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C251 User's Guide
C251 Introduction Compiling Programs 
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Keywords Memory Areas Program Memory Internal Data Memory External Data Memory Near Memory Far Memory Huge Memory SFR Memory Memory Models Tiny Model XTiny Model Small Model XSmall Model Large Model Memory Types Implicit Memory Types bdata code data ebdata far huge idata near xdata Data Types Special Function Registers sfr sfr16 sbit Bit-Addressable Objects bit Standard C Types void char int enum float double Type Modifiers signed unsigned short long Type Qualifiers const volatile Storage Classes auto register extern static Absolute Variable Location Pointers Memory Type in Pointer Declarations Pointer Conversions Function Declarations Memory Models Register Banks Interrupt Functions Reentrant Functions Alien (PL/M-51) Function Real-Time Function Tasks Preprocessor Advanced Programming Error Messages Library Reference AppendixInternal Data Memory
Internal data memory resides within the 251 MCU and is read/write. The data and idata memory areas reside within the 251 MCU and can be read and written. The data memory are the first 128 bytes of the on chip memory. The idata memory are the first 256 bytes of the on chip memory. There is also a 16 byte area starting at 20h that is bit addressable with short 8051 compatible instructions.
Access to internal data memory is very fast because it can be accessed using an 8-bit address. However, internal data memory is limited to a maximum of 256 bytes.
Internal data memory may be broken down into three distinct data types: data, bdata, and ebdata.
- The data memory specifier always refers to the first 128 bytes of internal data memory. Variables stored here are accessed using direct addressing.
- The bdata memory specifier refers to the 16 bytes of bit-addressable memory in the internal data area (20h to 2Fh). This memory type specifier allows you to declare data types that may also be accessed at the bit level.
- The ebdata memory specifier refers to the extended 251 bit addressable memory in the internal data area (20H to 7FH). This memory type specifier allows you to declare data types that can also be accessed at the bit level. However compared to the bdata memory area, bit access to ebdata memory requires a longer opcode encoding.
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