Zilog ZUSBOPTS - Manual (Page 202)

Calling Assembly Functions from C UM017105-0511
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Zilog Developer Studio II – ZNEO™
User Manual
Special Cases
Some function calls do not follow the mechanism described in the Function Call Mecha-
nism section on page 172. Such cases are described in this section.
Returning Structure
If the function returns a structure, the caller allocates the space for the structure and then
passes the address of the return space to the called function as an additional and first argu-
ment. To return a structure, the called function then copies the structure to the memory
block pointed to by this argument.
Not Allocating a Local Frame
The compiler does not allocate a local stack frame for a function in the following case:
•
The function does not have any local stack variables, stack arguments, or compiler-
generated temporaries on the stack
and:
•
The function does not return a structure
and:
•
The function is compiled without the debug option.
Calling Assembly Functions from C
The ZNEO C-Compiler allows mixed C and assembly programming. A function written in
assembly can be called from C if the assembly function follows the C calling conventions
as described in the
Calling Conventions section on page 172.
This section covers the following topics:
•
Function Naming Convention
•
Argument Locations
•
Return Values
•
Preserving Registers
Function Naming Convention
Assembly function names must be preceded by an underscore ( _ ). The compiler prefixes
the function names with an underscore in the generated assembly. For example, a call to
myfunc() in C is translated to a call to _myfunc in assembly generated by the compiler.