Sometimes it is necessary to write an interrupt service routine in assembly language. This is the case as well for the ARM Cortex-M0+ which is found in the KL25Z on my Freedom board. But there is something important about the ARM Cortex architecture: Thumb Mode.
Thumb mode the ‘ARM way’ to reduce the code size with a reduced (16bit wide) instruction set. The ARM architecture can implement a ‘mixed’ mode, on a function level. To distinguish between ‘normal’ ARM functions and ‘thumb’ functions, the processor is checking if the LSB (Least Significant Bit) of a function pointer (or function call destination) is set. So a jump address of 0x410 is for a ‘normal’ function, while a function jump to the address 0x411 (even if the function is located at the address 0x410) denotes a ‘thumb’ function.
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MCU on Eclipse 