PiDP-8/I Software

OS/8 Console TTY Setup
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How to show character deletion on the command line?

PDP-8s started with printing terminals as the primary console interface. It was only very late in the PDP-8 product cycle that video terminals became available, so there was an evolution of the display of command lines when one deleted characters. Back then, this was called "rubout processing".

For printing terminals, OS/8 offers a clever presentation where the characters removed are enclosed in backslashes, and the linefeed key can give a refreshed line.

So, typing "hexy<delete><delete>llo" looks like this:


Typing linefeed gives the clean line back:


Printing terminals are now the exception, and the vast majority of people are used to having deletion handled on their video terminals with <backspace><space><backspace> so that the clean line is always visible.

The DEC OS/8 TTY handler (KL8E) offers both options with the command:


to use the video terminal mode, and


to use the printing terminal mode.

Upper case or lower case by default?

PDP-8s are primarily — indeed, almost exclusively — upper case beasts, but the Pi/Linux platform is case sensitive and expresses commands primarily in lower case.

Early PiDP-8/i distributions simply used the SIMH ksr mode to force force all keyboard input to upper case. Bringing document preparation and the C compiler to the PDP-8 platform provokes exploration of a richer approach.

A couple of batch scripts patch the OS/8 keyboard monitor and command decoder to force lower case input to upper case:

LCSYS.BI patches the system to support a lower case default, and force command input into upper case.

UCSYS.BI undoes the action of UCSYS.BI and returns the keyboard monitor and command decoder to their "factory defaults."

When the LCSYS.BI patch adds in the upper case translation code, it does so by sacrificing the "show the clean command line" linefeed command. (UCSYS.BI puts everything back as it was, and so it restores that linefeed command.) A hand disassembly of these patches can be found at: OS/8 LCSYS.BI Disassembled

PiDP-8/i OS/8 TTY Defaults

The OS/8 system packs and startup script associated with boot option 0 and 7 provide the following defaults:

If you have a printing terminal and want to go back to old-style traditional OS/8 rubout processing and upper case only operation, change the tti mode in SIMH as follows:

set tti ksr

Change the tty setup in OS/8 for printing terminal upper case only behavior as follows:



Copyright © 2017 by Bill Cattey. This document is licensed under the terms of the SIMH license.