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Re: Create Amiga diskettes on Mac

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Am 11.07.2014 13:53, schrieb Mikael Fors:

>> I've read in a Swedish usenet groups (se.dator.sys.amiga,
>> swnet.sys.amiga) that an old Mac could write the first track in
>> Amiga-format. He didn't remember what program he used on the Mac though
>> but had used it himself to transfer small files.
>
> You can't use a Apple 800k drive to make Amiga disks. Amiga uses a completely different type of approach. The Apple 800k drive also uses variable speeds to make it even worse.
>
> Likewise, a PC uses hard index (rotation signal pulse) for sectoring.

A PC *may* use the sync pulse. Otherwise, PCs use MFM encoding very much 
like the Amiga does, except that the sector encoding is different, and 
the gap between sectors is in general larger.

>
> The Amiga uses a syncword ($4489) which is equal to MC680x0 ILLEGAL instruction.

$4489 is not an 68K illegal instruction (that would be 0x4afc). It is a 
sync word that cannot be created by any data using the MFM encoding. 
Actually, PC controllers use the same sync word.

> That's why Amiga drives unlike a PC, in SD (Single density) fits 880k vs a PC's 720k (raw data, excluding filesystem) and a HD (High density) fits 1.76MB vs a PC's 1.44MB.

Actually, the reason for that is that the PC has a larger gap between 
sectors. The Amiga encodes an entire track in one go, then writes it to 
the disk, using the blitter and the CPU. The PC has a dedicated 
controller which does the encoding and decoding, and can address 
sectors. The sector gap is larger to allow it to pick out an individual 
sector, something the Amiga does not do.

> There IS hardware differences between Amiga and PC floppy drives, but there exists hardware, as previously mentioned, that can be used to read and write disks to and from native Amiga, like CatWeasel

A lot of "mix-up" here. Yes, there are differences between Amiga and PC 
drives, but these are minor and irrelevant for the encoding. It's mostly 
related to how diskchange operates and how the motor is controlled. A 
selection of PC drives can be modified to fit into the Amiga just by 
picking up the necessary extra signals from the ICs.

The real difference is in the controller. The Amiga does not have a disk 
controller. Everything happens in software, with a little help from the 
custom chips: Positioning the drive head, encoding and decoding the MFM 
data is all done in software. The PC has a more advanced dedicated 
floppy disk controller, typically part of the southbridge, which is 
programmed by higher-level instructions like "read that sector from disk 
and place it in this memory buffer". The MFM encoding and the 
track-layout is defined by this chip.

Greetings,
	Thomas

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Re: Create Amiga diskettes on Mac Mikael Fors <swextal@gmail.com> - 2014-07-11 04:53 -0700
  Re: Create Amiga diskettes on Mac Thomas Richter <thor@math.tu-berlin.de> - 2014-07-14 14:43 +0200

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