Episode: 3470
Title: HPR3470: External Commands and Emergency Boot Disk
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr3470/hpr3470.mp3
Transcribed: 2025-10-25 00:02:26

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This is Hacker Public Radio Episode 3474 Friday, the 19th of November 2021.
Today's show is entitled, External Commands and Emergency Boot Disc,
and is part of the series DOS it is hosted by AUKA and is about 17 minutes long and carries a clean flag.
The summary is, more on DOS this time it is External Commands and Emergency Boot Disc.
This episode of HPR is brought to you by archive.org.
Support universal access to all knowledge by heading over to archive.org forward slash donate.
Hello, this is AUKA, welcoming you to Hacker Public Radio in another exciting episode in our DOS series.
At the last episode, we took a look at the internal commands of DOS.
These are the commands that are contained within commands.com and are loaded during the boot sequence.
Because of this, a certain amount of the limited DOS memory is committed to holding internal commands.
Obviously, some commands do not need to be loaded into memory every time you boot,
because they are used infrequently or possibly because they are large and take up a lot of space.
These commands are stored as separate files in your C colon backslash DOS directory,
or that's usually the places, I mean you have a hard drive.
My first XT computer was a twin floppy configuration.
Again, C colon backslash DOS is the normal default during installation.
It's possible to specify a different directory, but I'm going to assume defaults.
So, let's take a look. What are these external commands?
Append, assign atrib that sets the attributes of the file. Backup, check disk, command,
comp, double space, debug, defrag, del old DOS, del tree,
disk comp, disk copy, DOS key, DOS shell, edit, emm 386,
xe2bin, expand, fast help, fast open, fc, fdisk, find, format, graftable,
graphics, help, high men, interlink, interserver, join,
key b, label, mem, mem maker, mirror, mode, more, emsafe, MS backup, MSCDX, MSCDX, MSD,
and LSFUNC. Power, print, replace, restore, scan disk, select, server, share, sort,
sub-st, cis, tree, undelete, unformat, vsafe, xcopy. That's quite a lot of them.
Again, if you go to the web page, link in the show notes, they're all there,
and there's probably plenty of documentation on the web about all of this stuff anyway.
Now, please note that you might not find all of these files on every computer. This will
depend on things like which version of DOS you have installed. In some cases, Microsoft replaced
one file with a different file in later versions. An example of this is the check disk CHKDSK.
This name survives in Windows NT, but in DOS and later Windows, this program or applet was
replaced by the later scan disk. So, look at this as a list of some of the files you might find
on your computer. You may have some that are not on this list as well. Also, note that other
programs will sometimes install files in the DOS directory. In particular, Windows 3.x,
really, installed many of its files in this directory, and sometimes updated the versions
that DOS originally installed. This can complicate the discussion a bit, but the basic files and
how they operate is still going to follow the outline we will discuss. Now, interesting little
side note on what's DOS and what's Windows. Even with Windows 95, I remember that some of the
internals referred to it as a version of DOS. I remember some very interesting discussions about,
is this just relabeled DOS? It's all old by now, but it was fun at the time.
One file that is very important and is not a DOS command is the included copy of basic.
Now, in earlier versions of DOS, this was called GW basic in later versions, Q basic.
Now, one of the important external commands, edit, is actually a basic program,
and will not run unless basic is present. This is important to remember if you're preparing
an emergency boot disk for DOS, and we'll get to that in just a moment.
Now, extensions help in associated files.
External commands are files, and as such they have file extensions. Generally, they will be either
.exe or .com. A .exe file is what's called an executable file. This is a file that is compiled
to run as a program. To this day, any Windows program is run from a .exe file.
The other type, a .clm, is the original .com. This refers to a command file. A few of these files
will be .sys.sys, which are system files, such as the io.sys and ms.dOS.sys files we discussed in
lesson 2. Another file type is the .ovl file, which means overlay. This file type is found with
backup programs, particularly. Now, often these external files will come with help files as well.
Help files have the extension .hlp. If you have a dual boot system with both DOS and Windows,
and in the day, I actually did have a lot of dual boot systems. You can open these .hlp files
using Windows built-in help file viewer. Now, emergency boot disk. It's an important
concept. You're going to be working with these old computers. One of the things that anyone
learning about DOS or about computers in general, a lot to have in their arsenal is the DOS
emergency boot disk. Let's assume you have DOS installed already on your C drive and you can boot
into DOS. How do you create an emergency boot disk? Well, you take a blank floppy disk, put it
in your A drive, and this is a good place to just stop for an A, B, C. Why is the hard drive
the C drive? These days, that is something that confuses a lot of people in the Windows world.
