I did a lot of that on my CPC464 and TRS-80 too. Thanks for rekindling the memory. Relative jumps were the easiest way to make code relocatable, but you were limited to +/- 128 for an offset.

I used the "Programming the Z-80" book by Rodney Zaks, which is still on my bookshelf. Some manuals just can't be disposed of!

From memory I recall my favourite instruction - LDIR (Z80 block memory move) is EDB0, and JMP is CA.

Slightly more on topic, I piggybacked a 2716 EPROM onto the back of the TRS-80 ROM so I could get to the previous hard-coded RST locations and interrupt vector of the Z80 processor. In a nutshell, that allowed me to stop/debug/hack running programs without software being able to prevent me.

Fun times.