Hah, we commented on the exact same paragraph within a minute of each other! My memory agrees with your memory, although I think that should be 3E 00. Let me look that up:
One of the random things burned into my memory for 6502 assembly is that LDA is $A9. I never separated the instruction from the register; it's not like they were general purpose. But that might be because I learned programming from the 2 books that came with my C64, a BASIC manual and a machine code reference manual, and that's how they did it.
I learned assembly programming by reading through the list of supported instructions. That, and typing in games from Compute's Gazette and manually disassembling the DATA instructions to understand how they worked. Oh, and the zero-page reference.
> One of the random things burned into my memory for 6502 assembly is that LDA is $A9. I never separated the instruction from the register; it's not like they were general purpose.
You had LDA and LDX and LDY as separate instructions while the Z80 assembler had a single LD instruction with different operands. It's the same thing really.
Right, though the LD? and ST? instructions were kind of exceptions. You could only do arithmetic and stack and bitwise ops (and, or, eor, shift, rotate) with A, never X nor Y. Increment and decrement were X/Y only. You couldn't even add two registers together without stashing one in memory.
On the 6502 you had three instructions LDA, LDX, LDY where the register name is essentially part of the instruction name. On the Z80 you had a lot of "load" instruction so you had LD and then many different operands: loading 8-bit registers, loading 16-bit, writing to memory, reading from memory, reading/writing from memory using a register as an index. So, made more sense on Z80 to have "LD" whereas LDA/LDX/LDY worked fine on 6502.
> What is this "LD A, 0" syntax? Is it a z80 thing?
Well, I never wrote any 6502 so I can't compare, but yes, you could load immediate values into any register except the flag register on the Z80. Was that not a thing on the 6502?
The 6502 instruction set was really limited but there were three registers: A, X, Y and there were immediate load instructions for each: LDA #0, LDX #0, LDY #0.
https://jnz.dk/z80/ld_r_n.html
https://jnz.dk/z80/xor_r.html
Yep, if I'm reading this right that's 3E 00, since the second byte is the immediate value.
One difference between XOR and LD is that LD A, 0 does not affect flags, which sometimes mattered.