A 6502 assembler, in your browser.

Assembly is the human-readable form of the individual instructions a CPU executes. go6asm turns 6502 assembly into the exact bytes the chip would run, places them in a target machine's memory, and checks the result for mistakes — all in this page, nothing to install. What is assembly? · Pick an example and press Assemble. Hover any iInfo. A circled i marks a term — hover or focus it for a plain-language definition. for a plain-language definition of a term.

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What happens when you press Assemble

text → machine, in six stages

1Scan & expand Text becomes tokens; macros, .if and .repeat expand.

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2Parse Tokens become a syntax tree — one statement per line.

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3Resolve Every label and expression gets a value (ca65 32-bit math).

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4Generate Pick addressing modes, emit bytes — two passes so forward references work.

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5Link Place segments at real addresses for the target; patch relocations.

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6Analyze Trace reachable paths; flag bugs against the device map.

You can write just instructions and let go6asm fill in the rest, or spell out every detail yourself — same language, same result, you decide how much to hand off. The language reference walks through it.

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Source

write it · target it · assemble it

Step 1 — the program pick an example to start from, or edit it freely

Example iExamples. Small programs to start from. The Catch a bug one deliberately misbehaves, so you can watch the static analyzer point it out.

Step 2 — the target the machine you're building for — it decides the memory layout below

Target iTarget. A machine definition: where code loads, where the CPU's reset/interrupt vectors are, and which addresses are RAM, ROM or I/O. The bar below shows it.

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$0000$4000$8000$C000$FFFF

RAM I/O (VIC/VIA) ROM unmapped code load / reset vector

Step 3 — assemble turn the source into bytes for that target, and check it

emit .o6 i.o6 object. A relocatable object — assembled code whose addresses aren't fixed yet, ready to be combined and placed by the linker (like a .o file). Rarely needed; leave it off for a normal build.

Loading WebAssembly…

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Assembled result

Static analysis iStatic analysis. Without running the program, go6asm traces every path the CPU can take from the reset/IRQ vectors and flags trouble: writing to ROM, touching unmapped memory, hitting a BRK or an illegal opcode. It's honest about what it can't follow (indirect jumps, self-modifying code).

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Annotated disassembly iDisassembly. The finished bytes decoded back into instructions, with your labels and — the point — your own source comments threaded back in. Data is shown as .byte, not mis-decoded, because the analyzer knows what's code. addr · bytes · instruction · your comments

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Image (hex) iImage. The actual bytes that would be loaded into the machine's memory, shown as a hex dump starting at the load address.

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Symbols iSymbols. Named locations — code labels and exported constants — with their resolved addresses. These feed the disassembly and a debugger's memory view.

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Debug-info sidecars iSidecars. .lbl is a VICE-monitor label file; .sym is go6asm's canonical symbol JSON. Tools and debuggers consume these to show your names instead of raw addresses. click to expand

out.lbl

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out.sym

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