I'm not so sure you're right about that, Jez ...

The Z80 was designed as a general=purpose processor, though, and not as a control-oriented processor. It had I/O instructions, all the registers of the 8085, plus an alternate set (... and you couldn't test to see which one you were using at the time!), plus a couple of additional index registers which few people used. The main reason few people used the entire resource set was in order to maintain "backward-compatibility with the 8080. The Z80 would execute any 8080 code that the user wanted to feed it. The Z80 was available in 1976, which was pretty early, and had ~156 (?) instructions. When last I looked, the 805x had >>200.

The Z80 had block-move instructions, by means of which a single instruction, with some setup, could move an entire 256-byte block of data from here to there, whether to/from I/O space or memory space, and, thanks to its nWAIT line, could synchronize those moves with an external device, e.g. a disk drive.

For its time, it was very useful. It made very efficient use of the available memory bandwidth, too, unlike, say, the Motorola 680x series, or the MOS-Technology 650x series. It was, after all, designed for general-purpose computing, as had previously been done with the likes of the PDP-8 or PDP-11, similar offerings form Data General or other vendors.

The Z80 was not a microcontroller, but, rather, was a general purpose CPU. While I believe it had more resources than one could practically use, due to its architecture, and the compromises that had to be made to maintain 8080-object-code-compatibility, it was the best the era had to offer, as witnessed in the marketplace.

RE