Re: :( 12 Partition Limit!?!?!??? :(

[nparker@cie-2.uoregon.edu (Neil Parker)]

In article <4rhp72$s2h@nnrp1.news.primenet.com> johnlb@Primenet.Com (John
Bowling) writes:
>[...]
>The reason large drives are not recommended is that there is limited space
>for the bit map.  There is one bit for each block on the disk.  Increasing
>the number of blocks requires one of two methods:  You can make the bit
>map larger to allow more blocks, or you can make the blocks bigger.
>Either method takes away some usable space.  In ProDOS, either version,
>the bit map and the block size are both hard coded into it, and if they
>were changed, the new version of ProDOS would not read disks created with
>the old version.

No, the bitmap isn't the problem.  The bitmap has always been able to grow
or shrink as much as necessary.

The problem is that ProDOS has to store block numbers in directory entries
and in file index blocks.  In each case, only two bytes are available for
storing block numbers, which means that the largest useable block number
on a ProDOS disk is 65535.  With 512-byte blocks (an assumption which is
implicit in just about everything ProDOS does), that's a maximum of 32
megabytes of useable space.


Macintosh HFS has the same block number limitation.  It gets around the
32-megabyte limit by allowing blocks to be larger than 512 bytes.  There is
a field in the HFS master directory block that indicates what the disk's
block size is.

               - Neil Parker
-- 
Neil Parker, nparker@{cie-2,cie}.uoregon.edu, http://cie-2.uoregon.edu/~nparker

     "Evolution is vastly overrated."  -- Ambassador Delenn, _Babylon_5_