Re: 3.5" interleave

[mjmahon@aol.com (Michael J. Mahon)]

Joel replied:

>There is interleave on Apple 3.5 disks.
>4:1 was standard for 800k disks on the Unidisk.
>2:1 was standard for 800k disks on the IIgs and Apple 3.5 drive.
>IIgs formatter recommends 2:1 for 1.44M disks  for optimum speed.
>But a 1:1 option is given, which I believe is the standard for the IBM.
>
>"Dosius" <steve@dosius.zzn.com> wrote in message
>9307085f.0209240525.1aec7d4f@posting.google.com">news:9307085f.0209240525.1aec7d4f@posting.google.com...
>> In my emulator Dapple 0.8, I have implemented a driver to read/write
>> 1.44MB disks.
>>
>> The disk format follows this pattern, Head 0 Track 0, Head 1 Track 0,
>> Head 0 Track 1, Head 1 Track 1, Head 0 Track 2, ..., Head 1 Track 80,
>> with interleaving dealt with by the PC BIOS (I think there is none).
>>
>> How is the layout of a real ProDOS disk, created on a Mac or a Mark
>> ][?

Interleave should not be applied to disk images--sectors should be placed
in logical order without reference to physical order.

I think the question is, what is the mapping of physical order to logical
order for a 1.44MB ProDOS disk?  Of course, this will be handled
transparently if the image-creating program runs on ProDOS, but
becomes an issue if the image-creating program runs on another
OS/hardware base.

The mapping you suggest sounds reasonable.  A simple examination
of a ProDOS 1.44MB disk with a low-level block utility on a PC or Mac
should verify this assumption.

-michael

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