Re: ProDOS Directory

[dempson@actrix.gen.nz (David Empson)]

A2MG <a2mg@aol.comNomail> wrote:

> To the people who say that the Ram disk can only hold 12 files... could you
> clarify?  I have been running Ram disks on //c's for years, and they hold 51
> files. I use ProSel's Ram Driver to access 1 meg of memory in slot 3,1. I've
> used both Z-Rams (Aux) and Apple (Slinky) cards. I have never seen a limit of
> 12 files.

The standard ProDOS auxiliary memory /RAM volume (S3,D2) is created with
a single block for its root directory.  Each block in a ProDOS directory
can hold 13 file entries, except the first, where one is taken up by the
directory header.  Hence the /RAM volume is limited to 12 files in the
root directory.

Other RAM disks (such as the IIgs's /RAM5 and the RAM disk created in a
RamFactor, RamWorks, Apple II memory expansion card or PC Transporter)
will be normal volumes with four blocks for the root directory.  4 * 13
- 1 = 51 files limit.

/RAM is handled specially by formatting software - the FORMAT call to
its device driver will automatically create the one block root directory
and volume bitmap.  This differs from other volumes, where the FORMAT
call simply prepares the volume for use, and the formatting software is
responsible for creating the boot blocks, root directory (normally four
blocks) and volume bitmap.

There is one other common exception to the four block root directory:
AppleWorks 5.25" floppy disks have been using a single block root
directory at least as far back as version 3.0.  This saves 3 blocks on
the disk, freeing up enough space to fit the essential AppleWorks
startup files.

In theory, volume formatting software could set up the root directory
with any number of blocks, but some applications and utility programs
cannot handle unusual directory sizes.  There is probably also a limit
in the ProDOS boot code - it may not be able to locate the PRODOS file
if it is not in the first four blocks of a larger root directory.

-- 
David Empson
dempson@actrix.gen.nz