IBM 5170 Technical Reference page 163

If
no, go to previous address
It
is the responsibility of the application to preserve the value in
all registers, except AX, upon return. Sys is handled internal to
the keyboard routine.
Other Characteristics
The keyboard routine does its own buffering, and the keyboard
buffer is large enough to support entries by a fast typist.
However, if a key is pressed when the buffer is full, the key
will
be ignored and the "alarm" will sound.
The keyboard routine also suppresses the typematic action of the
following keys: Ctrl, Shift, Alt, Num Lock, Scroll Lock, Caps
Lock, and Ins.
Special Handling
~
System Reset
The combination of the Alt, Ctrl, and Del keys results in the
keyboard routine that starts a system reset or reboot. System
reset is handled by BIOS.
System BIOS 5-21