Differences between SISD, SIMD and MIMD architecture (Flynn classification)
Michael J. Flynn is one of the all-time great computer architects.  He was the design manager of the IBM 360/91, and made numerous foundational contributions in the areas of pipelining, and in computer arithmetic.

But the Flynn Taxonomy has not stood the test of time, and is no longer useful.

The SI/MI part of the taxonomy is easy to differentiate on the surface: how many hardware instruction pointers (sometimes called program counters) does the machine have?  If there is one hardware instruction pointer the machine is "SI" (either SISD or SIMD).  If there is more than one hardware instruction pointer the machine is "MI" (either MISD or MIMD).

If you go a bit deeper, it becomes less clear though.  Does a vector machine with one instruction pointer but vector predicate registers count as SI or MI?  Why is a machine with only one fetch unit that can quickly (just a few cycles) switch back and forth between two different instruction pointers MI, but a machine with only one fetch unit that can context switch once ever few-hundred or thousand cycles considered SI?  Is a machine with two hardware instruction pointers, but that only context switches at long-latency cache misses SI or MI?

The SD vs. MD distinction is just a complete mess.  When Flynn made up the classification I believe he was referring to whether the machine had instructions that could operate on multiple independent pieces of data or not.  That does not mean that the operations happen simultaneously.  It could mean that the machine has instructions that operate on vectors of numbers, one at a time (or maybe pipelined), like the CDC STAR-100. The vector instructions in the CDC STAR-100 were pretty complex, giving two vectors start addresses to load, a start address for storing the result vector back to memory, and an arithmetic operation.  The vector lengths were controllable on the CDC STAR-100.  The CDC STAR-100 designers could have leveraged the implicit independence of the operations in the vector instructions to make some of those operations happen truly simultaneously, but they did not.

It may be more appropriate to talk about whether certain instructions in an instruction set architecture are "SIMD" or not.  The ARM NEON instructions are SIMD instructions, but most of the other ARM instructions are not SIMD.

I don't know whether the Z-80's copy instruction was SIMD or not.  I guess I'd call it SIMD because I can't think of any reason not to call it SIMD.