Nios® II Processor Reference Guide

ID 683836
Date 8/28/2023
Document Table of Contents

8.5.45. flushd

Instruction flush data cache line

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

Assembler Syntax

flushd IMM16(rA)


flushd -100(r6)


If the Nios® II processor implements a direct mapped data cache, flushd writes the data cache line that is mapped to the specified address back to memory if the line is dirty, and then clears the data cache line. Unlike flushda, flushd writes the dirty data back to memory even when the addressed data is not currently in the cache. 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 data cache line, flushd 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 flushd ignores the cache line tag, flushd flushes the cache line regardless of whether the specified data location is currently cached.
  • If the data cache line is dirty, write the line back to memory. A cache line is dirty when one or more words of the cache line have been modified by the processor, but are not yet written to memory.
  • Clear the valid bit for the line.

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


Use flushd to write dirty lines back to memory even if the addressed memory location is not in the cache, and then flush the cache line. By contrast, refer to “flushda flush data cache address”, “initd initialize data cache line”, and “initda initialize data cache address” for other cache-clearing options.

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



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 0x3b