Dec 102017
 
Program shrink COM files to save space, yet they run normally.
File SHRINK.ZIP from The Programmer’s Corner in
Category Utilities for DOS and Windows Machines
Program shrink COM files to save space, yet they run normally.
File Name File Size Zip Size Zip Type
SHRINK.COM 1038 793 deflated
SHRINK.DOC 4771 2039 deflated

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Contents of the SHRINK.DOC file


SHRINK 1.0 Copyright (c) 1988 Thomas G. Hanlin III

Program Compression Utility
06/01/88



Using SHRINK:

This utility compresses .COM files without affecting the way you run them.
Its compression scheme is simple and will not work on all files, but may
produce impressive results depending on the particular file. If SHRINK is
unable to compress the file, it will abort without doing anything.
Otherwise, it will overwrite the old file with the smaller, compressed
file. Note that this will save only disk space, not memory. You must
have at least 64K of memory available to run a compressed program. The
larger the program is, the better chance SHRINK has to compress it... but
SHRINK can only handle programs up to 30,000 bytes long.

NOTE that SHRINK is not compatible with every program. In some cases, a
program compressed with SHRINK will no longer run. Do not use SHRINK on a
program unless you have a backup copy, just in case! Some files known not
to compress properly with SHRINK are: BURGER.COM, the Landmark Speed Test,
PKARC, and PKXARC.

The average compression rate seems to be around 7% for files that SHRINK
is able to compress, but I've seen compression rates of 46% and higher.
It depends entirely on the program.

To use SHRINK, just type "SHRINK filename" (without the quotes), where
"filename" is the name of the .COM file to shrink. SHRINK will only work
properly on .COM files, so if you do not type a file extension, ".COM" is
assumed. If SHRINK can compress the file, it will do so in place,
modifying the file you specified... so be sure to make a backup copy of
the file before SHRINKing it, in case something goes wrong.



Distributing SHRINK:

The SHRINK utility is protected by copyright, and may be distributed only
according to the following limitations:

1) No price may be charged, other than a disk copying/mailing fee which
may not exceed $6.00.

2) The utility and documentation must be included together, in unmodified
form. This includes the SHRINK.COM and SHRINK.DOC files.

Programs compressed with SHRINK may be distributed without royalties or
licensing. Please do not distribute a compressed program without the
author's permission, however, as this may result in a violation of his
copyright.



Warning:

I've warned you before, now let me say it again... back up your programs
before using SHRINK on them! Although SHRINK works fine in most cases, it
is known to be incompatible with some programs. You will simply have to
find out for yourself whether your particular programs will work with
SHRINK. Please notify me of any incompatibilities, and I'll try to keep
track of them (and find out what is causing the problem)... but I won't
guarantee that I'll be able to fix it, and I won't be responsible for any
damages you may accumulate through your carelessness.



Tech Notes:

The SHRINK utility works by compressing a COM-format executable file,
using the inefficient but simple Run Length Encoding compression scheme to
turn the code into an encoded data stream. The flag used for RLE is
chosen by scanning the file for the least-frequently-used character, for
maximum efficiency. The encoded program is written back to disk under the
original file name, with a relocating decoder attached to the start of the
file.

When the .COM file executes, control passes to the relocating decoder,
which relocates itself (and the encoded data) higher in memory. It then
decodes the program into the same place it would have been loaded if it
were not encoded... resets the registers and flags to their original
form... and starts the program just as if there had never been any
encoding.

So why doesn't it work for all .COM files? I really don't know. Any
suggestions would be appreciated.

Why did I choose RLE compression? I did so for a number of reasons. One,
it's fast, both for encoding and decoding. The SHRINK utility imposes no
noticeable loss of speed on executing a program. Two, it's simple, which
keeps it small. The compression overhead, so far as memory is concerned,
is equal to the length of the relocating decoder. With RLE compression,
this decoder is only 82 bytes, which is insignificant. Other coding
schemes might be more efficient, but would have a far greater amount of
overhead to overcome. Since no compression method I've seen appears to be
terribly effective on .COM files anyway, RLE is good enough.


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