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Retro emulation works by recreating a console in software, then running an unmodified game image inside that model. A ROM is a dump of the program and graphics chips on a cartridge (or a disc image for CD and DVD systems); it is not a PC conversion. The emulator models the original CPU, video and audio chips, memory map, timers, controllers, cartridge mappers and save hardware. In-game saves are the small data files the game would have written to SRAM, EEPROM, flash or a memory card. Save states are separate emulator snapshots that freeze the whole virtual machine, including registers and RAM. That distinction explains why a save-state file may fail after changing emulator cores while an .srm or .sav file usually remains portable.
How do emulators work?
An emulator supplies a software model of the original console. The game image remains unchanged and executes against that model; it is not rebuilt for Windows, Linux, Android or another host. As IndieKings’ 2026 summary of the Dolphin README puts it: “The emulator provides a simulated console, and the unmodified game image runs inside it.”
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A typical 8-bit or 16-bit emulator is a group of synchronized components:
- CPU model: fetches an opcode at the program counter, decodes its addressing mode, performs the operation, updates registers and flags, and accounts for the instruction’s clock cycles.
- System bus and memory map: connects RAM, ROM windows, memory-mapped I/O registers and bank-switching hardware.
- PPU or GPU model: handles video memory, sprites, backgrounds, scanline timing, blanking periods and interrupts.
- APU model: reproduces sound registers, channels and sample output.
- Cartridge hardware: implements mappers, coprocessors and writable save memory.
Klika’s description of NES emulation uses a 6502 interpreter loop: read the instruction at the program counter, decode its type and operands, call the matching operation, then update memory and registers. The NES address space is 64 kB, divided among ROM, RAM and registers. Instructions consume different numbers of clock cycles, so the emulator must preserve those timings or the CPU, video and audio chips drift out of alignment.
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Interpretation, dynamic recompilation and static recompilation
Interpretation executes one original instruction at a time. It is straightforward to count cycles and reproduce unusual behavior, but it requires more host-CPU work. Dynamic recompilation (JIT or dynarec) translates frequently executed blocks into native host instructions while the game runs, usually improving speed while making exact timing harder. Static recompilation translates a program ahead of time; it can be fast but must account for code that changes itself or depends on hardware side effects.
Older systems such as the NES and SNES are unusually timing-sensitive because the CPU must access video-related memory at precise moments. A fast translator that gets those moments wrong can produce graphical glitches, missing interrupts or audio errors even when the game appears to boot.
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High-level emulation (HLE) substitutes a higher-level model for a complex device, often gaining speed at the cost of edge-case accuracy. Low-level emulation (LLE) models the device more literally and usually costs more processing power. Dolphin’s README, as summarized by IndieKings in 2026, describes its HLE audio engine as faster and its LLE engine as slower but close to perfect. The same documentation lists a host baseline of an SSE2-capable CPU, a modern dual-core processor around 3 GHz and graphics support for Direct3D 11.1 or OpenGL 3.3; these are requirements for that emulator, not requirements for every retro system.
What is a ROM file?
On original cartridge hardware, the game program lived in mask-ROM chips. NES boards commonly separated PRG ROM, which the CPU reads for code, from CHR ROM, which the PPU reads for graphics. As games grew, mapper chips switched different ROM banks into the address space. Boards could also add RAM and a battery for saves.
A ROM file is a digital readout of those memory chips. NesHacker describes a program ROM as the chip containing the game’s code and graphics, but copying a board is not always one linear read: banks can differ in size and number, and extra chips can control bank switching or special effects.
The iNES and NES 2.0 formats
A common NES dump uses the iNES container:
- A 16-byte header identifying the format and cartridge properties.
- An optional 512-byte trainer block, a remnant of some Famicom copiers that is normally skipped.
- PRG ROM in 16 KB units.
- CHR ROM in 8 KB units.
The header records PRG and CHR sizes, mapper number and nametable mirroring (horizontal, vertical or four-screen). NES 2.0 keeps backward compatibility while carrying more cartridge metadata, reducing guesswork for larger or unusual boards. The header is not decorative: it tells the emulator which mapper implementation and memory layout to attach.
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Disc images
CD- and DVD-based systems use images such as BIN/CUE, ISO, CHD or RVZ. Compression changes storage, not the basic idea: the file is still a dump of the original sectors and filesystem, not a native port made for a modern operating system.
