Difference of core registers and peripheral registers

The difference has more to do with history and product differences than functionality. The early model(s) of the processor define the core architecture. This would include all the working registers for computing and maybe some I/O registers, if the original processor was an embedded type, such as ports 0 - 3 and timer 0 for an 8051 processor. Later models have more features. The added features would be described as peripheral features.

A good example of this, also based on the Intel 8051 architecture, is the ST Microelectronics Micro-PSD line of processors, which has an

8051 core, including 8051 I/O ports, plus added memory and ports mapped into the XDATA (external to original processor design) memory space, but still on chip. The way of addressing the original peripherals and added peripherals is different, because they wanted to maintain the core processor the same. Also, the added features were originally in a separate IC, now they are in the same package, but the technique of addressing them remains similar to the earlier split IC design.

What it means on a practical basis is that you might have separate documentation for the different sections of the processor. Also, if you integrate a previously separate IC, you might see vestiges of old features that are not supported in the combined chip, such as a separate selectable mode or I/O bit that is not relevant to the integrated part.

Thad