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A251 Introduction

This manual describes the macro assembler for the 251 microcontroller family and explains the process of developing software in assembly language for these microcontroller families.

A brief overview of the classic 8051, the extended 8051, and the 251 architectures may be found in the Instruction Set User's Guide, Architecture Overview. In this overview, the differences between the classic 8051, the extended 8051 variants and the 251 processors are described. For the most complete information about the microcontroller architecture refer to the hardware reference manual of the microcontroller derivative that your are using.

For optimum support of the different 8051 and 251 variants, Keil provides the following development tools:

Development Tools Support Microcontrollers Description
A51 Macro Assembler
BL51 Linker/Locater
LIB51 Library Manager
Development Tools for classic 8051.
Includes support for 32 x 64KB code banks.
AX51 Macro Assembler
LX51 Extended Linker/Locater
LIBX51 Library Manager
Development Tools for classic and extended 8051 versions (NXP 80C51MX, Dallas 390, etc.) Supports up to 16MB code and xdata memory.
A251 Macro Assembler
L251 Linker/Locater
LIB251 Library Manager
Development Tools for the 251.

The AX51 and A251 assemblers are supersets of the A51 assembler. This user's guide only covers the 251 development tool variant. However, for general reference to all tool variants and microcontroller architectures the terms listed in the following table are used:

Term Refers to ...
A*51 Macro Assembler A51, AX51 and A251 Macro Assembler .
C*51 Compiler C51, CX51 and C251 ANSI C Compiler.
L*51 Linker/Locator BL51, LX51 and L251 Linker/Locator.
LIB*51 Library Manager LIB51, LIBX51 and LIB251 Library Manager.
OH*51 Object-Hex Converter OH51, OHX51 and OH251 Object-Hex Converter.
*51 Architecture or *51 Device All classic 8051, extended 8051 and 251 device variants.
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