Altera Nios II Custom Manuel d'utilisateur télécharger pdf (Page 9)

Chapter 1: Nios II Custom Instruction Overview 1–5

Custom Instruction Types

January 2011 Altera Corporation Nios II Custom Instruction User Guide

Table 1–2 describes the combination custom instruction ports.

The only required port for combinational custom instructions is the

result

port. The

dataa

and

datab

ports are optional. Include them only if the custom instruction

functionality requires input operands. If the custom instruction requires only a single

input port, use

dataa

Figure 1–4 shows the combinational custom instruction hardware port timing

diagram.

In Figure 1–4, the processor presents the input data on the dataa and datab ports on

the rising edge of the processor clock. The processor reads the result port on the

rising edge of the following processor clock cycle.

The Nios II processor issues a combinational custom instruction speculatively; that is,

it optimizes execution by issuing the instruction before knowing whether it is

necessary, and ignores the result if it is not required. Therefore, a combinational

custom instruction must not have side effects. In particular, a combinational custom

instruction cannot have an external interface.

You can further optimize combinational custom instructions by implementing the

extended custom instruction. Refer to “Extended Custom Instructions” on page 1–7.

Multicycle Custom Instructions

Multicycle or sequential, custom instructions consist of a logic block that requires two

or more clock cycles to complete an operation. Additional control ports are required

for multicycle custom instructions, as shown in Table 1–3.

Table 1–2. Combinational Custom Instruction Ports

Port Name Direction Required Description

dataa[31:0]

Input No Input operand to custom instruction

datab[31:0]

Input No Input operand to custom instruction

result[31:0]

Output Yes Result of custom instruction

Figure 1–4. Combinational Custom Instruction Timing Diagram

clk

T0 T1 T3T2 T4

dataa[ ]

datab[ ]

result[ ]

dataa[ ] valid

datab[ ] valid

result valid

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 49 50

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Altera Nios II Custom Manuel d'utilisateur Page 9

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