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:

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.

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sfink 4 hours ago | parent | next [-]

What is this "LD A, 0" syntax? Is it a z80 thing?

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.

Good times.

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Narishma 2 hours ago | parent [-]

> 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.

	▲	sfink an hour ago \| parent [-]
		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.

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jgrahamc 10 hours ago | parent | prev [-]

You're right. Of course, it's 3E 00. Not sure how I remembered 3E 01. My only excuse is that it was 40 years ago!