Hi all,
Excellent explanation, Peter!
Please let me add on more bit of information: since many concepts from Apple's MacOS came from Apple II's ProDOS, I'd recommend reading chapter 3 of the "Beneath Apple ProDOS" book, available at:
ftp://ftp.a2central.com/pub/documents/beneathprodos.pdf
If you take the time to read Chapter 3, you'll get a very good explanation of concepts like "soft sectoring" and physical formatting. Please consider that this book was written on a time where the common Apple format was the 140KB 5,25" floppy diskettes, but ProDOS itself was the first Apple OS to support the 400KB and 800KB 3,5" floppies.
Although this is not Mac specific, I believe it's not too much off-topic because it applies to the concepts discussed herein. There is probably some good resources specific for the MacOS, also, which I'm not aware of.
Regards,
Celso.
Peter da Silva escreveu:
It's because of sector formatting.
Here's a section of track, full of 1s and 0s:
-------------------------------------------------------------------- 10101010101010101010101010101010101010101010101010101010101010101010 --------------------------------------------------------------------
Here's the end of a sector and the beginning of the next:
-------------------------------------------------------------------- 0101010101010XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX11101010110 --------------------------------------------------------------------
First, part of that data is the sector header, which uses up a bit of space. Second, part of that data is not data...
Those "X"es are unused "potential bits". Why? because the next time you write a sector the timing may be slightly different and it may end up looking like this:
-------------------------------------------------------------------- 0000000000000000101010100101010101010XXXXXXXXXXXXXXXXXXXX11101010110 --------------------------------------------------------------------
If the previous sector had started right after it, you'd have just overwritten the sector header and destroyed the sector.
On the Amiga, they wrote the entire track in one pass, so you only had to put the inter-sector gap at the end of the track. That gave you 1760K on a 2Mo flippy. You could also drop most of the sector headers by modifying AmigaDOS and get 1920K in a 2Mo floppy, so long as you didn't care that you could only read it on a similarly modified AmigaDOS.
-- Celso Kopp Webber [EMAIL PROTECTED]
(41) 7811-3858 (Nextel CD 55*47*428) (41) 9121-5188 (41) 284-3035
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