The makefile organization is displayed in the below box
The macro USE_THUMB_MODE is defined near to beginning of the file. The ARM966 implements the ARMv5TE architecture. To improve the compiled code density it provides the Thumb mode. The Thumb instruction set features a subset of the most commonly used 32-bit ARM instruction which have been compressed into 16-bit wide opcodes.
These steps explain how to compile some or all modules in Thumb mode: