MFM and gcr. I've never actually seen it explained anywhere how the affects the actual bits written to the disk. I cant really see why you would need to set any flags if GCR is just a different way of encoding the bits compared to MFM but you do, so there must be more to it than that.

Those multi format disks were typically amiga formatted tracks on one side and st/pc formatted on the other. Fortunately side 0 was the opposite side on both machines, so they both booted up nicely from their respective boot blocks.
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