RAM
There are 512 bytes RAM built in MPC82x54A.
The user can visit the leading 128-byte RAM via direct addressing instructions, and we name
those RAM as direct RAM that occupies address space 00h to 7Fh.
Followed 128-byte RAM can be visited via indirect addressing instructions, and we name
those RAM as indirect RAM that occupied address space 80h to FFh.
There are extra 256 bytes RAM can be visited via MOVX @Ri or @DPTR instructions which
are named external RAM. When using MOVX @DPTR, the content in DPH is ignored. None
of P0 status and P2 status is affected during MOVX instruction.