DS87C530/DS83C530 EPROM/ROM Microcontrollers with Real-Time Clock
device will immediately jump to external program execution because program code from 4kB to 16kB (1000h–3FFFh) is no longer located on-chip. This could result in code misalignment and execution of an invalid instruction. The recommended method is to modify the ROMSIZE register from a location in memory that will be internal (or external) both before and after the operation. In the above example, the instruction which modifies the ROMSIZE register should be located below the 4kB (1000h) boundary, so that it will be unaffected by the memory modification. The same precaution should be applied if the internal program memory size is modified while executing from external program memory.
Off-chip memory is accessed using the multiplexed address/data bus on P0 and the MSB address on P2. While serving as a memory bus, these pins are not I/O ports. This convention follows the standard 8051 method of expanding on-chip memory. Off-chip ROM access also occurs if the EA pin is a logic 0. EA overrides all bit settings. The PSEN signal will go active (low) to serve as a chip enable or output enable when Ports 0 and 2 fetch from external ROM.
Figure 4. ROM Memory Map
DATA MEMORY ACCESS
Unlike many 8051 derivatives, the DS87C530/DS83C530 contain on-chip data memory. The devices also contain the standard 256 bytes of RAM accessed by direct instructions. These areas are separate. The MOVX instruction accesses the on-chip data memory. Although physically on-chip, software treats this area as though it was located off-chip. The 1kB of SRAM is between address 0000h and 03FFh.
Access to the on-chip data RAM is optional under software control. When enabled by software, the data SRAM is between 0000h and 03FFh. Any MOVX instruction that uses this area will go to the on-chip RAM while enabled. MOVX addresses greater than 03FFh automatically go to external memory through Ports 0 and 2.
When disabled, the 1kB memory area is transparent to the system memory map. Any MOVX directed to the space between 0000h and FFFFh goes to the expanded bus on Ports 0 and 2. This also is the default condition. This default allows the DS87C530/DS83C530 to drop into an existing system that uses these addresses for other hardware and still have full compatibility.