The DS80C390 incorporates three internal memory areas:
§ 256 bytes of scratchpad (or direct) RAM
§ 4 KB of SRAM configurable as various combinations of MOVX data memory, stack memory, and
MOVC program memory
§ 512 bytes of RAM reserved for the CAN message centers.

Up to 4 MB of external memory is addressed via a multiplexed or demultiplexed 20-bit address bus/8-bit
data bus and four chip enable (active during program memory access) or four peripheral enable (active
during data memory access) signals.

Three different addressing modes are supported, as selected by the AM1, AM0 bits in the ACON SFR.

16-bit address mode
16-bit address mode accesses memory similarly to the traditional 8051. It is opcode compatible with the
8051 microprocessor and identical to the byte and cycle count of the Dallas Semiconductor High-Speed
Microcontroller family. A device operating in this mode can access up to 64 KB of program and data
memory. The device defaults to this mode following any reset.

22-bit paged address mode
The 22-bit paged address mode retains binary code compatibility with the 8051 instruction set, but adds
one machine cycle to the ACALL, LCALL, RET and RETI instructions with respect to the Dallas
Semiconductor High-Speed Microcontroller family timing. This is transparent to standard 8051
compilers. Interrupt latency is also increased by one machine cycle. In this mode, interrupt vectors are
fetched from 0000xxh.

22-bit contiguous address mode
The 22-bit contiguous addressing mode uses a full 22-bit program counter, and all modified branching
instructions automatically save and restore the entire program counter. The 22-bit branching instructions
such as ACALL, AJMP, LCALL, LJMP, MOV DPTR, RET and RETI instructions require an assembler,
compiler and linker that specifically supports these features. The INC DPTR is lengthened by one cycle
but remains byte count compatible with the standard 8051 instruction set.

Internally, the device uses a 22-bit program counter. The lowest order 22 bits are used for memory
addressing, with a special 23 rd bit used to map the 4KB SRAM above the 4 MB memory space in
bootstrap loader applications.