Altera Nios II Custom - Manual (Page 18)

2–2 Chapter 2: Software Interface
Built-in Functions and User-defined Macros
Nios II Custom Instruction User Guide January 2011 Altera Corporation
Example 2–2 illustrates how most applications use custom instructions. The macros
defined by the Nios II software build tools use C integer types only. Occasionally,
applications require input types other than integers. In those cases, you can use a
custom instruction macro to process non-integer return values.
1 You can define custom macros for Nios II custom instructions that allow other 32-bit
input types to interface with custom instructions.
Built-in Functions and User-defined Macros
The Nios II processor uses gcc built-in functions to map to custom instructions. By
default, the integer type custom instruction is defined in a system.h file. However, by
using built-in functions, software can use non-integer types with custom instructions.
Fifty-two built-in functions are available to accommodate the different combinations
of supported types.
Built-in function names have the following format:
__builtin_custom_ <return type> n <parameter types>
Table 2–1 lists the 32-bit types supported by custom instructions as parameters and
return types, as well as the abbreviations used in the built-in function names.
Example 2–3 shows the prototype definitions for two built-in functions.
The built-in function __builtin_custom_nf takes a float as an input, and does not
return a value. In contrast, the built-in function __builtin_custom_fnp takes a pointer
as input, and returns a float .
To support non-integer input types, define macros with mnemonic names that map to
the specific built-in function required for the application.
f Refer to Appendix B, Custom Instruction Built-in Functions for detailed information
and a list of built-in functions.
Table 2–1. 32-bit Types Supported by Custom Instructions
32-bit Type Built-in Function Abbreviation
int i
float f
void * p
Example 2–3. Two Example Built-in Function Prototypes
void __builtin_custom_nf (int n, float dataa);
float __builtin_custom_fnp (int n, void * dataa);