IT block for conditional execution (Thumb-2) in ARM Architecture

Introduction

The IT block lets you run up to four instructions only if certain conditions are true, without using jumps. This makes your code smaller and faster.

When you want to run a few instructions only if a condition is met, like checking if a number is positive.

When you want to avoid using multiple jump instructions to keep code compact.

When writing small decision-making code in embedded systems with limited memory.

When you want to improve performance by reducing branching overhead.

When you want to keep your code easier to read by grouping conditional instructions.

Core Concept

ARM Architecture

IT{x{y{z}}} condition

IT stands for "If-Then" and starts the block.

Up to 4 instructions can be conditionally executed after the IT instruction.

Key Points

Run the MOVE instruction only if the Equal (EQ) condition is true.

ARM Architecture

IT EQ
MOVEQ R0, #1

Run two instructions if Not Equal (NE) condition is true.

ARM Architecture

ITT NE
MOVNE R1, #5
MOVNE R2, #10

Run three instructions with mixed conditions: first if Greater Than (GT), next two if Then Else (TE).

ARM Architecture

ITTE GT
MOVGT R3, #3
MOVTE R4, #4
MOVTE R5, #5

Detailed Explanation

This code compares R0 with zero. If R0 is greater or equal (GE), it sets R1 to 1. Otherwise, it sets R1 to 0.

ARM Architecture

CMP R0, #0
IT GE
MOVGE R1, #1
MOVLT R1, #0

Important Notes

The IT instruction affects only the next 1 to 4 instructions.

Each instruction after IT can have a condition suffix like EQ, NE, GT, LT, etc.

Using IT blocks helps reduce the number of branch instructions, making code faster and smaller.

Summary

IT blocks let you run a few instructions conditionally without jumps.

They improve code size and speed in Thumb-2 assembly.

Use IT with conditions like EQ, NE, GT, LT to control execution.