Retro UNIX 8086 uses CTRL+BRK keystroke interrupt to terminate
running process (program) without a sys exit call by the program's itself.

For that...
Ctrl+ScrollLock keys must be pressed together.

For example:

# cat -
skjkjdkjwekjekwe

when the /bin/cat program input is keyboard (tty)
'cat' never exits as own because always waits for keyboard input.
(infinitive loop, read from keyboard and then write to video page)
((/dev/tty is keyboard when reading, it is also video page when writing))

# cat, # cat -, # cat /dev/tty do same because all of them uses /dev/tty

after ctrl+brk:

# cat -
skjkjdkjwekjekwe
#

When user presses ctrl+scrolllock together
(It is CTRL+BRK for Retro UNIX 8086 and IBM PC/AT ROMBIOS)
Retro Unix 8086 v1 kernel terminates 'cat' via its ctrl+brk handler.

But /etc/init and /bin/sh can not be terminated via CTRL+BRK keystroke
because they disable CTRL+BRK (user interrupt) for theirselves.

Other programs (binaries) can be terminated by using CTRL+BRK interrupt
if they do/can not exit to the shell via their own 'sys exit' code.
(CTRL+BRK prevents infinitive running of a process on a pseudo tty.)

Note: Retro UNIX 8086 v1 kernel contains IBM PC/AT KEYBOARD BIOS code v2.
(multi screen feature has been obtained via modified PC/AT keyboard bios)


(Erdogan Tan, 30/01/2022)