Up arrow / down arrow command history in simh

[Steve Tockey]

I’ve done a couple of complete rebuild from the ground up, including reflashing Trixie onto the uSim card and reloading all of the PiDP-8/I software from Tangentsoft. For some reason, when in simh prompts, for example after typing control/e, using the up arrow and down arrow keys to scroll through the simh command history doesn’t work. The up arrow shows as

sim> ^[[A

And down arrow shows as

sim> ^[[B

Does anybody know what’s going on and how to get command history scrolling to work?

Thanks,

— steve

[Warren Young]

It appears you aren’t using a proper terminal program.

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[Steve Tockey]

I’m using the Screen program exactly that comes as installed using the defined PiDP-8/I software install process:

1) re-flash a uSim with Trixie

2) install the dependencies, pexpect, yaml, libncurses, fossil

3) clone the repository

4) cd into the trunk directory

5) ./configure

6) make clean

7) tools/mmake

8) sudo make install

If it’s not the right terminal program (Screen), then the above install process is installing the wrong one.

I spent some time searching around and Google’s search engine seemed to suggest it might actually have something to do with the switch to Wayland. I tried editing the ~/.screenrc file as it suggested but that didn’t help.

So if it’s really the wrong terminal/Screen program then the defined software install process needs to be modified to get the right one, or if it’s a Wayland issue then maybe something has to be done with a ~/.screenrc file.

— steve 

[Ian Schofield]

Hi All,

This function in simh used to depend upon readline but has now been updated to use editline. 

Neither of these options are initialised in the current build.

I will leave this with Steve to comment further.

Regards, Ian.

[Steve Tockey]

Ian and all,

I'm not sure what further commenting I could make. It's not at all clear to me why the change from readline to editline was made. Was there some advantage in doing so? All I can say is that being able to use the arrow keys in simh is mighty handy in a lot of cases and I would really like to get it back into the current version.

-- steve

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[Ian Schofield]

Hi Steve, to enable this function requires an update to Makefile.in. Am on hol at the moment so  can't sort it now. Will be back in touch as soon as.

[Clem Cole]

On Thu, Jun 11, 2026 at 2:14 PM Steve Tockey <steve...@gmail.com> wrote:

Ian and all,

I'm not sure what further commenting I could make. It's not at all clear to me why the change from readline to editline was made. Was there some advantage in doing so?

First off, it has changed its name a few times, so Oscar's installation notes, when written for one version of the RPi OS, do not 100% a newer one.

That said, GNU Readline is licensed under the GNU General Public License v3 (GPLv3) [libreadline* is what simh links against]. Where as editline uses a BSD/Apache license thus any of proprietary, commercial, and permissively licensed open-source projects can integrate editline without any risk of license infection.   Depending on your legal advice, many people interpret library linking of a GPL-3 binary as creating a "derivative work," and believe that any software linking against GNU Readline—even dynamically—is legally required to adopt a GPL-compatible license.  If you don't edit it, use it; the legal question goes away. [Note: OpenSIMH is 100% BSD licensed]

That said, there is a technical reason:  code bloat/stability: GNU Readline is a massive codebase (~30,000 lines of C) that forces an external dependency on terminal-handling libraries like ncurses or termcap. Where as  editline is a lot simpler, a fraction of the size (less than 30 KB). It relies strictly on standard POSIX termios, completely eliminating the need for ncurses.