Greetings Everybody
I am currently working on Zilog Encore XP Z8F642X kit. For learing purposes I am using some documentation and its code for my kit. Not all codes are for my kit. Some are for microcontrollers such as F083A. My question is how zilog microcontrollers are backward compatible?
Thank you for your time.

Adam,
There is no simple answer to your question.
When staying within the global families (Encore/ZNEO/Acclaim), most of the compatibility issues remain with the different peripherals and registers. For example, the Z8F083A you mentioned has an Internal Precision Oscillator, where as the Z8F642X does not. The Z8F642X has UARTs where the Z8F083A does not. These are each designed for different types of applications. If you change from Encore to ZNEO or Acclaim, then there are many differences.