Nios® II Processor Reference Guide

ID 683836
Date 10/22/2020
Public
Document Table of Contents

8.5.54. ldb / ldbio

Instruction load byte from memory or I/O peripheral
Operation

rB ←  σ(Mem8[rA + σ(IMM16)])

Assembler Syntax

ldb rB, byte_offset(rA)

ldbio rB, byte_offset(rA)

Example

ldb r6, 100(r5)

Description

Computes the effective byte address specified by the sum of rA and the instruction's signed 16-bit immediate value. Loads register rB with the desired memory byte, sign extending the 8-bit value to 32 bits. In Nios® II processor cores with a data cache, this instruction may retrieve the desired data from the cache instead of from memory.

Usage

Use the ldbio instruction for peripheral I/O. In processors with a data cache, ldbio bypasses the cache and is guaranteed to generate an Avalon® -MM data transfer. In processors without a data cache, ldbio acts like ldb.

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

Exceptions

Supervisor-only data address

Misaligned data address

TLB permission violation (read)

Fast TLB miss (data)

Double TLB miss (data)

MPU region violation (data)

Instruction Type

I

Instruction Fields

A = Register index of operand rA

B = Register index of operand rB

IMM16 = 16-bit signed immediate value

Table 95.  ldb
Bit Fields
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
A B IMM16
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
IMM16 0x07
Table 96.  ldbio
Bit Fields
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
A B IMM16
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
IMM16 0x27

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