(1) SPROM non-secured mode (the last byte is 0xFF). The access behavior of SPROM is the same
with APROM and LDROM. All area can be read by CPU or ISP command, user can be erased and
programmed by ISP command.
(2) SPROM secured mode (the last byte is not 0xFF). In order to conceal SPROM code in secured
mode, CPU only can perform instruction fetch and get data from SPROM when CPU is run at SPROM
area. Otherwise, CPU will get all 00H for data access. In order to protect SPROM, the CPU instruction
fetch will also get zero value when ICE (OCD) port is connected in secured code. At this mode,
SPROM doesn’t support ISP program, read or erase.