> A subset of an ISA will be incompatible with the full ISA and therefore be a new ISA. No existing software will run on it. So this won't really help anyone.

> A subset of an ISA will be incompatible with the full ISA and therefore be a new ISA. No existing software will run on it. So this won't really help anyone. This isn't an issue in any way. Vendors have been routinely taking out rarely used instructions from the hardware and simulating them in the software for decades as part of the ongoing ISA revision. Unimplemented instruction opcodes cause a CPU trap to occur where the missing instruction (s) is then emulated in the kernel's emulation layer. In fact, this is what was frequently done for «budget» 80[34]86 systems that lacked the FPU – it was emulated. It was slow as a dog but worked.