Altera Nios II C2H Compiler Manuel d'utilisateur télécharger pdf (Page 60)

3–20 9.1 Altera Corporation

Nios II C2H Compiler User Guide November 2009

Memory Accesses

Example 3–13 demonstrates the use of volatile to guarantee multiple,

distinct reads from a constant address.

Example 3–13. volatile Type Qualifier

volatile char *DataFIFO = FIFO_BASE;

char Byte0 = *DataFIFO;

char Byte1 = *DataFIFO;

char Byte2 = *DataFIFO;

char Byte3 = *DataFIFO;

By comparison, Example 3–14 demonstrates two sections of code that are

equivalent, due to the consolidation of equivalent pointers. In this case,

the type of *DataFIFO is not declared volatile.

Example 3–14. Equivalent Pointers

char *DataFIFO = FIFO_BASE;

char Byte0 = *DataFIFO;

char Byte1 = *DataFIFO;

char Byte2 = *DataFIFO;

char Byte3 = *DataFIFO;

// The code above is equivalent to the following:

char *DataFIFO = FIFO_BASE;

char dereferenced_DataFIFO = *DataFIFO;

char Byte0 = dereferenced_DataFIFO;

char Byte1 = dereferenced_DataFIFO;

char Byte2 = dereferenced_DataFIFO;

char Byte3 = dereferenced_DataFIFO;

Avalon-MM Master Port Signal Generation

A dereference operation, such as *(ptr_to_int + i), translates to an

Avalon-MM master port on the accelerator. This section describes how

hardware accelerator logic generates the signals that drive the master

port.

Avalon-MM master ports created by the C2H Compiler comprise the

following fundamental elements:

■ Logic to compute the address signal

■ For write transfers only, logic to compute the write-data signal

■ Logic to control the read-enable or write-enable signal

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Altera Nios II C2H Compiler Manuel d'utilisateur Page 60