I totally get why you'd want mem-to-mem instructions, it just seemed more interesting in a blitting context (and others) to have the operation not be static but rather register-based. But perhaps in practice it matters not, been a long time since I wrote blitting code.
RISC-V already has what you want in terms of R-type instructions[1], ie "dest = src1 op src2" where "op" is effectively an immediate value, but being based on a load-store architecture it's only register-to-register of course.
Though I suppose ISA-wise there's nothing in the way of making an extension that introduces "M-type instructions" which act like the R-type instructions but are mem-to-mem instead. How much that messes with everything else I have no idea.
edit: ah, forgot about you wanting it to behave like movsb. Still, something that could be handled by the extension instruction.
But the “two instruction” part isn’t important there on most risc-v hardware, is it? That “memory to memory” instruction still routes the data through the CPU, so what does replacing a load instruction and a store instruction by a more complex but probably/likely slightly shorter load-store instruction bring if your CPU is much faster than your memory?
