Re: Apple2 disk controller questions

[supertimer@aol.com (Supertimer)]

"Fabrice Frances" <frances@ensica.NOSPAM.fr> wrote:

>Supertimer a �crit dans le message:
>
>>Actually, Apple II disks are GCR encoded.  The 5.25" ones
>>are single sided, double density.  That's why they store only
>>144k, a little less than half of a double sided, double density
>>DOS FAT 5.25" disk (320k).
>
>right, 160 or 180 KB for a single sided 5"1/4 DOS disk (resp.
>eight and nine 512-bytes sectors per track).
>
>> Apple II double sided, double
>>density 3.5" disks are also GCR (800k).  Apple II double
>>sided, double density disks are MFM (1.44MB).
>>
>>GCR is no less efficient than MFM, unlike FM which is less
>>efficient.  In fact, Apple II and Macs squish 800k into a disk
>>that Win/DOS PCs format to 720k!
>
>there's still something confusing me...
>It seems 343 GCR-encoded bytes are transfered through the
>disk controller for a single 256-bytes sector... I would say
>GCR efficiency lies between FM (512) and MFM (256),
>am I wrong ?

Come to think about it, you are right.  It must be the variable
spin drives that give Apple 3.5" disks the extra 80k, but GCR
is probably a little less efficient.

What I don't understand is the high density 3.5" Apple II
format.  Like the Mac, high density 3.5" Apple II disks are
1.44MB MFM.  However, there is a special 1.6MB GCR
format that is used by AE's driver for the IIGS.  I'm not
sure how the AE driver squishes those extra few kilobytes
into the disk.

>Oh, just an idea... I know that Macs use CLV and not CAV
>with double density 3"1/2 disks, is it the same for Apple II
>5"1/4 drives ?

Not for 5.25" drives, but yes for 3.5" drives.  The Apple II
uses the same 3.5" drive mechanisms and encoding as the
Mac.

>This would mean more sectors are stored in the outer tracks
>than the inner tracks, is it the case ?

Oui.