 Cx51 User's Guide | |
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Cx51 User's Guide
Cx51 Introduction Compiling Programs Language Extensions
Keywords Memory Areas Program Memory Internal Data Memory External Data Memory Far Memory SFR Memory Memory Models Small Model Compact Model Large Model Memory Types bdata code data far idata pdata 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 Generic Pointers Memory-Specific Pointers Pointer Conversions Abstract Pointers Function Declarations Parameters and the Stack Parameters and Registers Return Values Memory Models Register Banks Register Bank Access Interrupt Functions Reentrant Functions Alien (PL/M-51) Function Real-Time Function Tasks Preprocessor Advanced Programming 8051 Derivatives Error Messages Library Reference AppendixInternal Data Memory
Internal data memory resides within the 8051 MCU and is read/write. Up to 256 bytes of internal data memory are available depending upon the 8051 derivative. The first 128 bytes of internal data memory are both directly and indirectly addressable. The upper 128 bytes of data memory (from 0x80 to 0xFF) can be addressed only indirectly (this address space, when accessed directly, is mapped to SFRs). There is also a 16 byte area starting at 20h that is bit-addressable.
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 can be broken down into three distinct memory types: data, idata, and bdata.
- The data memory specifier always refers to the first 128 bytes of internal data memory. Variables stored here are accessed using direct addressing.
- The idata memory specifier refers to all 256 bytes of internal data memory; however, this memory type specifier code is generated by indirect addressing which is slower than 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.
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