I'd recommend you consult the datasheet, or "the bible" (reference materials located on this site).

The 805x architecture is a modified Harvard architecture, meaning that it has separate code and data spaces, which therefore permits concurrent accesses to both in the same machine instruction cycle.

It provides 64 kBytes of code space, which is accessed by its assertion of the nPSEN signal, and 64kBytes of esternal data space, accessed by means of the nRD and nWR signals. In addition, the standard configuration has up to 256 bytes of internal scratchpad RAM. No externally visible signals are presented for the purpose of accessing that memory block. Stack and registers reside within those 256 bytes.

Code space can not ordinarily be written under program control, and data space cannot be used to execute code.

Modern variants on this architecture sometimes, in some cases, violate these general rules.

RE