| ▲ | rocqua 2 hours ago | |
There's also a lot of specialized instructions like AES ones. But the main thing that makes x86 CISC to me is not the actual instruction set, but the byte encoding, and the complexity there. | ||
| ▲ | 201984 2 hours ago | parent | next [-] | |
The classic distinction is that a CISC has data processing instructions with memory operands, and in a RISC they only take register parameters. This gets fuzzy though when you look at AArch64 atomic instructions like ldadd which do read-modify-write all in a single instruction. | ||
| ▲ | Joker_vD 2 hours ago | parent | prev [-] | |
Eh, that's really just a side effect of almost 50 years of constant evolution from a 8-bit microprocessor. Take look at VAX [0], for instance: its instruction encoding is pretty clean yet it's an actual example of a CISC ISA that was impossible to speed up like, literally: DEC engineers tried very hard and concluded that making a truly pipelined & super-scalar implementation was basically impossible; so DEC had to move to Alpha. See [1] for more from John Mashey. Edit: the very, very compressed TL;DR is that if you do only one memory load (or one memory load + store back into this exact location) per instruction, it scales fine. But the moment you start doing chained loads, with pre- and post-increments which are supposed to write back changed values into the memory and be visible, and you have several memory sources, and your memory model is actually "strong consistency", well, you're in a world of pain. | ||