Why are ROM dumps not enough by themselves?
Custom cartridge logic is part of the console design. Emulation General Wiki states: “Because NES cartridges contain custom hardware, ROM dumps alone are not enough to accurately describe how a game should be emulated.” The mapper number in a header points to code in the emulator that reproduces bank switching, interrupts, timing and any special registers. A newly dumped board with an unknown mapper may not run until someone implements that behavior.
The same issue appears in other systems. Super NES cartridges can contain coprocessors; Game Boy Advance cartridges use different save-memory interfaces. bsnes.org explains that emulators combine ROM images with headers, checksums and models of coprocessors to decide how each title is wired. An unheadered NES dump, a damaged header or an incorrect mapper assignment can therefore produce a black screen, scrambled graphics or a game that starts but cannot save.
How do I dump a cartridge I own?
You need the physical cartridge, a compatible reader, a computer or single-board host, storage for the dump and, if you want to preserve progress, a way to read the save chip before servicing the battery. A ROM downloader is not a substitute for this process and ownership of a cartridge does not automatically authorize downloading someone else’s copy.
- Identify the system and board. Check whether the cartridge uses a standard connector, a special mapper, a coprocessor or a disc format. Compatibility depends on the board, not just the game’s title.
- Use a cartridge dumper. The open-source Sanni Cart Reader documented by Hackaday uses an Arduino Mega 2560 Pro, custom adapter PCBs and firmware to read ROMs to an SD card. It can also dump and restore save data, including certain Satellaview and Nintendo Power flash-media formats. Older readers such as the Retrode existed, but availability and support vary.
- Read the save memory first. If the board has a battery-backed SRAM, dump the save before opening the cartridge or replacing its cell. A battery swap without an external power source can erase the only copy.
- Verify the dump. Read the cartridge more than once when possible and compare file sizes, checksums and header fields. A consistent image with the expected PRG/CHR sizes is less likely to contain a loose connection or bad contact.
- Keep the original files untouched. Store the ROM and SRAM dump in separate, backed-up folders. Use a working copy for header repair, translation patches or emulator-specific conversions.
- Load it in an emulator that supports the hardware. If it fails, inspect the mapper, header version, coprocessor support and save type before assuming the ROM itself is bad.
Hackaday commenters note that some older floppy-based copiers placed an extra 512-byte block at the start of an SRAM dump; removing that copier-specific prefix may be necessary before an emulator recognizes the save. That is a format-cleanup issue, not evidence that the game has no save.
How do original cartridges save progress?
The game program is read-only; progress is written to a separate memory device. The game’s code decides when to write, what data to write and whether to validate it on the next boot.
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Battery-backed SRAM
Static RAM is fast and freely writable but volatile. While the console is on, the cartridge supplies normal power. When it is off, a lithium coin cell maintains the RAM in low-power retention mode. Diodes isolate the battery from the console supply and prevent charging, while resistors keep the chip deselected during shutdown.
Classic examples include The Legend of Zelda on NES and Super Mario World on SNES. Nintendo Power explained the design in 1991: “By putting a battery in the Game Pak, game data can be stored as long as the life of the battery—about five years.” Many cartridges are now decades beyond that design estimate. A common NES, SNES or N64 cell is a tabbed CR2032; Game Boy, Game Boy Color and some GBA boards commonly use tabbed CR1616 or CR2025 cells. A bare store-bought coin cell lacks the welded tabs and should not simply be taped onto the original pads.
Games often store a checksum alongside the save. If the bytes no longer match, the game may treat the slot as corrupt and erase it. Hex-editing an .srm file therefore requires the game-specific checksum to be recalculated or the next boot can show a blank file.
On SNES SxROM/MMC1 boards, an 8 KB RAM chip could be fitted with or without battery backup. A solder pad marked SL selected the battery-backed configuration; boards without the cell, diodes and resistors behaved as ordinary volatile RAM.
EEPROM and flash memory
EEPROM and flash retain data without a coin cell. Nintendo 64 cartridges used a mixture of EEPROM, battery-backed SRAM and flash, so one game can have a battery while another has no battery to fail. GBA save interfaces documented by Screwtape include no save device, 512-byte EEPROM, 8 KB EEPROM, 32 KB SRAM, 64 KB flash and 128 KB flash. A game written for one interface cannot use another because the devices occupy the same memory-map region.
Why GBA save detection can be wrong
The GBA internal header does not identify the save type. Nintendo SDK libraries left marker strings such as EEPROM_V, SRAM_V, FLASH_V, FLASH512_V and FLASH1M_V in the ROM. Emulators scan for those strings, but developers sometimes changed them, included several libraries or triggered copy-protection behavior. The result can be a wrong save device, a game that never writes correctly or a flash cartridge that appears to be damaged.
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Emulators address this with internal databases, defaults and user overrides. VBA-M uses defaults plus exceptions, while MAME aims for a more exhaustive database. Manual override screens can be confusing because they may ask for a flash-chip vendor or device ID that is not visible without opening the cartridge. Emulating EEPROM does not wear out the physical chip; wear is a concern for actual flash or SRAM hardware.
Bootlegs and batteryless patches
Some bootleg cartridges include SRAM but no battery. Flash Cart DB lists Game Boy SRAM capacities of 4K (512 bytes), 64K (8 KiB), 256K (32 KiB) and 512K (64 KiB, used by Japanese Pokémon Crystal). Such boards may need a batteryless patch that redirects writes into flash; an unmodified ROM expecting battery-backed SRAM can lose progress when power is removed.
What is a save state?
A save state is an emulator-created snapshot, not a feature of the original game. The core serializes CPU registers, RAM, video memory, timers, controller state and any other internal data it tracks, then writes the result to a file. Reloading returns the emulated machine to that frame, even in the middle of a jump, cutscene or boss fight.
MVG’s Game Boy example includes CPU registers, LCD position, scrolling registers, palettes, sound, timers, VRAM tiles, sprites, backgrounds and joystick I/O. A state can therefore restore a point the cartridge never could. It can also preserve a glitch or inconsistent state if taken while the emulator is already malfunctioning.
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Rewind keeps a rolling sequence of states in memory instead of asking you to name files. One MVG implementation pushes a state every frame onto a last-in, first-out stack; holding rewind pops earlier frames. Its example buffer contains 360 states, or six seconds at 60 frames per second. Other emulators use compressed differences or less frequent checkpoints, so that figure is an implementation example, not a universal specification.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Save states versus in-game saves: which should I use?
Use the game’s own save as the archival copy and save states as convenience checkpoints. Held Games summarizes the rule: “Use in-game saves as your primary save method. Use save states as a convenience layer on top.”
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| Method | What it stores | Exact-frame restore | Portability | Version lock-in | Risk to later in-game saves |
|---|---|---|---|---|---|
In-game save / SRAM / .srm or .sav |
Only data the game designers wrote | No; restores the game’s normal save point | Usually portable across compatible emulators and, with suitable hardware, the original cartridge | Low | Low if backed up; the game can still overwrite its slots normally |
Save state / .state |
Whole emulated machine, including registers and RAM | Yes, at the captured frame | Not portable to real hardware; often core-specific | High; a state may fail after changing cores or versions | High if it is the only copy and you later load an old state |
| Rewind buffer | Short rolling sequence of emulator snapshots | Yes, within the retained window | Usually session-specific and not an archival file | High | It can return you to an earlier point and obscure newer progress |
A Beetle PSX state will not load in another PlayStation core, and Held Games gives PPSSPP 1.16 versus 1.17 as an example of states that may not remain compatible across versions. By contrast, an .srm file generally carries the game’s intended save data. Handheld firmware can place both kinds of files under one saves directory—for example, KNULLI uses /userdata/saves/—so back up both, but treat the SRAM or memory-card file as the long-term copy.
A safe routine
- After a difficult section, trigger the game’s normal save.
- Exit or reset once to confirm that the in-game save reloads.
- Create a named save state for a convenient checkpoint.
- Back up the save directory before updating an emulator core, changing a BIOS or installing handheld firmware.
- When moving to another emulator, copy the
.srm,.savor memory-card image first; do not expect the.statefile to follow.
Why did my Zelda or Pokémon save disappear?
The most common cause is a dead battery in a cartridge that uses SRAM. Once the cell can no longer retain power while the console is off, the next boot may show an empty or corrupted file. A checksum failure can produce the same symptom even when some bytes remain.
