IBM AT 5170 Technical Reference page 195

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.
During each interrupt 09H from the keyboard, an interrupt 15H,
function (AH)=4FH is generated by the BIOS after the scan
code is read from the keyboard adapter. The scan code is passed
in the (AL) register with the carry flag set. This is to allow an
operating system to intercept each scan code prior to its being
handled by the interrupt 09H routine, and have a chance to
change or act on the scan code.
If
the carry flag is changed to 0
on return from interrupt 15H, the scan code
will
be ignored by
the interrupt handler.
System BIOS 5-23