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251 Architecture

The 251 architecture is a superset of the classic 8051 architecture. The 251 is the most advanced variant and provides the following key features:

  • Completely code compatible with the standard 8051 microcontroller.
  • Powerful 8/16/32 bit instructions and flexible 8/16/32 bit registers.
  • 16MB linear address space and CPU support for 16-bit and 32-bit pointers.
  • True stack-oriented instructions with 16-bit stack pointer.

The following table shows the memory classes used for programming a 251 microcontroller. These memory classes are available when you are using the A251 macro assembler and the L251 linker/locater.

Memory Class Address Range Description
DATA 00:0000 00:007F Direct addressable on-chip RAM.
BIT 00:0020 00:002F 8051 compatible bit-addressable RAM; can be accessed with short 8-bit addresses.
IDATA 00:0000 00:00FF Indirect addressable on-chip RAM; can be accessed with @R0 or @R1.
EDATA 00:0000 – 00:FFFF Extended direct addressable memory area; can be accessed with direct 16-bit addresses available on the 251.
ECONST 00:0000 – 00:FFFF Same as EDATA - but allows the definition of ROM constants.
EBIT 00:0020 00:007F Extended bit-addressable RAM; can be accessed with the extended bit addressing mode available on the 251.
XDATA 01:0000 – 01:FFFF
(default space)
8051 compatible DATA space. Can be mapped on the 251 to any 64 KB memory segment. Accessed with MOVX instruction.
HDATA 00:0000 – FF:FFFF Full 16 MB address space of the 251. Accessed with MOV @DRK instructions. This space is used for RAM areas.
HCONST 00:0000 – FF:FFFF Same as HDATA - but allows the definition of ROM constants.
ECODE 00:0000 – FF:FFFF Full 16 MB address space of the 251; executable code accessed with ECALL or EJMP instructions.
CODE FF:0000 - FF:FFFF
(default space)
8051 compatible CODE space; used for executable code or RAM constants. Can be located with L251 to any 64 KB segment.
CONST FF:0000 – FF:FFFF
(default space)
Same as CODE – but can be used for ROM constants only.

Colons are used to improve the readability only.
The addresses are entered in the tools as numbers without the colon.

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