Altera Nios II C2H Compiler Manuel d'utilisateur télécharger pdf (Page 56)

3–16 9.1 Altera Corporation

Nios II C2H Compiler User Guide November 2009

Memory Accesses

behavior of a C function accessing memory is the same, regardless of

whether the function is implemented as hardware logic or software

instructions.

f For more information on SOPC Builder, Avalon interfaces, and how

SOPC Builder generates system interconnect fabric, refer to the

Quartus II Handbook, volume 4: SOPC Builder and the Avalon Memory-

Mapped Interface Specification.

In order to maximize bandwidth, the C2H Compiler creates a master port

on the accelerator for every C operator that accesses external memory.

Multiple master ports allow the accelerator to read and write data to an

unlimited number of locations simultaneously, thereby reducing the

bandwidth limitations inherent in a CPU with a single data master port.

In some cases, the C2H Compiler can determine that master ports can be

shared between several external memory operations without sacrificing

performance. However, as a general rule, an Avalon-MM master port is

created for each of the following:

■ Pointer dereference (* operator)

■ Index into an array ([ operator)

■ Index into a struct or union (. or -> operator)

■ Usage of a global or static variable

Example 3–8 demonstrates various lines of code that generate a master

port in hardware.

Example 3–8. C Statements that Generate Avalon-MM master ports

*my_ptr = 8;

data_in = *src;

dst[index] = data_out;

pixel = pixel_array[i][j];

buffer.input = 0x80000400;

current = s->next;

The following sections describe each case in detail.

Indirection Operator (Pointer Dereference)

The indirection operator (*) is the fundamental expression of

dereferencing and indirection. This section describes how the

C2H Compiler handles pointer dereferencing.

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Altera Nios II C2H Compiler Manuel d'utilisateur Page 56