nes-rom-debug

Testing & Quality

Disassemble and debug NES ROM files (.nes) produced by the dotnes transpiler. Use this skill whenever the user wants to disassemble a .nes ROM, inspect 6502 machine code, compare two NES ROMs side-by-side, debug transpiler output, investigate byte differences between a cc65 reference ROM and a dotnes ROM, or understand what 6502 instructions the transpiler emitted. Also use when the user mentions disasm, disassembly, PRG ROM, ROM bytes, NES addresses, or wants to look at the hex/binary output of a build. Even if the user just says something like "the ROM looks wrong" or "what did the transpiler emit", this skill applies.

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NES ROM Debug

Tools and knowledge for disassembling NES ROMs and debugging dotnes transpiler output.

Available Scripts

`disasm.py` — 6502 Disassembler

Disassembles the PRG ROM section of an iNES (.nes) file into human-readable 6502 assembly.

python scripts/disasm.py <file.nes> [start_hex] [end_hex]

file.nes — Path to the NES ROM file
start_hex — (Optional) Start NES address in hex, default 8000
end_hex — (Optional) End NES address in hex, default end of PRG

Examples:

Note: .nes files are build outputs — run dotnet build in the sample directory first.

# Disassemble the entire PRG ROM
python scripts/disasm.py samples/hello/hello.nes

# Disassemble only the main() region
python scripts/disasm.py samples/hello/hello.nes 8500 8600

# Disassemble the interrupt vectors area
python scripts/disasm.py samples/hello/hello.nes FF00 FFFF

Output format:

$8500: A9 02     LDA #$02
$8502: 20 61 85  JSR $8561
$8505: 85 17     STA $17
$8507: D0 F7     BNE $8500

Each line shows: $ADDR: HEX_BYTES MNEMONIC OPERAND

`compare_rom.py` — Side-by-Side ROM Comparison

Compares two NES ROMs with byte-level and instruction-level analysis. Use this to verify dotnes output against a cc65 reference ROM.

python scripts/compare_rom.py <reference.nes> <dotnes.nes>

Output includes:

File sizes
Total byte differences (header / PRG / CHR breakdown)
Contiguous diff groups with NES addresses
Side-by-side disassembly of the largest diff region with match/mismatch markers
Instruction mismatch count (ignoring absolute addresses, since the two compilers may place subroutines at different offsets)

`ildump.cs` — IL Opcode Dumper

Dumps the .NET IL opcodes from a compiled DLL — useful for understanding what IL the transpiler will process.

dotnet run scripts/ildump.cs -- <path-to-dll>

NES ROM Format (iNES)

Understanding the binary layout helps interpret disassembly output:

Offset    Size     Content
0x00      16       iNES header (starts with "NES\x1A")
0x10      32768    PRG ROM (mapped to NES $8000-$FFFF)
0x8010    8192     CHR ROM (pattern tables for tiles/sprites)

Key NES addresses:

$8000-$85AD — neslib runtime and built-in subroutines (palette, PPU, NMI handler, stack ops)
$85AE+ — main() and user code (exact layout varies per sample)
$FFFA — NMI vector (2 bytes, little-endian)
$FFFC — RESET vector (2 bytes, little-endian — entry point)
$FFFE — IRQ vector (2 bytes, little-endian)

File offset → NES address: nes_addr = 0x8000 + (file_offset - 16) NES address → file offset: file_offset = (nes_addr - 0x8000) + 16

Common Debugging Workflows

"The ROM doesn't match the reference"

Run compare_rom.py to identify where bytes differ
Use disasm.py on both ROMs targeting the diff region to see the instructions
Check if differences are just address relocations (compare_rom normalizes these) or actual logic differences

python scripts/compare_rom.py reference.nes output.nes
# If diff at $8600-$8620:
python scripts/disasm.py reference.nes 85F0 8630
python scripts/disasm.py output.nes 85F0 8630

"What did the transpiler emit for my code?"

Build the sample: cd samples/hello && dotnet build
Dump the IL to see what C# compiled to: dotnet run scripts/ildump.cs -- samples/hello/bin/Debug/net10.0/hello.dll
Disassemble the ROM to see the 6502 output: python scripts/disasm.py samples/hello/bin/Debug/net10.0/hello.nes 8500

"Where is main() in the ROM?"

The RESET vector at $FFFC points to the startup code, which eventually jumps to main. To find it:

# Look at the last few bytes for the vectors
python scripts/disasm.py myrom.nes FFF0 FFFF

# The RESET vector value tells you where startup is
# Then disassemble from there to find the JMP to main

"Comparing test output against verified snapshot"

When TranspilerTests.Write fails, the test produces a .received.bin alongside the .verified.bin. Compare them:

# The test DLLs are in src/dotnes.tests/Data/
# Build the test project to get the .received.bin
python scripts/compare_rom.py path/to/verified.bin path/to/received.bin

6502 Quick Reference

The most common instructions you'll see in dotnes output:

Opcode	Mnemonic	Meaning
`A9`	`LDA #imm`	Load accumulator with immediate value
`A5`	`LDA zpg`	Load accumulator from zero page
`AD`	`LDA abs`	Load accumulator from absolute address
`85`	`STA zpg`	Store accumulator to zero page
`8D`	`STA abs`	Store accumulator to absolute address
`20`	`JSR abs`	Jump to subroutine
`60`	`RTS`	Return from subroutine
`4C`	`JMP abs`	Jump to address
`D0`	`BNE rel`	Branch if not equal (Z=0)
`F0`	`BEQ rel`	Branch if equal (Z=1)
`C9`	`CMP #imm`	Compare accumulator with immediate
`E6`	`INC zpg`	Increment zero page location
`C6`	`DEC zpg`	Decrement zero page location

Zero page addresses used by dotnes:

$17 — TEMP
$22-$23 — sp (cc65 software stack pointer)
$0325+ — Local variables