 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 AppendixParameters and Registers
The Cx51 Compiler passes up to three function arguments in MCU registers. This mechanism significantly improves system performance as arguments are not written to and read from memory. Argument or parameter passing can be controlled by the REGPARMS and NOREGPARMS directives.
The following table details the registers used for different argument positions and data types.
| Argument Number | char 1-byte ptr | int 2-byte ptr | long float | generic ptr |
|---|---|---|---|---|
| 1 | R7 | R6 & R7 | R4-R7 | R1-R3 |
| 2 | R5 | R4 & R5 | R4-R7 | R1-R3 |
| 3 | R3 | R2 & R3 | R1-R3 |
Note
- If the first parameter of a function is a bit type, then other parameters are not passed in registers. This is because parameters that are passed in registers are out of sequence with the numbering scheme shown above. For this reason, bit parameters should be declared at the end of the argument list.
- If no registers are available for argument passing, fixed memory locations are used for function parameters.
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