IBM XT 5160 Technical Reference page 138

Save the previous address.
Overlay interrupt vector hex 15.
Check AH for a value of hex 85:
If yes, process may begin.
If no, go to previous address.
The application program must preserve the value in all registers,
except AX, upon return. System Request 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.
During each interrupt hex 09 from the keyboard, an interrupt hex
15, function (AH)=hex 4F 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 hex 09 routine, and have a chance
to change or act on the scan code. If the carry flag is changed to
o
on return from interrupt hex 15, the scan code will be ignored
by the interrupt handler.
lOl/102-Key Keyboard 4-43