Altera Nios II Custom Instrukcja Użytkownika Pobierz pdf (Strona 19)

Chapter 2: Software Interface 2–3

Custom Instruction Assembly Software Interface

January 2011 Altera Corporation Nios II Custom Instruction User Guide

Example 2–4 shows user-defined custom instruction macros used in an application.

On lines 2 through 6, the user-defined macros are declared and mapped to the

appropriate built-in functions. The macro UDEF_MACRO1 takes a float as an input

parameter and does not return anything. The macro UDEF_MACRO2 takes a pointer as

an input parameter and returns a float. Lines 14 and 15 show code that uses the two

user-defined macros.

Custom Instruction Assembly Software Interface

The Nios II custom instructions are also accessible in assembly code. This section

describes the assembly interface.

Custom instructions are R-type instructions, containing:

■ A 6-bit opcode

■ Three 5-bit register index fields

■ Three 1-bit fields for the

readra

readrb

, and

writerc

signals

■ An 8-bit

field, used for the custom instruction index (opcode extension), and

optionally including a function select subfield

Example 2–4. Custom Instruction Macro Usage Example

1. /* define void udef_macro1(float data); */

2. #define UDEF_MACRO1_N 0x00

3. #define UDEF_MACRO1(A) __builtin_custom_nf(UDEF_MACRO1_N, (A));

4. /* define float udef_macro2(void *data); */

5. #define UDEF_MACRO2_N 0x01

6. #define UDEF_MACRO2(B) __builtin_custom_fnp(UDEF_MACRO2_N, (B));

8. int main (void)

9. {

10. float a = 1.789;

11. float b = 0.0;

12. float *pt_a = &a;

13.

14. UDEF_MACRO1(a);

15. b = UDEF_MACRO2((void *)pt_a);

16. return 0;

17. }

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Altera Nios II Custom Instrukcja Użytkownika Strona 19