UPD784938 Datasheet, PDF (605/733 Pages) NEC – 16-Bit Single-Chip Microcontrollers

CHAPTER 24 LOCAL BUS INTERFACE FUNCTION
24.1.2 Memory map with external memory expansion
The memory map when memory expansion is used is shown in Figures 24-2 and 24-3. External devices at the same
addresses as the internal ROM area, internal RAM area and SFR area (excluding the external SFR area (0FFD0H to
0FFDFH)) cannot be accessed. If an access is made to these addresses, the memory or SFR in the µPD784938 has access
priority and no ASTB signal, RD signal, or WR signal is output (these pins remain at the inactive level). The address bus
output level remains at the level output prior to this, and the address/data bus output becomes high-impedance.
Except in 1-Mbyte expansion mode, the address output externally is output with the upper part of the address specified
by the program masked.
Example 1:
In 256-byte expansion mode, when address 54321H is accessed by the program, the output address is 21H.
Example 2:
In 256-byte expansion mode, when address 67821H is accessed by the program, the output address is 21H.
Preliminary User’s Manual U13987EJ1V0UM00
605