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Retro emulation recreates a console in software so the original game program can run on a computer or other device. A ROM is a dump of that program—not a PC conversion—and an emulator models the console hardware the game expects. An in-game save stores only the progress the game was designed to save; a save state instead captures the emulator’s broader machine state at a particular moment. Knowing the difference helps explain why a game may fail to load, why an old cartridge forgets progress, and which save file to keep when changing emulators.
How do emulators work?
An emulator models a console’s components in software and runs the game’s original instructions against that model. The game is not converted into a new PC game: the emulator supplies a simulated machine, while the game image remains the original program. IndieKings’ 2026 summary of Dolphin’s README describes that contract this way: “The emulator provides a simulated console, and the unmodified game image runs inside it.”
A typical 8-bit or 16-bit emulator has several cooperating parts. The CPU model reads and executes instructions; the system bus and memory map connect the CPU to RAM, ROM, and hardware registers; the graphics processor model handles such things as video memory, sprites, backgrounds, and interrupts; and the audio model responds to sound registers. Cartridge-specific hardware, including bank-switching chips and save memory, may need its own emulation too.
How does the emulator execute instructions?
In interpretation, the emulator reads one original instruction, decodes it, performs the equivalent operation, updates registers and flags, and accounts for the instruction’s clock cycles. Klika’s 2023 explanation of NES emulation describes this approach for the 6502 CPU. The NES has a 64 kB address space, and its CPU, graphics hardware, and other components must remain in step. The Picture Processing Unit (PPU), for example, can depend on video-related memory being accessed at precise times.
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Other approaches trade simplicity or timing accuracy for speed. Dynamic recompilation, also called JIT compilation, translates frequently used blocks of the original code into native host instructions while the game runs. Static recompilation translates the program ahead of time. These techniques can be faster, but keeping the emulated machine’s timing faithful is more complicated than counting cycles in an interpreter. The right balance depends on the console and game; there is no single accuracy setting that suits every system.
What do HLE and LLE mean?
High-level emulation (HLE) recreates the behavior of a component at a higher level rather than reproducing every low-level detail. Low-level emulation (LLE) models the component more closely. Dolphin’s README, as summarized by IndieKings in 2026, describes its HLE audio engine as faster and less accurate, and its LLE audio engine as slower and close to perfect. That is a trade-off in one emulator, not a universal speed or accuracy guarantee for all games.
Hardware requirements also depend on the console and emulator. Dolphin’s 2026 README baseline, reported via IndieKings, lists a CPU with SSE2 support, a modern dual-core processor running around 3 GHz, and a graphics card supporting Direct3D 11.1 or OpenGL 3.3. Those are host-computer requirements for Dolphin, not requirements for creating or converting a game file. A simpler console emulator may have different needs.
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What is a ROM file?
A ROM file is a digital dump of the read-only game data stored on a cartridge; disc-based games use disc images instead. On a classic NES cartridge, for example, PRG ROM held program code for the CPU, while CHR ROM held graphics data for the PPU. The two banks served different parts of the console, so the physical cartridge was more than one undifferentiated block of data. NesHacker put it simply in 2025: “Game boards have a chip inside called a program ROM, and it contains all the code and graphics.”
Why does a ROM file have a header?
A dump needs to tell an emulator how to interpret the data and what cartridge hardware to model. The common NES iNES format uses a 16-byte header describing information such as PRG and CHR sizes, mapper number, and screen mirroring. An optional 512-byte trainer block may appear before the ROM data; it originated with older Famicom copiers and is usually skipped. In this format, PRG ROM is sized in 16 KB chunks and CHR ROM in 8 KB chunks, as documented by bugzmanov in 2026. NES 2.0 extends iNES compatibly and carries more cartridge metadata, reducing the guesswork an emulator must do.
Disc systems use other image formats, including BIN/CUE and ISO, as well as compressed formats such as CHD or RVZ. These are still images of original media, not native ports. Their format and the emulator’s support for it determine whether the emulator can read the image.
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Why might a ROM dump fail to work in an emulator?
