Z80 Disassembler Online Full Best May 2026

I can’t post directly to forums or social media for you, but here’s a ready-to-copy forum post/reply you can use when asking for a full-featured Z80 disassembler online.

Title: Looking for a full Z80 disassembler online (not just a limited web demo)

Post:

Does anyone know a fully-featured online Z80 disassembler that handles:

• All undocumented Z80 opcodes (IXH, IXL, SLL, etc.)
• Multiple file formats (raw binary, Intel HEX, Z80 snapshot .z80, .sna)
• Label management / comments (so I can annotate addresses)
• Origin/base address setting (ORGs)
• Output as text or assembly source

I’ve tried the small ones on GitHub Pages, but they usually miss undocumented instructions or only work for tiny code snippets.

Ideally something like z80dasm (command-line) but accessible in a browser. z80 disassembler online full

So far the closest I’ve found:

• Masswerk’s Z80 disassembler – good, but no label support
• Online Z80 Disassembler by defuse.ca – simple, missing undocumented opcodes
• www.z80.info/decod.htm – too basic

Does anyone know a better full online tool? Or a way to run z80dasm in a browser (WASM, JS port)?

Thanks!

If you want just the two best online full-featured options to check out right now:

1. Masswerk Z80 Disassembler
   https://www.masswerk.at/z80/disassembler/
   – Handles many undocumented instructions, supports origin, can embed labels via syntax. I can’t post directly to forums or social

2. Z80 Disassembler by Simon Morris (web port of z80dasm)
   https://simonmorris.uk/z80-disassembler/
   – Closest to a “full” command-line experience in the browser.

Best Practices for Using Online Z80 Disassemblers

To maximize effectiveness, a user should follow a few guidelines:

1. Know Your Origin: Specify the correct starting address (ORG). If the binary is a raw ROM intended to be mapped at 0x0000, tell the disassembler. Otherwise, address references will be offset incorrectly.
2. Use Manual Annotation: After an initial linear pass, identify sections that are clearly data (strings, graphics, lookup tables) and mark them as such. The best online tools allow toggling bytes between code and data.
3. Combine with an Emulator: Disassembly tells you what the code does; an emulator tells you how it behaves. Running the code under a debugger like Fuse (for ZX Spectrum) or MAME while referring to the disassembly is a powerful workflow.
4. Verify Checksums: Before disassembly, ensure the uploaded ROM or binary has not been corrupted. A simple online hex checksum tool can confirm file integrity.

1. Game Hacking and Cheats

Want infinite lives in Manic Miner on the ZX Spectrum? Load the game ROM into a full online disassembler. Search for the number 3 (starting lives) in the hex dump. Look for a DEC A (decrement accumulator) or SUB 01 instruction near that data. Replace it with NOP or INC A. Reassemble and patch the ROM.

Limitations & Pitfalls

• Disassembly ambiguity: data vs code cannot always be reliably distinguished; manual verification is often needed.
• Self-modifying code and encrypted/compressed ROMs require extra handling.
• Some online tools limit file size or strip metadata; offline tools may be better for large projects.
• Undocumented opcodes and prefix combinations may be inconsistently supported across tools.

The Epilogue: From Static to Dynamic

The "Z80 Disassembler Online Full" does more than just convert code; it preserves culture. It turns a static, unreadable binary blob into a readable textbook.

For the hobbyist translating a Japanese RPG, this tool finds the text strings. For the speedrunner looking for glitches, it finds the collision-checking logic. For the student, it reveals how software is built from the ground up. Title: Looking for a full Z80 disassembler online

As you stare at the resulting code on your browser, the barrier between the past and the present dissolves. You aren't just looking at hex codes anymore; you are looking at the thoughts of a programmer from 40 years ago, preserved in silicon and finally unlocked by a tab in your web browser.

2. Online Z80 Disassembler (Dissy)

URL: https://www.antoniosalas.com/software/z80dis/

Features:

• Simple paste-hex or upload-file interface
• Disassembles to Zilog syntax
• Output includes address, hex dump, and instruction
• Can handle up to 64KB

Best for: Quick disassembly of small code blocks.

Key Features to Expect

• Input formats: raw binary, Intel HEX, Motorola S‑records, or plain hex/text.
• Configurable start address and CPU mode (Z80, undocumented opcodes enabled/disabled).
• Support for extended opcodes (CB, ED, DD, FD prefixes) and IX/IY register decoding.
• Label generation, branch target resolution, and basic flow analysis (jumps, calls, ret).
• Interactive disassembly: clickable addresses, cross-references, and byte highlighting.
• Export options: assembly source (asm file), annotated hex dump, or copyable text.
• Options for commenting, renaming symbols, and applying custom symbol tables.
• Syntax selection (e.g., TASM, z80asm, SjASMPlus) and configurable formatting.
• Search, filters, and byte-level editing for quick patches.
• Performance: handles ROM images from a few KB to several MB (depending on site limits).
• Privacy: local-only processing is preferable — check whether uploads leave your browser.