Hi,
Is your HCS12 code written in the C programming language?
You are in for a decent amount of work to port your application code. You will have a different memory map, and the registers will be different. The peripherals will also need to be revisited, as the basic functionality may be similar (ADC, Timer, ...) but the registers will most likely be different. If you have application code, written in C, that does not deal with any registers, it may port pretty cleanly if you make sure the data type sizes match. On the upside, you have a flat memory model with 32 bit address space and 32 bit data bus on the ColdFire V2, V3 & V4 processors.
There are some tools available in CodeWarrior which can help generate code for your project. A rapid application development tool called Processor Expert is included with CodeWarrior for ColdFire v7.0, and can generate C code for many of the processors in the ColdFire family.
Thanks,
Rick