All bits are affected by an interrupt occuring. When an interrupt occurs, these bits will indicate which device caused the interrupt:

Bit 0 - this will be set if the interrupt resulted from the ON key being pressed
Bit 1 - Set if the interrupt was caused by the normal hardware timer
Bit 2 - unknown, something to also do with the hardware timer
Bit 3 is an ON key status. It's reset when the key is depressed.
Bit 4 - the link cable was pushed in the port (I think?)
Bit 5 - the first crystal timer generated an interrupt
Bit 6 - the second crystal timer generated an interrupt
Bit 7 - the third crystal timer generated an interrupt

The appropriate bit must be reset in the interrupt routine, otherwise the interrupt will occur repeatedly.

Also, the hardware timer speed can be adjusted. Bits 1-2 control the speed at 4 different levels. Setting these bits to 00h causes the fastest speed, and 11h the slowest. The normal speed is 11h.