The game data alone may not describe all the hardware that was on the cartridge. Emulation General Wiki explained in 2026: “Because NES cartridges contain custom hardware, ROM dumps alone are not enough to accurately describe how a game should be emulated.” A mapper chip might switch banks of ROM as the game runs; the emulator needs code that reproduces that behavior. The file’s header identifies the mapper, but it does not implement it. If an emulator does not support a mapper—or the dump lacks a usable header—the game may not load or may behave incorrectly.
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- Check the image format: Confirm that the emulator supports the file type and the console it came from.
- Check cartridge metadata: A missing or incorrect header can prevent the emulator from identifying hardware such as a mapper.
- Check emulator support: Newly dumped or unusual cartridge hardware may not be implemented in that emulator.
- Keep the original dump: A compatibility problem does not by itself mean the game data needs to be converted; the missing piece may be the emulator’s hardware model or metadata.
How did original cartridges save progress?
Game progress was stored separately from the read-only program. Depending on the game, the cartridge could contain SRAM, EEPROM, or flash memory. The game wrote its save data to this writable memory, which is why an emulator’s in-game save file is generally much closer to the original cartridge’s save than a save state is.
How does battery-backed SRAM work?
Static RAM (SRAM), also called Save RAM or battery-backed RAM, is fast and can be rewritten freely, but it is volatile: it loses its contents when power is removed. A coin cell keeps it in a low-power retention mode while the console is off. In the circuit described by Code Like It’s 198x in 2026, the console’s 5 volts power the RAM while the game is running; when the console is off, the battery supplies the tiny current needed to retain the data. Diodes isolate the battery from the rest of the cartridge and prevent console power from charging the cell, while a resistor helps keep the memory deselected as power falls.
Nintendo Power explained the consumer-facing idea 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.” About five years was the stated design life, not a guarantee that each battery would last exactly that long. Many cartridges from that era are now well past it. The Legend of Zelda on NES and Super Mario World on SNES are examples of games using battery-backed saves, according to Data Crystal (2025) and VGCL (2026).
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CR2032 cells with vertical through-hole tabs are typical on NES and SNES boards, as well as some N64, Sega, and GameCube boards. Game Boy, Game Boy Color, and some GBA boards may use CR1616 or CR2025 cells with horizontal tabs, according to Calgary Games (2026). A bare retail coin cell does not have the welded tabs needed for a cartridge PCB. If preserving the existing save matters, dump it before replacing a battery: removing the old cell without keeping the RAM powered can erase the data. Repair sources Nostos (2026) and Calgary Games (2026) describe using a tabbed cell, a 3.8 mm Gamebit screwdriver where applicable, and proper desoldering rather than taping a coin cell onto the board.
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Do EEPROM and flash need a battery?
No. EEPROM and flash retain data without a coin cell. Some Nintendo 64 cartridges used EEPROM, while others used battery-backed SRAM or flash, so the absence of a battery is normal for many games. For documented official GBA cartridge types, Screwtape (2026) lists the following capacities:
| GBA save-memory type | Documented capacity | Battery needed to retain data? |
|---|---|---|
| No save memory | None | Not applicable |
| EEPROM | 512 bytes or 8 KB | No |
| SRAM | 32 KB | Battery-backed SRAM needs a battery; ferroelectric RAM does not |
| Flash | 64 KB or 128 KB | No |
These devices are not interchangeable from the game’s perspective. Screwtape notes that GBA save devices share a memory-map region, so an emulator cannot emulate more than one at a time for a given game. Choosing the wrong type can prevent a save from working.
Why can GBA save detection be confusing?
The GBA’s internal header does not identify the save type. Nintendo SDK libraries left strings such as EEPROM_V, SRAM_V, FLASH_V, FLASH512_V, and FLASH1M_V inside ROMs, and emulators may scan for them. But developers sometimes altered the strings or included multiple libraries, which could cause detection to select the wrong memory type, trip copy protection, or leave a game unusable on an emulator or flash cart. bsnes.org (2023) and Screwtape (2026) describe databases of known games and user overrides as ways to address the problem. An override can be difficult for players because it may ask for flash-chip details they cannot identify without opening the cartridge. Emulating EEPROM does not wear out the physical chip; that wear concern applies to the cartridge hardware, according to Pudgycat (2026).
