Nios II Classic Processor Reference Guide

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

8.5.90. sub

Instruction subtract

rC ←  rA – rB

Assembler Syntax

sub rC, rA, rB


sub r6, r7, r8


Subtract rB from rA and store the result in rC.


Carry Detection (unsigned operands):

The carry bit indicates an unsigned overflow. Before or after a sub operation, a carry out of the MSB can be detected by checking whether the first operand is less than the second operand. The carry bit can be written to a register, or a conditional branch can be taken based on the carry condition. Both cases are shown in the following code:

sub rC, rA, rB

cmpltu rD, rA, rB

sub rC, rA, rB

bltu rA, rB, label

# The original sub operation (optional)

# rD is written with the carry bit

# The original sub operation (optional)

# Branch if carry generated

Overflow Detection (signed operands):

Detect overflow of signed subtraction by comparing the sign of the difference that is written to rC with the signs of the operands. If rA and rB have different signs, and the sign of rC is different than the sign of rA, an overflow occurred. The overflow condition can control a conditional branch, as shown in the following code:

sub rC, rA, rB

xor rD, rA, rB

xor rE, rA, rC

and rD, rD, rE

blt rD, r0, label

# The original sub operation

# Compare signs of rA and rB

# Compare signs of rA and rC

# Combine comparisons

# Branch if overflow occurred



Instruction Type


Instruction Fields

A = Register index of operand rA

B = Register index of operand rB

C = Register index of operand rC

Bit Fields
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
A B C 0x39
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0x39 0 0x3a