The lower four mask various devices which can cause interrupts. For each bit, a 1 indicates that the device can cause an interrupt. A 0 masks out the device and no interrupt is triggered. The bits:

Bit 0 - ON key (the on key triggers an interrupt when it is pressed)
Bit 1 - The hardware timer (it generates the normal interrupts that occur ~150 times per second)
Bit 4 - The link port (when a cable is pushed in the link port, an interrupt is generated)