Tonight I completed chapter 4 of the Mastering Machine Code on your Spectrum. This section dealt with "POKE" commands in machine code, and introduced the LDI, LDD, and LDIR op-codes. LDIR is especially interesting as it uses BC explicitly as a counter for memory moves. It's a pretty primitive way of copying blocks of memory but prior to the invention of blitters it was likely the preferred way to copy blocks of memory quickly.

Ever onward.