Unimplemented Opcode Trap

The CPU12 has opcodes in all 256 positions in the page 1 opcode map,
but only 54 of the 256 positions on page 2 of the opcode map are used.
If the CPU attempts to execute one of the 202 unused opcodes on
page 2, an unimplemented opcode trap occurs. The 202 unimplemented
opcodes are essentially interrupts that share a common interrupt vector,
$FFF8:$FFF9.
The CPU12 uses the next address after an unimplemented page 2
opcode as a return address. This differs from the M68HC11 illegal
opcode interrupt, which uses the address of an illegal opcode as the
return address. In the CPU12, the stacked return address can be used
to calculate the address of the unimplemented opcode for
software-controlled traps.