Altera Nios II Custom Instrukcja Użytkownika Pobierz pdf (Strona 12)

1–8 Chapter 1: Nios II Custom Instruction Overview

Custom Instruction Types

Nios II Custom Instruction User Guide January 2011 Altera Corporation

Figure 1–7 is a block diagram of an extended custom instruction with bit-swap,

byte-swap, and half-word swap operations.

The custom instruction in Figure 1–7 performs swap operations on data received at

the

dataa

port. It uses the two-bit-wide

port to select the output from a multiplexer,

determining which result is presented to the

result

port.

1 This logic is just a simple example, using a multiplexer on the output. You can

implement function selection based on an index in any way that is appropriate for

your application.

Extended custom instructions can be combinational or multicycle custom

instructions. To implement an extended custom instruction, simply add an n port to

your custom instruction logic. The bit width of the

port is a function of the number

of operations the custom logic block can perform.

Extended custom instructions occupy multiple custom instruction indices. For

example, the custom instruction illustrated in Figure 1–7 occupies 4 indices, because

is two bits wide. Therefore, when this instruction is implemented in a Nios II system,

256 - 4 = 252 available indices remain.

For information about the custom instruction index, refer to “Custom Instruction

Assembly Software Interface” on page 2–3.

All extended custom instruction port operations are identical to those for the

combinational and multicycle custom instructions, with the exception of the

port,

which is not present in combinational and multicycle custom instructions. The

port

timing is the same as that of the

dataa

port. For example, for an extended variable

multicycle custom instruction, the processor presents the index value to the

port on

the same rising edge of the clock at which

start

is asserted, and the

port remains

stable during execution of the custom instruction.

Figure 1–7. Extended Custom Instruction with Swap Operations

dataa[31..0]

n[1..0]

result[31..0]

bit-swap

operation

byte-swap

operation

half-word-swap

operation

Custom

Instruction

1 2 ... 7 8 9 10 11 12 13 14 15 16 17 ... 49 50

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