Nios II Classic Processor Reference Guide

ID 683620
Date 10/28/2016
Document Table of Contents

8.5.49. initd

Instruction initialize data cache line

Initializes the data cache line associated with address rA + σ(IMM16).

Assembler Syntax

initd IMM16(rA)


initd 0(r6)


If the Nios® II processor implements a direct mapped data cache, initd clears the data cache line without checking for (or writing) a dirty data cache line that is mapped to the specified address back to memory. Unlike initda, initd clears the cache line regardless of whether the addressed data is currently cached. This process comprises the following steps:

  • Compute the effective address specified by the sum of rA and the signed 16-bit immediate value.
  • Identify the data cache line associated with the computed effective address. Each data cache effective address comprises a tag field and a line field. When identifying the line, initd ignores the tag field and only uses the line field to select the data cache line to clear.
  • Skip comparing the cache line tag with the effective address to determine if the addressed data is currently cached. Because initd ignores the cache line tag, initd flushes the cache line regardless of whether the specified data location is currently cached.
  • Skip checking if the data cache line is dirty. Because initd skips the dirty cache line check, data that has been modified by the processor, but not yet written to memory is lost.
  • Clear the valid bit for the line.

If the Nios® II processor core does not have a data cache, the initd instruction performs no operation.


Use initd after processor reset and before accessing data memory to initialize the processor’s data cache. Use initd with caution because it does not write back dirty data. By contrast, refer to “flushd flush data cache line”, “flushda flush data cache address”, and “initda initialize data cache address” for other cache-clearing options. Intel recommends using initd only when the processor comes out of reset.

For more information on data cache, refer to the Cache and Tightly Coupled Memory chapter of the Nios II Software Developer’s Handbook.


Supervisor-only instruction

Instruction Type


Instruction Fields

A = Register index of operand rA

IMM16 = 16-bit signed immediate value

Bit Fields
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
A 0 IMM16
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
IMM16 0x33