Re: RISC/CISC Hack Architecture

Posted by WBahn on
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/RISC-CISC-Hack-Architecture-tp4034432p4036653.html

While I agree that Patterson and Hennessy are authorative voices in this area, I think that the context of their criteria need to be considered as well. They were trying to distinguish between real-world approaches to practical implementations of real-world architectures. So I don't know to what degree their criteria can be blindly applied to something like the Hack which is far from real-world or practical -- or arguably even complete as far as having something that you could drop into a physical design as is.

I think part of the context for their first two criteria was to contrast it with the CISC norm where you had a variety of instructions that all did the same basic operation but where one of them operated between two registers, while another operated between a register and memory, and so on and so forth. So I don't know that the underlying concept is really that RISC instructions must operate only on registers as it is that the architecture should strive for a single instruction for each operation.

Because of real-world practical considerations, the division of the world between registers and memory makes a lot of sense and having operations work only on one side of that divide and load/store instructions be the sole instructions that bridge the divide between registers and memory also makes a lot of sense in terms of simplifying architecture design. But I don't know how applicable that division is for the Hack since the architecture is able to ignore those real world practicalities and largely treat memory as nothing more than a big register space.

It would be very interesting to know what Patterson and/or Hennessy would have to say about where the Hack should sit on the RISC/CISC spectrum -- and their reasoning and to what degree they agreed with each other, for that matter. I suspect they would have a few observations that would be anything but obvious to the rest of us.