 Cx51 User's Guide | |
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Cx51 User's Guide
Cx51 Introduction Compiling Programs Language Extensions Preprocessor Advanced Programming Customization Files
Startup Code Variable Initialization Code XBANKING.A51 Basic I/O Memory Allocation Optimizer Optimizer Options General Optimizations 8051-Specific Optimizations Global Register Coloring Segment Naming Conventions Data Objects Program Objects Parameters and Local Variables Interfacing C to Assembler Global Variables Function Parameters Passing in Memory Passing in Registers Function Return Values Using the SRC Directive Register Usage Overlaying Segments Small Model Example Compact Model Example Large Model Example Interfacing C programs to PL/M-51 Data Storage Formats Bit Variables 1-Byte Scalars 2-Byte Scalars 4-Byte Scalars Generic and Far Pointers Floating-point Numbers Floating-point Errors Absolute Memory Locations Absolute Access Macros Linker Location Controls The _at_ Keyword Debugging 8051 Derivatives Error Messages Library Reference AppendixFunction Return Values
Function return values are always passed using MCU registers. The following table lists the possible return types and the registers used for each.
| Return Type | Register | Description |
|---|---|---|
| bit | Carry Flag | Single bit returned in the carry flag. |
| char, unsigned char, 1-byte pointer | R7 | Single byte type returned in R7. |
| int, unsigned int, 2-byte ptr | R6 & R7 | MSB in R6, LSB in R7. |
| long, unsigned long | R4-R7 | MSB in R4, LSB in R7. |
| float | R4-R7 | 32-Bit IEEE format. |
| generic pointer | R1-R3 | Memory type in R3, MSB R2, LSB R1. |
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