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On-Line Manuals
C251 User's Guide
C251 Introduction
Compiling Programs
Language Extensions
Preprocessor
Advanced Programming
Customization Files
Startup Code
Variable Initialization Code
Basic I/O
Memory Allocation
Optimizer
General Optimizations
251-Specific Optimizations
Options for Code Generation
Global Register Coloring
Segment Naming Conventions
Segment Types
Initialization Segments
Function Symbols
Parameter Passing Symbols
Hardware Stack
Calculating Stack Size
Interfacing C Programs to Assembler
Global Variables
Function Parameters
Passing in Memory
Passing in Registers
Function Return Values
Using the SRC Directive
Register Usage
Overlaying Segments
TINY Model
XTINY Model
SMALL Model
XSMALL Model
LARGE Model
Data Storage Formats
Bit Variables
1-Byte Scalars
2-Byte Scalars
4-Byte Scalars
Floating-point Numbers
Floating-point Errors
Double Numbers
Double Errors
Absolute Memory Locations
Absolute Access Macros
Linker Location Controls
The _at_ Keyword
Debugging
Error Messages
Library Reference
Appendix
C251 User's Guide
Register Usage
In an assembler subroutine, the following registers may be used without prior saving of their contents: R0 to R11, WR16 to WR30, PSW, PSW1, and DPTR. DR60 is the STACK POINTER; at the end of the assembler subroutine the value of DR60 must be the same as at the entry of the routine. If the subroutine uses the registers R12 to R15, the contents of the registers must be saved and restored at return to the calling C function.
When calling a C function from assembler, it must be assumed that the register contents of R0 to R11, WR16 to WR30, PSW, PSW1, and DPTR are destroyed during the processing of the C function. The PSW flag UD is not used in C251 code and may be freely used.
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