MC68HC812A4 Datasheet, PDF (95/342 Pages) Motorola, Inc – 16-bit device composed of standard on-chip peripheral modules connected by an intermodule bus. Modules include

6.4 Registers
Bus Control and Input/Output (I/O)
Registers
Not all registers are visible in the memory map under certain conditions.
In special peripheral mode, the first 16 registers associated with bus
expansion are removed from the memory map.
In expanded modes, some or all of port A, port B, port C, port D, and port
E are used for expansion buses and control signals. To allow emulation
of the single-chip functions of these ports, some of these registers must
be rebuilt in an external port replacement unit. In any expanded mode,
port A, port B, and port C are used for address and data lines so registers
for these ports, as well as the data direction registers for these ports, are
removed from the on-chip memory map and become external accesses.
Port D and its associated data direction register may be removed from
the on-chip map when port D is needed for 16-bit data transfers. If the
MCU is in an expanded wide mode, port C and port D are used for 16-bit
data and the associated port and data direction registers become
external accesses. When the MCU is in expanded narrow mode, the
external data bus is normally 8 bits. To allow full-speed operation while
allowing visibility of internal 16-bit accesses, a 16-bit-wide data path is
required. The emulate port D (EMD) control bit in the MODE register
may be set to allow such 16-bit transfers. In this case of narrow special
expanded mode and the EMD bit set, port D and data direction D
registers are removed from the on-chip memory map and become
external accesses so port D may be rebuilt externally.
In any expanded mode, port E pins may be needed for bus control (for
instance, ECLK and R/W). To regain the single-chip functions of port E,
the emulate port E (EME) control bit in the MODE register may be set.
In this special case of expanded mode and EME set, PORTE and DDRE
registers are removed from the on-chip memory map and become
external accesses so port E may be rebuilt externally.
Advance Information
MOTOROLA
Bus Control and Input/Output (I/O)
MC68HC812A4 — Rev. 3.0
95