GNU Arm Embedded Toolchain | arm-none-eabi-gcc options: What is a difference between Thumb (-mthumb) and Arm (-marm) state?

Question

I have a maybe trivial question, but what is a difference between Thumb (-mthumb) and Arm (-marm) state and why most of the tutorials recommend to use Thumb state?

I am curious what exactly does it mean? What it is related to?

Best!

Answer 1

I would suggest to read those two articles, one from Arm, Instruction Set Architecture (-marm means that GCC will generate arm32/A32 code, -mthumb means that it will generate thumb/T32 one), and this research paper, Profile Guided Selection of ARM and ThumbInstructions .

Basically, the two instruction sets differ in the set of instructions available as well as their encoding. You should therefore get a smaller and faster executable by using thumb/T32 than by using arm/A32.

This is the reason why most of the tutorials recommend to use the thumb/T32 instruction set.