 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 AppendixPointers
The Cx51 Compiler supports the declaration of variable pointers using the * character. The Cx51 Compiler pointers may be used to perform all operations available in standard C. However, because of the unique architecture of the 8051 and its derivatives, the Cx51 Compiler provides two different types of pointers:
Comparison: Memory Specific & Generic Pointers
You can significantly accelerate an 8051 C program by using memory specific pointers. The following sample program shows the differences in code & data size and execution time for various pointer declarations.
| Description | idata Pointer | xdata Pointer | Generic Pointer |
|---|---|---|---|
| Sample Program | char idata *ip; char val; val = *ip; | char xdata *xp; char val; val = *xp; | char *p; char val; val = *p; |
| 8051 Program Code Generated | MOV R0,ip MOV val,@R0 | MOV DPL,xp +1 MOV DPH,xp MOV A,@DPTR MOV val,A | MOV R1,p + 2 MOV R2,p + 1 MOV R3,p CALL CLDPTR |
| Pointer Size Code Size Execution Time | 1 byte 4 bytes 4 cycles | 2 bytes 9 bytes 7 cycles | 3 bytes 11 bytes + library call 13 cycles |
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