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Copyright (C) 1987, the Disk Doctor.
First published in the Rochester (PC)^3 News:
Picture City PC Programming Club
PO BOX 20342
Rochester, NY 14602
The Disk Doctor may be contacted at this
address, or via CIS [73147,414].
This material may be reproduced for internal
use by other not-for-profit groups, provided
this copyright notice is included.
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[When we last left our hero, he was in a
sticky situation. He had no idea what was
wrong with the 3 disks. What has happened
to the program that it no longer works?]
2:32pm
A montage of thoughts flashed through my
mind as I struggled to find the answer. I
opened my eyes and stared at my screen for
some clue....
** Four files of 65,536 bytes....
** A filesize of 65,536. That number
jogged something in my memory....
** 64K is exactly 65,536 bytes. It must
be more than a coincidence that there
were four files of exactly 64K....
** What was the significance of 64K?
Something to do with copying to the
hard disk....
** COPYing! I've got it!
I asked one more question to confirm my
hypothesis. "S., what happened to the
copy you made on your computer? Did you
try to run the program on your hard disk?"
"Yeah. But there was an error. Something
about 'Bad command or file missing'."
"Of course!" I proclaimed loudly as I
spun in the chair to face her. "The files
were missing because they were never
copied on to your hard disk!"
S. was startled by my sudden burst of
insight. "Huh?"
"What exactly did your brother tell you to
do, to install the software on your
computer?" I asked S.
"Let's see. He made me memorize the list
because he didn't trust me. 1) turn on
the computer
2) insert the disk in drive A:
3) close the drive door
4) type A:INSTALL and press <Enter>."
"And is that what you did?"
"Yeah, of course." She looked at me, then
reconsidered, "Oh, wait a minute, I think
I might have gone over to drive A: to do a
DIRectory first, to make sure I had disk
#1, ya' know."
"That clinches it. These files were
copied on top of themselves!" I was
difficult to control my excitement. "When
your brother wrote INSTALL.BAT, he was
expecting it would be run from drive C:.
If you switched to drive A:, you've got
big trouble."
"And what harm does that do?" S. asked as
she folded a stick of gum into her mouth.
"With the DOS COPY command, files are
copied in 64K blocks. DOS opens the
source file, and reads the first 64K
block. Next, it finds a file by the same
name on the target disk, erases it, and
copies the first block on top of it. Now,
for any files bigger than 64K, DOS will go
back to the source file to read the second
block. It finds the file is now only one
block long, so it quits. There probably
wasn't even an error message. That
explains the 64K file size and all the
free space on your diskettes."
"Let's look at that batch file again." I
did a Shift-PrtScreen, and tore off the
page:
MKDIR C:\RX
CHDIR C:\RX
COPY A:\*.*
PAUSE INSERT DISK #2
COPY A:\*.*
PAUSE INSERT DISK #3
COPY A:\*.*
2:35pm
"The problem is here with the line 'COPY
A:\*.*. Internally, this gets expanded
into this instruction..." I wrote on the
right side of the printout, 'COPY A:\*.*
A:' . "This copies everything from the
root directory of A: into the default
directory of the default drive.
Unfortunately, in this case, it is the
same directory."
"Oh." She responded, showing mild
concern, "I thought you weren't supposed
to be able to do that."
"The operating system is supposed to
prevent such a blunder, but it is possible
for DOS to get confused. DOS simply
compares the source and destination file
specification strings. In this case," I
pointed to my handwritten instruction, "It
is not an exact match, so DOS proceeds."
"Well, can't you un-erase the file, or
something? After all, you ARE the Disk
Doctor."
I shook my head. "Too late. When DOS did
the damage, it also covered its tracks.
That's why it took me so long to figure it
out. On each disk, the largest file was
first on the disk. All the subsequent
files were copied over the tail end of the
first erased file. There's no way to get
them back!"
S. obviously didn't share my satisfaction
that the problem was solved. "I just paid
$150 for this software. You mean I erased
it?"
"Unfortunately, yes. But let's find
someone here using the same program. This
software is not copy-protected, and you
have the original disks to prove that you
bought it, so we should be able to restore
your disks without violating anyone's
license agreement."
[This story you have just heard is true.
The names have been changed to protect the
ignorant.]
