Newsgroups: comp.sys.amiga.programmer
Path: utzoo!utgpu!watserv1!watdragon!rose!ccplumb
From: ccplumb@rose.uwaterloo.ca (Colin Plumb)
Subject: Re: FFS
Message-ID: <1991Apr5.101335.11607@watdragon.waterloo.edu>
Sender: news@watdragon.waterloo.edu (News Owner)
Organization: University of Waterloo
References: <1991Mar26.211632.24246@watdragon.waterloo.edu> <1991Apr1.220945.24885@unislc.uucp>
Date: Fri, 5 Apr 1991 10:13:35 GMT
Lines: 36

dave@unislc.uucp (Dave Martin) wrote:
>From article <1991Mar26.211632.24246@watdragon.waterloo.edu>, by ccplumb@rose.uwaterloo.ca (Colin Plumb):
>>
>> The FFS doesn't enforce any track size, blocks per track, or whatever.
>> It just wants blocks numbered x through y, for some x>0.  As of 1.3, it
>
>                                                      y-x > 0 ?

No, actually.  The file system uses "0" as a null vlaue, so it can't
use block number 0.  Typically, the first two blocks of a partition
are left as dummy "boot blocks", so x=2.

>> In any case, if the last usable block is y, then both the old and new
>> file systems use block (y+1)/2 to store the root block, and usually
>> allocate bitmap blocks immediately thereafter, but they can go anywhere.
>
>  I assume that you mean (x+y+1)/2 ?   I would hate to think that if I have
>  a partition from block # 4000 to 5000, that the root block is 2500 and
>  not 4500.   1/2 :-)

No, actually I meant (y+1)/2, because I assumed the partition started
with block 0.  If you look at your partition from blocks 4000 to 5000,
you'll find that DOS uses logical block numbers 0 through 1000.  These
were the block numbers I was referring to.

You can, in the mountlist or partition block, select the number of
reserved blocks at the beginning and end of a partition.  For
historical reasons (namely, floppies, where the root directory is block
880, while block 881 is the middle of the usable blocks 2..1759),
the low-end "reserved" blocks aren't counted in the partition-centre
computation, but when the high-end "preallocated" blocks were
added, they were allowed for, and the partition is deemed to end where
usable space ends, which is <# of preallocated blocks> before the
end of the partition proper.
-- 
	-Colin