Trying to understand ITS on PiDP-10

[Francis King]

Hello.

My first computers were the ZX Spectrum and IBM PC-XT, and so I'm finding ITS to be a fairly steep learning curve. I would like to ask some fairly basic questions, please.

I don't have the lights/switches kit, yet - I'm waiting to see how much I used the PiDP-10 first - and I'm running SimH on a Raspberry Pi 5, outputting an image to a desktop monitor via HDMI. 

When I boot the Raspberry Pi 5, it boots the PiDP-10 software. I use pdpcontrol stop and pdpcontrol start 1 to set the system up for ITS. I have already attached a tele-typewriter using CON. The system boots to DSKDMP, and then I type ITS<Esc>G to continue the boot.

I also attached a Knight TV Console. Once the system is up I press F1 once to get service from PiDP-10. I login as LARS using :LOGIN LARS. This deposits me in a folder called LARS, which is shown at the bottom of the Knight TV Console. From here I can type commands prefixed with a colon against the left edge of the screen; except, sometimes there is a mysterious asterisk prompt.

Questions, please:

1. I am having to specify ITS twice (and the same basic system is used if I had the lights/switches kit). What is happening on the original hardware which leads to specifying ITS twice?
2. What is the mysterious asterisk prompt? Why does it appear, and what do I do about it?
3. Is it generally the case that a user account of NAME will have a default folder called NAME, as account LARS has a default folder of LARS?

Regards,

Francis.

[Eric Swenson]

Hi Francis,

See below.

On Aug 2, 2025, at 13:38, Francis King <francis...@gmail.com> wrote:

Questions, please:

1. I am having to specify ITS twice (and the same basic system is used if I had the lights/switches kit). What is happening on the original hardware which leads to specifying ITS twice?

I’m not sure what you mean here. When you boot you should end up in DSKDMP. From there you should type:

ITS<enter>

That will load the file:

 .;@ ITS

Into memory. Then you type 

<esc>G

Which will start it running.

That only amounts to typing “ITS” once.  Where do you think you are having to specify it twice?

2. What is the mysterious asterisk prompt? Why does it appear, and what do I do about it?

That is DDT’s prompt. Think of it as a command interpreter prompt. To that prompt you can enter “colon” commands, like:

:PRINT foo;bar bar<enter>

Or shortcuts like:

<control>R foo;bar baz<enter>

3. Is it generally the case that a user account of NAME will have a default folder called NAME, as account LARS has a default folder of LARS?

Yes. If you login as FOO, ITS will look for a home directory named FOO.

— Eric

[Eric Swenson]

It occurs to me what you might mean by specifying ITS twice. Perhaps you meant that by typing “pdpcontrol start 1”, that is the first time, and typing ITS<enter> to DSKDMP is the second time.

DSKDMP is a loader program that allows running programs from disk files. It is used to start ITS, the salvager (SALV) and any other program in the root directory (.) whose first filename (FN1) is “@“.

When you run “pdpcontrol start 1”, that really means set up the simulator to run ITS and start DSKDMP. You could run any number of programs under DSKDMP. If you want to start ITS, you have to load it in memory first. That is the ITS<enter> input. You could have multiple versions of ITS in the “.” directory, in which case you would type one of them’s name to DSKDMP.

The PiDP10 setup that is provided has been not one version of ITS in the “.” directory. It’s filename is “.;@ ITS”.  So that input is selecting the ITS image to load.

Does that answer your question(s)?

— Eric

On Aug 2, 2025, at 13:38, Francis King <francis...@gmail.com> wrote:

Hello.

--
You received this message because you are subscribed to the Google Groups "PiDP-10" group.
To unsubscribe from this group and stop receiving emails from it, send an email to pidp-10+u...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/pidp-10/e1574f9d-e5c3-4dd6-b18a-1283ac9b8668n%40googlegroups.com.

[Lars Brinkhoff]

Eric wrote:

3. Is it generally the case that a user account of NAME will have a default folder called NAME, as account LARS has a default folder of LARS?

Yes. If you login as FOO, ITS will look for a home directory named FOO.

It's also possible to set another directory in INQUIR.  E.g. user FOO could have the home directory (HSNAME) BAR if so desired.

HSNAME means "home SNAME".  SNAME means "system name".  Here, "system" is an older name for directory.

[Andrew Barron]

Welcome aboard Francis.

I think that everything you asked has been answered. Just a note. When you login to ITS as "Francis" the system will allow you to create a Francis directory. If you put files into it, it will not be deleted. If you leave it empty it is automatically deleted when you log out. So, it won't be there when you login again.  

It is easiest, at least initially, to login as Lars and have a look around the Lars directory. ITS has no folder protection, so you can see the whole disk.

I am busy writing a book about the PiDP-10 incl ITS, TOPS-10, and TOPS-20. I wont be a big seller, but it does consolidate what I have learned so far.

Like you, I am very new to the PiDP-10 software.

BTW "FOO" just means something you are required to type in. It could be a name, filename, directory etc.

cheers

AndrewB

[Eric Swenson]

This is all true, except that ITS uses a maximum of 6 characters for its directory names, file names, and lots of other names. So your “Francis” example would result in a login name of FRANCI — 6 characters and only uppercase. The character set used for these names is called SIXBIT, and, you guessed it, uses 6 bits per character (and as a result, no lowercase letters). The reason for the use of a 6 sixbit characters for these names is that they all fit in a single 36-bit word.

I’d be happy to review your book about PiDP-10 and ITS. I’m an old ITS users from the 70s and 80s, and still use it daily.

On Aug 3, 2025, at 03:38, Andrew Barron <zl...@outlook.co.nz> wrote:

Welcome aboard Francis.

--

You received this message because you are subscribed to the Google Groups "PiDP-10" group.
To unsubscribe from this group and stop receiving emails from it, send an email to pidp-10+u...@googlegroups.com.

To view this discussion visit https://groups.google.com/d/msgid/pidp-10/57502931-d92d-47fd-9b18-b6f0ff134d85n%40googlegroups.com.

[Francis King]

I would like to thank everyone for their assistance.

In the course of which I found this file, which contains a lot of useful beginner material: https://github.com/PDP-10/its/blob/master/doc/_info_/its.primer

Regards,

Francis.