'Proper' Compact
(C) John Robinson 1989

This program does what *COMPACT doesn't do - it moves ALL the free space
on an ADFS disc to the end of the disc. This may seem rather obvious, but
when using the normal *COMPACT command, the free space is not always
completely collected together - it just does exactly what the manual says
it does though; free space tends to migrate towards higher disc addresses.
Apart from being a nuisance, I find it absolutely infuriating (having been
used to the DFS' *COMPACT command)!

Other programs have been written to do this by copying everything to
another disc, but this wastes discs, is obviously useless with a hard
disc, and is a terrible slog for users with only a single drive. People
have suggested repeatedly using *COMPACT until there's only one space
left, but this takes an excruciatingly long time.

To run the program, first do everything you can to get more memory - turn
off ROMs which push page up (not the ADFS!); put shadow RAM on; or, on a
6502 second processor or Master Turbo CoPro, start up HiBasic. The extra
memory is advisable so that you have a larger data buffer while the data
is being moved around the disc. This will make the process slightly
faster. Then just type
  CHAIN "Compactor"
When the program loads, you will be presented with the message
  Drive?
when you should insert the disc you want compacted, and give its drive
number. Then leave the computer alone until you get the ALL FINISHED
message.

During the first stage of the program, when directories' information is
changed, you have no idea of how much of this process has been done.
However, it should not take very long; most discs don't have a lot of
directories on them. The time taken is roughly proportional to the number
of files and directories on the disc.

The second stage is the slowest one; here the data is actually moved
around on the disc. This is where the extra buffer comes in useful: it is
used so that fewer disc accesses have to be made (although the same amount
is read in all, the drive has to be prepared each time you access it -
this slows everything down), allowing the program to work more quickly. A
percentage count of how much of the disc has been completed is displayed;
this is calculated from the amount of data which is in the right place
divided by the total amount of data on the disc.

The last stage is a relatively fast one: the new free space map (with only
one entry) is created and written to the disc. However, strangely enough,
this is the stage that took the longest to write. Most of that time was
spent trying to find out the correct method for calculating the checksum.
I asked many people, including Acorn, and they didn't know! How's that for
customer service? I ended up hacking through the ADFS ROM code, and
extracted the following routine (to calculate the first sector's checksum):
  CLC
  LDY #&FF
  TYA
  ADC &DFF,Y
  DEY
  BNE P%-4
A similar routine is used for the second sector's checksum, but with the
value &EFF instead of the &DFF. Simple, when you know how! The public
should be told...

Well, may you have many happy hours compacting away!
€RJJohn Robinson.

PS. The program "FixFSM" will recover a disc from the dreaded "Bad FS map"
error or even a "Disc error 48 at :x/000000". It would be advisable to
copy all the files on that disc onto another disc as soon as possible if
you ever need to use this program (do NOT just use *BACKUP, instead use
something like DIRCOPY - and do it on a double disc drive), because the
FSM may be corrupt even though it looks OK and lets the disc work. Maybe
someone could write a program to check each sector of a disc to see if
it's filled (by directory checking recusively), and make a new FSM out of
that? Good luck to you if you try. Please send me a copy if you succeed.