The One-Page CPU

2^9 (512) registers. Isn't that a bit overkill?

Only reason to have lots of registers is so compiler can extract more instruction-level parallelism. But 512 is way past the amount needed for significant benefits.

Downside of lots of registers means that the multiplexers for register bank will have to be really big, thus slow, and that will dramatically limit the clock speed.

Another downside is that instructions are unnecessarily large. That causes unnecessary pressure on the instruction cache.