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If a cartridge used battery-backed SRAM, a depleted cell is a common reason its save vanished: once the cell can no longer retain the RAM while the console is off, the stored progress can be lost. EEPROM and flash do not depend on that battery, so a dead coin cell cannot explain every old cartridge’s save problem.
A game can also reject corrupted save data. Some games store a checksum over SRAM; if the checksum does not match, the game may treat the slot as damaged and erase it. Data Crystal (2025) notes that editing an SRAM file by hand can trigger this behavior unless the game-specific checksum is recalculated. On a physical cartridge, dump the save before battery work if the progress is important; on an emulator, preserve a copy of the save file before editing or troubleshooting it.
Battery-backed SRAM was also used for purposes other than player progress. Data Crystal (2025) describes Capcom CPS-2 arcade boards whose battery-backed SRAM stored decryption keys. When that battery died, the keys were lost and the game became unplayable. That is the same volatile-memory principle, but not an ordinary saved-game file.
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What is a save state, and how is it different from an in-game save?
An in-game save records the progress the game was designed to put in save memory. A save state is made by the emulator: it serializes the machine state it tracks and restores that snapshot when loaded. A state can capture a moment the original game had no way to save, but it is a different, less portable kind of file.
| Save method | What it contains | Restore point | Portability and compatibility | Main risk |
|---|---|---|---|---|
In-game save, SRAM, or a file such as .srm / .sav |
The progress the game wrote to its intended save memory | The game’s own save point or menu | Generally the more portable layer; compatible across many emulators and, with appropriate tools, real hardware | Battery-backed cartridge SRAM can lose data when its battery fails; game-specific save formats and checksums still matter |
Emulator save state, often a file such as .state |
A snapshot of CPU registers, RAM, video state, timers, and other tracked emulator state | The exact captured moment, including points where the game has no save system | Usually tied to a particular emulator core and often its version; not portable to original hardware | A state can preserve a glitch or broken emulator state; a software or firmware update may make it unusable |
Data Crystal (2025) summarizes the distinction: “SRAM differs from emulator savestates, which are unique to each emulator.” It also explains: “Savestate files hold all of the data in all the different parts of the emulated system, plus various status registers that the emulator author needs to save. While SRAM holds only data that the game designers intended to save.” Held Games (2026) gives the practical rule: “The golden rule: Use in-game saves as your primary save method. Use save states as a convenience layer on top.”
The larger snapshot is why a state can restore a mid-boss or mid-jump moment, but it also makes states dependent on how a particular emulator represents the machine. A Beetle PSX state, for example, will not load in a different PSX core; Held Games (2026) also notes that a PPSSPP 1.16 state may not load in PPSSPP 1.17. By contrast, an in-game save such as a Pokémon Center save represents the game’s own save data rather than the whole running console.
How does rewind work?
Rewind is commonly implemented by keeping a sequence of save states in memory and restoring earlier snapshots as the player holds a rewind control. MVG’s 2019 description gives one example: push a state every frame onto a last-in, first-out stack, then pop states in reverse. In that tutorial-sized example, a buffer of 360 states provides six seconds of rewind at 60 frames per second. That is an implementation example, not a universal emulator setting; other emulators can use compressed differences between states or capture them less frequently.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do you dump a cartridge you own?
A cartridge dumper reads the game ROM chips and, on supported systems, can also read or restore save memory. Hackaday’s 2023 account of the open-source Sanni Cart Reader describes a setup using an Arduino Mega 2560 Pro, custom PCBs, and firmware on the Mega, with ROM and save data written to an SD card. It also covers specialized media such as Satellaview 8M packs and Nintendo Power SF Memory cartridges. Linus Tech Tips and Save The Hero (2022) discuss Sanni v3 with a Raspberry Pi as a way to copy cartridges already owned. Older commercial dumpers such as the Retrode existed, but were expensive and largely discontinued, according to Hackaday.
- Identify the cartridge and its supported hardware. Check that the reader supports the console and cartridge type; unusual boards or add-on hardware may need specific support.
- Set up the reader according to its documentation. For the Sanni-style path described by Hackaday, that means the reader hardware, its compatible PCBs, firmware, and SD-card workflow.
