[crest]

There are lots of good reasons to make cryptographic operations instructions instead of a memory mapped peripheral, but I prefer something like VIA padlock which implemented cipher modes instead of just implementing the round function as instruction. Any implementation could even trap those and implement them in a peripheral. The problem with memory mapped peripherals is that access to them has to be multiplexed and their state preserved by context switches. Specialized instruction on existing registers avoid this problem. VIA padlock solved it by piggybacking on the existing x86 REP prefix for interruptible string instructions and only cached the cipher round keys in the crypto unit reloading them from memory (or repeating the key schedule) after a context switch.