MSATOST
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MSA-to-ST version 1.3
=====================
Copyright 1997 Damien Burke
email: st@jetman.demon.co.uk
WWW: http://www.jetman.demon.co.uk/st/index.html
DESCRIPTION
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MSA-to-ST converts Magic Shadow Archiver (MSA) files to .ST disk images for
use with the Atari ST emulator, PacifiST. Both compressed and uncompressed
archives are supported, as are disks that have been split into more than
one MSA file.
WHAT'S NEW IN VERSION 1.3?
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Wildcards and filename completion - i.e. you need no longer supply an output
filename (it'll guess it based on the input filename). This has the handy
side-effect that you can now use drag and drop in Win95! Just drag the MSA
file to convert onto the MSATOST.EXE icon (or a shortcut to it) and the file
is automatically converted. An icon is supplied if you want to use it; I also
recommend you set the properties of the EXE file so that the DOS box is
closed after execution.
You can now convert multiple files using wildcard characters in the input
filename. In this case, you *cannot* supply an output filename - no point
overwriting the same filename over and over again, is there?
Also fixed one small bug - if converting an MSA file of precisely 80 tracks
it would give a spurious 'not a full image' message. This never actually
caused a problem, it was just a message!
DISTRIBUTION
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MSA-to-ST is freeware and unsupported. I take no responsibility for any data
loss you may suffer (e.g. if you type the wrong filename in and overwrite
something important). If you find bugs, mail me and I'll fix them. You may do
what you want with the program provided you include this documentation
unaltered, and do not alter the program itself.
USAGE
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To use MSA-to-ST, type:
MSATOST <name of MSA input file> [name of ST output file] [/Q] [/O]
/Q - quiet mode - no screen output and no pauses for keyboard input
/O - overwrite - if output file exists, overwrite without prompting
If /Q is used without /O and an output file exists, it will *not* be
overwritten and the program will *not* extract the archive at all. Both /Q and
/O, if used, must be *after* the input/output filenames, for example:
MSATOST INFILE.MSA OUTFILE.ST /Q /O
...extracts the disk in INFILE.MSA to the disk image OUTFILE.ST without
any onscreen messages and overwriting the existing OUTFILE.ST file (if it
exists).
MSATOST INFILE.MSA
...extracts INFILE.MSA to INFILE.ST - the output filename is based on the
input filename if you don't supply a specific output filename.
MSATOST *.MSA
...extracts every MSA file in the current directory, using output filenames
based on the input filenames as described above.
While extracting, a progress bar is displayed. Each block in the bar indicates
one track converted. Large blocks indicate compressed tracks and smaller
blocks indicate uncompressed tracks.
NOTES
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MSA-to-ST will return status codes of 0 for success or 1 for failure; you can
check for these in a batch file by testing ERRORLEVEL or if you are running
MSA-to-ST from within another program you can check by testing the return code
you get from the execution.
MULTI-MSA DISKS
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Disks that have been split into more than one MSA file (e.g. an 800Kb disk
split into tracks 1 to 40 and 41 to 80 in order to keep two small MSA files
that can fit onto 720Kb disks) should be handled correctly by MSA-to-ST, but
if you find any problems please mail me. It doesn't matter what order you
convert the MSA files, just make sure you use the same output file, e.g.:
MSATOST 01_TO_40.MSA OUTFILE.ST
MSATOST 41_TO_80.MSA OUTFILE.ST
Any areas in the image not filled with data from an MSA file will be full of 0
bytes and such an incomplete image will no doubt not be of any use until you
complete it by converting all the appropriate MSA files. Also note that if you
convert only the first part of a disk, the image file will be too short and
PaCifiST may be unable to use it. I doubt this will be a problem but if it is
mail me and I'll make the next version add blank bits to the end of the image
in such a situation.
.MSA FILE FORMAT
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For those interested, an MSA file is made up as follows:
Header:
Word ID marker, should be $0E0F
Word Sectors per track
Word Sides (0 or 1; add 1 to this to get correct number of sides)
Word Starting track (0-based)
Word Ending track (0-based)
Individual tracks follow the header in alternating side order, e.g. a double
sided disk is stored as:
TRACK 0, SIDE 0
TRACK 0, SIDE 1
TRACK 1, SIDE 0
TRACK 1, SIDE 1
TRACK 2, SIDE 0
TRACK 2, SIDE 1
...and so on. Track blocks are made up as follows:
Word Data length
Bytes Data
If the data length is equal to 512 x the sectors per track value, it is an
uncompressed track and you can merely copy the data to the appropriate track
of the disk. However, if the data length value is less than 512 x the sectors
per track value it is a compressed track.
Compressed tracks use simple a Run Length Encoding (RLE) compression method.
You can directly copy any data bytes until you find an $E5 byte. This signals
a compressed run, and is made up as follows:
Byte Marker - $E5
Byte Data byte
Word Run length
So, if MSA found six $AA bytes in a row it would encode it as:
$E5AA0006
What happens if there's an actual $E5 byte on the disk? Well, logically
enough, it is encoded as:
$E5E50001
This is obviously bad news if a disk consists of lots of data like
$E500E500E500E500... but if MSA makes a track bigger when attempting to
compress it, it just stores the uncompressed version instead.
MSA only compresses runs of at least 4 identical bytes (after all, it would be
wasteful to store 4 bytes for a run of only 3 identical bytes!). There is one
exception to this rule: if a run of 2 or 3 $E5 bytes is found, that is stored
appropriately enough as a run. Again, it would be wasteful to store 4 bytes
for every single $E5 byte.
The hacked release of MSA that enables the user to turn off compression
completely simply stops MSA from trying this compression and produces MSA
images that are completely uncompressed. This is okay because it is possible
for MSA to produce such an image anyway, and such images are therefore 100%
compatible with normal MSA versions (and MSA-to-ST of course).
.ST FILE FORMAT
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The file format of the .ST image files used by PaCifiST is simplicity itself;
they are just straight images of the disk in question, with sectors stored in
the expected logical order. So, on a sector basis the images run from sector
0 (bootsector) to however many sectors are on the disk. On a track basis the
layout is the same as for MSA files but obviously the data is raw, no track
header or compression or anything like that.
.ST files can be written to real disks using DKC's STImage program for the PC,
available from my web site (see top of this document). If you have .ST files
on your real ST you can write them to a disk using ArghBlarg's STTODSK.TTP
program, also available from my web site.
You can also use MSAtoST's companion program, STtoMSA, to convert .ST files to
MSA files. That is also available from my web site.
--
Damien Burke
31st August 1997
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