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On-Line Manuals
Cx51 User's Guide
Cx51 Introduction
Compiling Programs
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Preprocessor
Advanced Programming
Customization Files
Startup Code
Variable Initialization Code
XBANKING.A51
Basic I/O
Memory Allocation
Optimizer
Optimizer Options
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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
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Absolute Memory Locations
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Linker Location Controls
The _at_ Keyword
Debugging
8051 Derivatives
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Library Reference
Appendix
Function 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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