Extract From the AT Technical Refrence:

IRQ's
"An interupt request is generated when an IRQ line is raised from low to high.
 The line must be held high until the microprocessor acknowledges the interupt
 request (Interupt Service routine). Interupt 13 is used on the system board and
 is not available on the i/o channel. Interupt 8 is used for the real-time
 clock."

I also found that the standard printer port dose not conform to this, as it has
 not got any latching device in the interface. It just passes the ACK pulse from
 the printer's ACK pin. This pulse width is typicaly 5us.