6502 Assembly: Lower vs Upper Case Style

1 December 2020 · Updated 26 February 2026

This is going to annoy some people, but I cannot stand lower case mnemonics in 6502 assembly. There, I said it.

How It Should Look

This is correct:

  LDA #$00
  STA $D020
  STA $D021

This makes me wince:

  lda #$00
  sta $d020
  sta $d021

They assemble to the same bytes, obviously. The CPU doesn’t care. But I care, and here’s why.

Historical Precedent

6502 mnemonics have been upper case since the beginning. Open the Commodore 64 Programmer’s Reference Guide — the canonical text that every C64 coder learned from — and every single mnemonic is upper case. LDA. STA. JSR. RTS. That’s how MOS Technology documented them. That’s how Commodore documented them. That’s how every magazine listing, every assembly tutorial, and every disassembler output rendered them for decades.

Upper case mnemonics visually distinguish instructions from labels and data. When you scan a block of code, the upper case mnemonics jump out. You can instantly see the structure — where the instructions are, where the operands are, where the labels break things up. Lower case turns everything into a grey mush of similar-looking text.

Where Did Lower Case Come From?

My theory: it crept in from modern web-era languages. People who learned JavaScript or Python first and then came to 6502 assembly brought their formatting habits with them. In those languages, lower case is the convention. Fair enough — in those languages. But 6502 assembly isn’t JavaScript. It has its own conventions, established over 40+ years, and those conventions exist for good reasons.

Some cross-assemblers default to lower case or are case-insensitive, which doesn’t help. Just because your assembler accepts lower case doesn’t mean you should use it.

How It Should Look

Historical Precedent

Where Did Lower Case Come From?

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