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:

1. Look the Makefile for the USE_THUMB_MODE macro, and define it to YES.
2. Add to the THUMB_SOURCE all source file to be compiled in Thumb mode.
3. Add to the ARM_SOURCE all source file to be compiled in ARM mode.
4. The following files are to be compiled in ARM mode:
• 91x_it.c
• $(RTOS_SOURCE_DIR)/portable/GCC/STR9x/port.c
• $(RTOS_SOURCE_DIR)/portable/GCC/STR9x/portISR.c
  
5. That is all.