Atari 400 Technical Reference Manual page 215

Special Functions
Start/Stop
Display Handler and Screen Editor output to the text or ~raphics
mode screen can be stopped and started (without losing any of the
output data) through the use of the [CTRLl 1 key combination.
Each key depression toggles a flag that is monitored by the above
mentioned Handlers. When the flag is nonzero, the handlers wait
for it to go to zero before continuing any output.
E4 SSFLAG [02FF,1l -- Start/stop flag
The flag is normally zero, indicating that screen out_put is not
to be stopped. The flag is complemented by every occurrence of
the [CTRLl 1 key combination by the keyboard IRG service routine.
The flag is set to zero upon power-up, [SYSTEM.RESETl or [BREAKJ
key processing.
[BREAKl KeV
E5 BRKKEY [0011,1l -- [BREAKl key flag
BRKKEY is used to indicate that the [BREAKl key has been pressed.
The value is normally nonzero and is set to
zero
whenever the
[BREAKl key is pressed. The code that detects and processes the
[BREAKl condition (flag
=
0) should set the flag nonzero again.
BRKKEY is monitored by the following OS routines: Keyboard
Handler, Display Handler, Screen Editor, Cassette Handler, xx?
The detection of a [BREAKl condition during an 110 operation
will cause the operation to be aborted and a status of $80 to be
returned to the user.
The flag is set to nonzero upon Power-up, [SYSTEM.RESETl or upon
aborting a pending 110 operation.
[SHIFTJ/[CONTROLJ Lock
The keyboard control has three different modes for code
generation that apply to the alphabetic keys A through Z:
1) normal, 2) caps lock, and 3) control lock.
OPERATING SYSTEM C016555 -- Appendix L
230