I think you're confusing instruction ROMs and instruction decode/sequencing ROMs.
What's being discussed is at the same level as microcode, whereas most of the distinctions you're making apply to the normal code storage ROMs. For instance modern microcode is nearly never a PROM.
Additionally combinatorial logic is generally made out of cheap gates, the two concepts aren't in conflict, but instead orthogonal.
What's being discussed is at the same level as microcode, whereas most of the distinctions you're making apply to the normal code storage ROMs. For instance modern microcode is nearly never a PROM.
Additionally combinatorial logic is generally made out of cheap gates, the two concepts aren't in conflict, but instead orthogonal.