This photo:
is taken from the book:
The Intel Microprocessor Family: Hardware and Software Principles and Applications,
page 298.
This is a 8284 (clock generator) for the 8088 microprocessor. What is the function of the capacitor and the diode here? The circuit seems to work without them.
The diode is used to quickly discharge the capacitor on a power-down spike so that the circuit will hold the reset low when powered back up.
Without the diode, then a transient dip that lasted less than the RC time constant would upset the electronics without providing a guaranteed reset pulse.
Peter M is correct saying the diode is used to quickly discharge the capacitor, however it is for more than just transients. Due to the long RC time constant, the circuit could fail to reset properly just by cycling the power with an on/off switch.
The 10 uF capacitor along with the 100K ohm resistor form a delay holding the 8284 and 8088 in reset allowing time for the power to stabilize and the oscillator to start-up.
