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C251 User's Guide
C251 Introduction Compiling Programs Language Extensions Preprocessor 
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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 AppendixOverlaying Segments
If the overlay process is executed during program linking and locating, it is important that each assembler subroutine be declared with the PROC / ENDP assembler controls or have a unique program segment. This is necessary so that during the overlay process, the references between the functions are calculated, using the references of the individual segments.
The data areas of the assembler subprograms may be included in the overlay analysis when the following points are observed:
- All segment names must be created using the C251 Compiler segment naming conventions.
- Each assembler function with local variables must be assigned its own data segment. This data segment must be declared with the relocation type OVERLAYABLE. This data segment may be accessed by other functions only for passing parameters. Parameters must be passed in order.
Example Routines
The following program examples show how to pass parameters to and from assembly routines. The equivalent C source for the assembler program is printed as assembler comment.
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