An 8080 has what, 40 opcodes? All doing simple things like "add" and "mov".

https://altairclone.com/downloads/manuals/8080%20Programmers...

I learned on an 8080 system (Interact) that also had MS basic. Almost every one of 256 op-codes is used on the 8080. That's not to say there were that many instructions. For example register-to-register move might be considered one instruction but the source and destination register are encoded into that one byte so there are many reg-reg moves. Same with add sub, etc... My father wrote a disassembler in basic to read out the ROM and he made his own mnemonics which amounted to probably 20-40 actual instructions (I still have it and could look it up), but there were maybe 8 entries in the opcode table that did nothing.

If we exclude addressing mode then I think there are less than 26 instructions. Our mnemonic format was one letter (verb/op), one for source (a,b,c,d,e,h,l,w,x,y) and one for destination. I dont recall the letter for immediate, probably i but MIA for move immediate to a doesnt ring a bell for me. :-)

Yes, around 8 spare opcodes, and they were the ones Zilog used to escape into a large set of extensions for the (backwardly compatible to 8080) Z80.

Can a Z80 be dropped into an 8080A socket? I'm thinking there were hardware differences.

No absolutely not. 8080 was a 3 chip solution from memory. The 8085 was, like a Z80, a 40 pin DIP, but even here the compatibility was strictly software not hardware. And in fact the software compatibility was a little less than 100% because Zilog decided to add an overflow flag by changing the 8080/8085 parity flag to actually reflect overflow instead of parity on arithmetic instructions.