- Read the game image and save separately where supported. Keep the game dump distinct from the save dump so you can preserve or restore progress without confusing it with the read-only game program.
- Verify the files with an emulator that supports the title’s hardware. A failed load can reflect missing mapper or coprocessor support, incorrect metadata, or a save-type issue rather than a need to modify the original cartridge.
- Back up the save before battery replacement. A battery-backed SRAM save may be erased if the cell is removed without maintaining power to the memory.
This describes a hardware-based copying route, not a blanket legal permission. Dumping an owned cartridge, downloading someone else’s commercial ROM, and distributing a dump are distinct actions with different legal questions.
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Are emulators and ROMs legal?
In the United States, independently written emulator code that does not include copyrighted ROMs, console BIOS, or encryption keys is widely described as the legal core of emulation. Commercial game images remain copyrighted works, and uploading or downloading a commercial ROM without rights can infringe copyright. Gizmodo’s 2024 summary says, “These hardware-to-software environments are fair game so long as they don’t contain any ROMs themselves,” and separately warns, “However, the games themselves, those ROMs, are direct copies of the original games. They are breaking copyright, and it is certainly illegal to distribute.” Those are broad journalistic statements, not a substitute for jurisdiction-specific legal advice.
Owning a cartridge does not automatically give you a license to download someone else’s ROM dump. Personal format-shifting—making a copy from a cartridge you own—is argued to be permitted in some jurisdictions, but its status is contested, particularly when copy protection is bypassed. The material cited here does not establish a current U.S. court holding that makes personal cartridge dumping categorically legal. A Linus Tech Tips presenter summarized the popular distinction in 2022 as “Whatever Nintendo says, emulators are legal – but ROMs are not,” while also saying the presenter was not a lawyer; treat that as a shorthand, not legal advice.
BIOS files and encryption keys raise separate issues from emulator code. Some emulators reimplement firmware behavior in native code, often called a high-level BIOS, rather than including the original firmware. A forum discussion of 17 U.S.C. § 117 and BIOS dumps is an argument, not a court holding. Nintendo’s objection to Dolphin’s attempted Steam distribution over Wii Common Key concerns, reported by Gizmodo in 2024, illustrates why circumvention questions can complicate the otherwise distinct issue of independently written emulator code. Official re-releases, remasters, and services such as Nintendo Switch Online are the rights-cleared route where a title is available; older games can remain unavailable when music, character, or publisher rights are split.
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Keep the game’s in-game save file as the transferable progress file, and treat save states as emulator-specific checkpoints. Before updating an emulator core, a standalone emulator, or handheld firmware, back up the relevant save directory and retain both kinds of file if you may need them. Held Games (2026) notes that handheld firmware such as KNULLI can store .srm saves and save states together under /userdata/saves/, organized by system; being in the same folder does not make the formats interchangeable.
When a save appears missing, first identify which file you were relying on. If it was a state, check whether the same core and a compatible version are available. If it was an in-game save, check that the emulator is using the expected save location and save-memory type. A new emulator may not automatically find a file stored under the previous emulator’s directory structure. Nintendo’s modern Switch setup is different from a coin-cell cartridge: Nintendo says Switch saves are kept in system memory whether a game is downloaded or played from a game card, with cloud backup—where supported—as a separate layer.
Frequently Asked Questions
Can a save state restore a save file that the game erased?
Not by itself: a state can only help if you created one before the erase and it still loads in a compatible emulator core. If the state was made after the game erased the slot, it may simply preserve that already-erased condition.
Does replacing a cartridge battery restore the old save?
No. A replacement cell can retain SRAM that is still present, but it cannot recreate progress that was already lost when the old battery failed. If the old save is important, dump it before removing the existing cell.
Can I play a dumped ROM on original hardware?
A ROM image is a computer file, so original hardware generally needs a compatible flash cartridge or another device that can present the game data to the console. Hardware support and save-memory behavior vary by console and cartridge.
Why does an emulator ask for a BIOS file?
Some systems rely on firmware as well as game data, and an emulator may require a BIOS image rather than reimplementing that behavior. Whether the emulator needs one depends on the system and its design; firmware files can also be copyrighted separately from emulator software.
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