MPLAB C Compiler for PIC24 MCUs and dsPIC DSCs ... - Microchip

MPLAB ® C COMPILER FOR
PIC24 MCUs AND dsPIC ® DSCs
USER’S GUIDE
Chapter 9. Mixing Assembly Language and C Modules
9.1 INTRODUCTION
9.2 HIGHLIGHTS
This section describes how to use assembly language and C modules together. It gives
examples of using C variables and functions in assembly code and examples of using
assembly language variables and functions in C.
Items discussed in this chapter are:
• Mixing Assembly Language and C Variables and Functions – Separate
assembly language modules may be assembled, then linked with compiled C
modules.
• Using Inline Assembly Language – Assembly language instructions may be
embedded directly into the C code. The inline assembler supports both simple
(non-parameterized) assembly language statement, as well as extended
(parameterized) statements, where C variables can be accessed as operands of
an assembler instruction.
9.3 MIXING ASSEMBLY LANGUAGE AND C VARIABLES AND FUNCTIONS
The following guidelines indicate how to interface separate assembly language
modules with C modules.
• Follow the register conventions described in Section 4.12 “Register
Conventions”. In particular, registers W0-W7 are used for parameter passing. An
assembly language function will receive parameters, and should pass arguments
to called functions, in these registers.
• Functions not called during interrupt handling must preserve registers W8-W15.
That is, the values in these registers must be saved before they are modified and
restored before returning to the calling function. Registers W0-W7 may be used
without restoring their values.
• Interrupt functions must preserve all registers. Unlike a normal function call, an
interrupt may occur at any point during the execution of a program. When returning
to the normal program, all registers must be as they were before the interrupt
occurred.
• Variables or functions declared within a separate assembly file that will be
referenced by any C source file should be declared as global using the assembler
directive.global. External symbols should be preceded by at least one
underscore. The C function main is named _main in assembly and conversely an
assembly symbol _do_something will be referenced in C as do_something.
Undeclared symbols used in assembly files will be treated as externally defined.
The following example shows how to use variables and functions in both assembly
language and C regardless of where they were originally defined.
The file ex1.c defines foo and cVariable to be used in the assembly language file.
The C file also shows how to call an assembly function, asmFunction, and how to
access the assembly defined variable, asmVariable.
© 2008 Microchip Technology Inc. DS51284H-page 119