 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 AppendixExternal Data Memory
External data memory is read/write. Since external data memory is indirectly accessed through a data pointer register (which must be loaded with an address), it is slower than access to internal data memory.
Several 8051 devices provide on-chip XRAM space that is accessed with the same instructions as the traditional external data space. This XRAM space is typically enabled via dedicated chip configuration SFR registers and overlaps the external memory space.
There may be up to 64K Bytes of external data memory; though, this address space does not necessarily have to be used as memory. Your hardware design may map peripheral devices into the memory space. If this is the case, your program would access external data memory to program and control the peripheral. This technique is referred to as memory-mapped I/O.
The C51 Compiler offers two memory types that access external data: xdata and pdata.
- The xdata memory specifier refers to any location in the 64K Byte address space of external data memory. The large memory model locates variables in this memory space.
- The pdata memory type specifier refers to exactly one (1) page (256 bytes) of external data memory. The compact memory model locates variables in this memory space.
Note
- External data memory may be accessed indirectly using the MOVX instruction. It may not be accessed directly.
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