Altera Nios II Custom Instrukcja Użytkownika Pobierz pdf (Strona 20)

2–4 Chapter 2: Software Interface

Custom Instruction Assembly Software Interface

Nios II Custom Instruction User Guide January 2011 Altera Corporation

Figure 2–1 shows a diagram of the custom instruction word.

Bits 5–0 are the Nios II custom instruction opcode. The opcode for a custom

instruction is 0x32.

f A list of opcodes appears in the “Instruction Opcodes” section in the Instruction Set

Reference chapter of the Nios II Processor Reference Handbook.

The

field, bits 13:6, is the custom instruction index. The custom instruction index

distinguishes between different custom instructions, allowing the Nios II processor to

support as many as 256 distinct custom instructions. Depending on the type of

custom instruction, the

field has one of the following functions:

■ A unique custom instruction index, for logic that implements a single custom

function

■ An extended custom instruction index, for logic that implements several custom

functions

Example 2–5 shows the assembly language syntax for the custom instruction.

In the assembly code instruction in Example 2–5,

is the custom instruction index,

is the destination for the

result[31:0]

port,

is the

dataa

port, and

is the

datab

port. To access the Nios II processor’s register file, replace

with

. To access a custom

with

. The use of

determines whether the custom

instruction has the

readra

readrb

, and

writerc

bits held high or low. Refer to

Figure 2–1 for the location of these three bits in the custom instruction low-level

format.

Example 2–6, Example 2–7, and Example 2–8 demonstrate the syntax for custom

instruction assembler calls.

Figure 2–1. Custom Instruction Format

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

uP OPCode = Custom

writerc

readrb

readra

A = Register index of operand A

B = Register index of operand B

C = Register index of operand C

N = 8-bit number that selects instruction

readra = 1 if instruction uses processor’s register rA, 0 otherwise

readrb = 1 if instruction uses processor’s register rB, 0 otherwise

writerc = 1 if instruction provides result for processor’s register rC, 0 otherwise

Instruction Fields:

Example 2–5. Custom Instruction Assembly Syntax

custom N, xC, xA, xB

Example 2–6. Assembly Language Call to Customer Instruction I

custom 0, r6, r7, r8

1 2 ... 15 16 17 18 19 20 21 22 23 24 25 ... 49 50

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