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- Original cartridge: dump the SRAM before replacing the cell. Use a tabbed CR2032, CR1616 or CR2025 appropriate to the board, a 3.8 mm Gamebit screwdriver where required, and proper desoldering equipment. Replacing a cell without keeping SRAM powered can erase it.
- Emulator: check that the ROM is paired with the correct
.srmor.savfilename, folder and save type. A wrong GBA EEPROM/flash setting can prevent writes. - After an update: restore the backed-up save directory and confirm that the same core and game-region revision are selected. Save states are especially vulnerable to core and version changes.
- After editing a save: restore the game’s checksum or the title may intentionally clear the slot.
Are emulators legal, and are ROMs legal?
Rules vary by country, and this is not legal advice. The widely described legal distinction is between independently written emulator software and copyrighted game images. Gizmodo writes: “These hardware-to-software environments are fair game so long as they don’t contain any ROMs themselves.” It then states: “However, the games themselves, those ROMs, are direct copies of the original games. They are breaking copyright, and it is certainly illegal to distribute.”
A Linus Tech Tips presenter summarized the popular view as “Whatever Nintendo says, emulators are legal – but ROMs are not,” while expressly noting that he was not a lawyer. Uploading or downloading a commercial ROM without the rights holder’s permission is not made lawful merely because you own the cartridge. Personal format-shifting of media you own is argued as permitted in some jurisdictions, but the position is unsettled in the United States, especially when bypassing copy protection.
BIOS files have a separate complication. Some emulators use a clean-room or high-level replacement; others can load a BIOS dumped from hardware you own. A forum discussion of 17 U.S.C. § 117 describes an archival and use argument for copies necessary to run a lawfully owned program, but that interpretation is not a court holding and does not settle game-copyright questions. Official re-releases, remasters and subscription libraries are the rights-cleared option where available. Older games may remain absent because music, character or publisher licenses are split among multiple owners.
Which way should I play an old game?
| Approach | What you get | Save behavior | Main trade-off |
|---|---|---|---|
| Official re-release | A rights-cleared package maintained by the publisher or platform | Uses the release’s supported save and backup system | Selection depends on current licensing and platform availability |
| Dump your owned cartridge or disc and emulate | Your own image running in a software model of the original hardware | Original-style save files plus optional emulator states | You must supply compatible dumping hardware and preserve files yourself |
| FPGA recreation | Hardware logic recreated in programmable silicon | Depends on the core and attached storage | More hardware-like behavior, but requires dedicated equipment and compatible cores |
| Original hardware | The original console and cartridge | Physical SRAM, EEPROM or flash; battery replacement may be needed | Authentic peripherals and timing, with aging components and maintenance |
What should I check when a ROM will not load?
- Confirm that the dump has the expected header and file size; an unheadered NES image often will not boot.
- Check the mapper, mirroring mode and NES 2.0 or iNES metadata.
- Verify that the emulator supports the cartridge’s mapper, coprocessor or special audio hardware.
- For disc images, keep the cue sheet with its BIN files and use a format the emulator supports.
- For GBA, select or verify the save type rather than relying only on marker-string detection.
- Make sure the save file belongs to the exact ROM revision and uses the emulator’s expected filename and directory.
- If only a save state fails, load the game’s
.srm,.savor memory-card file instead and recreate a state in the current core.
Frequently Asked Questions
Can I move an SRAM save between different emulators?
Usually, yes, when both emulators support the same console and save type. Copy the .srm or .sav, preserve the expected filename, and make sure the ROM revision matches; a save-state file is a different, core-specific format.
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Does emulating a game wear out its original EEPROM or flash chip?
No. Emulation writes a file on the host device and does not electrically cycle the chip in your cartridge. Wear matters when you repeatedly write physical flash or SRAM hardware, not when the emulator models it.
Why does a cartridge have a battery if the game is on ROM?
The battery does not power the read-only program. It supplies a tiny retention current to volatile SRAM while the console is off, allowing the game’s progress data to survive between sessions.
Can a save state be converted for use on original hardware?
Not directly. A state contains CPU, video, timer and other emulator internals that a cartridge never stores. To return to hardware, load the game’s normal save file and transfer that SRAM, EEPROM or memory-card data with compatible hardware.
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