That's 24 bytes because gas is using Thumb2 mov.w instructions to initialize r1 and r2. It took me a long time to figure out that the problem was mov vs. movs;

That's 24 bytes because gas is using Thumb2 mov.w instructions to initialize r1 and r2. It took me a long time to figure out that the problem was mov vs. movs; this is the correct 20-byte version: .cpu cortex-m4 .thumb .syntax unified .thumb_func .globl fib fib: movs r1, #1 movs r2, #1 1: subs r0, #1 itt lo movlo r0, r1 bxlo lr mov r3, r2 adds r2, r1 mov r1, r3 b 1b I swear, I keep making the same n00b mistakes, year after year! It might be possible to get it down to 18 bytes, but I don't see how.