It is possible, and likely, to have mapped, and unmapped memory at the same physical address. And, with offset registers set to zero, mapped addresses will match unmapped ones. The only difference is the NOMAP signal to tell whether the address is mapped or unmapped.

To program the mapper, the operating system must load the A, X, Y, and Z registers with the following information, and execute a MAP opcode,

Mapper Register Data

7	6	5	4	3	2	1	0	BIT
LOWER OFF15	LOWER OFF14	LOWER OFF13	LOWER OFF12	LOWER OFF11	LOWER OFF10	LOWER OFF9	LOWER OFF8	A
MAP BLK3	MAP BLK2	MAP BLK1	MAP BLK0	LOWER OFF19	LOWER OFF18	LOWER OFF17	LOWER OFF16	X
UPPER OFF15	UPPER OFF14	UPPER OFF13	UPPER OFF12	UPPER OFF11	UPPER OFF10	UPPER OFF9	UPPER OFF8	Y
MAP BLK7	MAP BLK6	MAP BLK5	MAP BLK4	UPPER OFF19	UPPER OFF18	UPPER OFF17	UPPER OFF16	Z

After executing the MAP opcode, all interrupts are inhibited. This is done to allow the operating system to complete a mapping sequence without fear of getting an interrupt. An interrupt occurring before the proper stack-pointer is set will cause return address data to be written to an undesired area.

Upon completing the mapping sequence, the operating system must remove the interrupt inhibit by executing a EOM (formerly NOP) opcode. Note that application software may execute NOPs with no effect.