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Each interrupt is assigned an 8-bit priority number, which is equal to the lower nibble of the lower byte of the interrupt vector address. A higher priority number indicates a higher priority. Interrupt priorities are listed in chip-specific technical reference manual, for example, we find the Interrupt Vector Map for the M68HC(9)12B in Table 4-1 of the Data Sheet for the Motorola M68HC12B Family.
The priority of one interrupt may be elevated to the highest priority level by writing its priority number into the HPRIO register. Bits 6 and 7 of all priority values are always one. Bit 0 of all priority values is zero. To avoid indeterminacy, the HPRIO register can be modified only when interrupts are disabled.