Why is it start with C? Because the oldest computers, like my first XT, came with two floppy drives,
and the floppy drives were A and B. You can't even buy a computer with a floppy drive these days,
and I wouldn't want one in any event. They're pretty much useless. Anything you would do with a
floppy, you would do these days with either a CD or a USB's on the drive or whatever. But
because the A and B were already assigned for floppy drives, when hard drives were first added
computers, the next letter was C. If you're ever wondering why Windows insists that your hard drive
is the C drive, that's why. Anyway, you place your floppy disk in your A drive and at the C,
colon, backslash, command prompt, use the command format, A colon, space, slash S.
This command has three parts. First, it uses the external command format.
Obviously, that's not something you'd be doing constantly, so that's why it's an external
command. You don't want it loaded in memory all the time. Second, it applies this command to the
A colon or floppy drive. Now, the last part is called a switch in DOS terminology.
The slash S. Many DOS commands have switches available to modify the command in some way. In
this case, the slash S switch tells it to place the system files on the disk. Recall from lesson
two that this means that the hidden system files IO, SIS and MS DOS SIS will be placed very
specifically in the first sectors of the disk. Also, command.com will be placed in the root
directory of the disk. This is enough to boot the computer. Now, you may wish to add a config.sIS
and auto-exec-bat on the disk as well. You can copy these from your hard drive, if you wish,
using the commands. Copy, C colon, backslash, config.sIS, space, A colon. And then the other will be
copy, C colon, backslash, auto-exec-bat, space, A colon. Now, you can modify those files later using
the edit command to invoke the DOS file editor as long as you've also got basic. Remember, edit
is a basic command. Now, if it's an emergency boot disk, editing files sounds like something you
just might want to do. So, that suggests to me you're going to want to do some of this stuff, right?
So, at this point, you should test your boot disk by turning off the computer if you're in DOS.
And, you know, in DOS, you didn't have to go through a complicated shutdown process. You just
cut the power. And you place the boot disk in the A drive, floppy disk drive, and turn on the
power. It follows, well, you'll boot in the disk. You'll boot into DOS. Although, you'll probably be
asked to enter the date and time when you boot. Now, if you boot to the C colon, backslash prompt,
that means the computer never even looked at the boot disk. And this is set in the BIOS.
Going to your BIOS, look for the setting for boot order or seek order and change it from
something like C, comma A to A, comma C, reboot with your floppy disk in the A drive and see if your
boot disk works properly. Now, once you know you have a good boot disk, you should add a few things.
All of the internal commands are available to you because you have command.com on your disk.
But you should also add some critical external commands. Atrib. Atrib is the command that lets you
set the file attributes on files. So you can make them hidden or display them or read only, I mean,
a number of attributes and we'll cover those some other point. Check disk or scan disk. Check
disk is CHKDSK. Remember that in DOS, we did not have long file names. Everything was 8.3.
You could have eight letters at the beginning and in a three letter extension. And that was it.
There was no room for anything else. So a lot of these DOS commands are a little bit cryptic looking.
Now, for later versions, you might have scan disk instead of check disk.
You might want to add the edit so that you can edit files. But remember, this will only work if
you add either GW basic or Q basic depending on which version of DOS you're working with.
F disk. Format. Undelete. Unformat. You know, these are things that are pretty handy.
Now, undelete is nice. If you want to recover files, it can get a little bit tricky. Basically,
in the DOS world, files were not deleted so much as the pointer to them in the file allocation
table was zeroed out so that they would just get written over at some point.
And what they did is they would just remove the first character of the file name. That was enough
to make it disappear. Now, if you've used disk compression, and again, that was something we did
because space was at a premium. You know, the first hard, I mean, the floppy disks,
the first ones that I had on my XT were 320K. And those are the big five and a quarter inch floppy
disks. 320K is not a lot of space. Later on, I got the 1.4 megabyte ones. Those are the
three and a half floppy disks with the plastic case, the hard plastic case, which confused people.
Why is it floppy when it's hard? Well, you know, inside was a floppy mylar disk.
The first hard drive that I had on my DOS computer was 20 megabytes. I remember thinking,
well, I'll never fill this. These days, I'm giving away some two terabyte drives because they're
too small for me to bother with. It just shows how things change. But, you know, in the day, we use
disk compression a lot. And rather than use disk compression these days, I just buy bigger hard drives.
So, I'm not going to get into disk compression in this. I don't think anyone does it anymore.
If you have a desperate need, you can look it up online. But the external commands listed above are
critical for solving possible software problems with your computer. I recommend anyone following this
series, you know, create such a boot disk if you have a floppy drive. If not, just file us away for
reference. That's a tool that a computer expert might have handy. You know, back when I was in the
IT department at the university. And I was training students to run the help desk and everything.
We, you know, this is one of the things we did was to create an emergency boot disk. And I had a
portfolio I carried around with me that had a number of disks in it for various reasons, including
some floppy emergency boot disks that I would carry with me in my organizer, so that they'd have them with me at all times.
So, that's enough on that for today. This is a hook up for hacker public radio signing off. And as always, encouraging you to support free software. Bye-bye.
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