I'm going to spoil the argument because I agree with you :) An off-FPGA CPU will be the right choice in just about 100% of the designs; and for that rare case where you do need a soft core, there are plenty of commercial 8051 out there, all of which are better and more reliable than any homegrown one.

A core like this only makes sense as a hobbyst project. I made this one during a long spell of unemployment. It was fun and rewarding but the end result is flawed in many ways; It could have been much better given the development time available, but I built it at a laisurely pace without much thought for the end product. I was actually surprised when I finished it and only published it so that I could mention it in my resume, to be totally honest.

So this particular core is not a good case for the soft-vs-hard-CPU argument. However, as I said, I think soft CPUs do have their place: tightly integrated, deep into some custom logic design. Such a system with its CPU built in is self-contained and thus easier to port to other boards or reuse in other designs.

Another attractive use case might be building an interconnected crowd of small CPUs as an alternative to a large CPU running a multi-threaded application on a RTOS. This can be done more conveniently inside an FPGA (*really small* soft CPUs that can be programmed in C are hard to come by but that's another story).


In short, as someone said in this forum, a soft core will go into places a hard CPU will not. That is their biggest (and maybe only) advantage, I think.