Altera Nios II Custom - Manual (Page 30)

3–8 Chapter 3: Implementing a Nios II Custom Instruction in SOPC Builder
Using the User-defined Custom Instruction Macro
Nios II Custom Instruction User Guide January 2011 Altera Corporation
Using the User-defined Custom Instruction Macro
The design example software uses a user-defined macro to access the CRC custom
instruction.
Example 3–2 shows the macro that is defined in the ci_crc.c file.
This macro takes a single int type input operand and returns an int type value. The
CRC custom instruction has extended type; the n value in the macro CRC_CI_MACRO()
indicates the operation to be performed by the custom instruction. The custom
instruction index is added to the value of
n . The n value is masked because the n port
of a custom instruction has only three bits.
To initialize the custom instruction, for example, you can add the initialization code in
Example 3–3 to your application software.
For details of each operation of the CRC custom instruction and the corresponding
index value
n , refer to the comments in the ci_crc.c file.
Example 3–2 and Example 3–3 demonstrate that you can define the macro in your
application to accommodate your requirements. For example, you can determine the
number and type of input operands, decide whether to assign a return value, and
vary the custom instruction index. However, the macro definition and usage must be
consistent with the port declarations of the custom instruction. For example, if you
define the macro to return an
int value, the custom instruction must have a result
port.
f
For details about writing software for Nios II custom instructions, refer to Chapter 2,
Software Interface .
Example 3–2. CRC Custom Instruction Macro Usage
#define CRC_CI_MACRO(n, A) __builtin_custom_ini(ALT_CI_CRC_INST_N + (n & 0x7), (A))
Example 3–3. Using the User-defined Macro to Initialize the Custom Instruction Logic
/* Initialize the custom instruction CRC to the initial remainder value: */
CRC_CI_MACRO (0,0);