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8051 Instruction Set Manual
Architecture Overview Memory Classes and Layout 
Classic 8051 Classic 8051 Memory Layout Extended 8051 Variants Extended 8051 Memory Layout NXP 80C51MX NXP 80C51MX Memory Layout 251 Architecture 251 Memory Layout CPU Registers 8051 Variants 251 Architecture Program Status Word (PSW) Opcodes 
Instructions251 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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