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Retro emulation works by rebuilding a console in software, not by converting a cartridge into a PC game. The emulator models the original CPU, memory bus, graphics chip, audio hardware, timers, controllers, and cartridge-specific electronics; the unmodified ROM or disc image then runs against that model. A ROM file is a dump of the program and graphics chips (or original optical media), while an in-game save is the small block the game writes to SRAM, EEPROM, flash, or a memory card. A save state is different: it freezes the emulator’s entire tracked machine state, including registers and RAM, and stores that snapshot on disk. Understanding those layers explains why an emulator can run without shipping games, why a ROM may need mapper metadata, and why a save state often fails after changing cores or versions.
How do emulators work?
An emulator executes the original game instructions against a software model of the console. It does not rebuild the game as native computer code and does not include the commercial game image by default. As the Dolphin project’s documentation is summarized by IndieKings (2026), “The emulator provides a simulated console, and the unmodified game image runs inside it.”
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The virtual console’s main parts
- CPU model: fetches an opcode at the program counter, decodes its addressing mode, performs the operation, updates registers and flags, and accounts for the required clock cycles.
- System bus and memory map: connects RAM, ROM windows, memory-mapped registers, and bank-switching hardware. The NES, for example, exposes a 64 kB address space divided among ROM, RAM, and I/O registers.
- PPU or GPU model: handles video memory, sprites, backgrounds, scrolling, blanking intervals, and video interrupts.
- APU model: reproduces sound registers, timers, channels, and sample output.
- Cartridge hardware: implements mappers, coprocessors, save memory, and other board-specific behavior.
Klika’s description of NES emulation captures the central loop: read the instruction at the program counter, decode its type and operands, run the matching routine, then update memory and registers. Instructions consume different numbers of cycles, so an emulator that gets timing wrong can make video, audio, input, or interrupts drift out of alignment.
Interpretation versus recompilation
With interpretation, the emulator decodes and executes one original instruction at a time. That makes cycle counting and debugging straightforward, but it costs host-CPU time. Dynamic recompilation (JIT or dynarec) translates frequently executed blocks into native host instructions while the game runs. Static recompilation translates a whole program ahead of time. The latter approaches can be faster, but self-modifying code, interrupts, timing-sensitive hardware, and unusual coprocessors make a simple conversion unreliable.
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Older systems such as the NES and SNES often require tight synchronization between CPU and video hardware. Dolphin exposes the same trade-off in its audio engines: high-level emulation (HLE) is faster, while low-level emulation (LLE) is slower but closer to the original hardware. Dolphin’s documented host baseline is a CPU with SSE2 support, a modern dual-core processor around 3 GHz, and graphics support for Direct3D 11.1 or OpenGL 3.3; those are requirements for running the emulator, not steps that convert a ROM.
What is a ROM file?
On a cartridge, the game normally lived in mask-ROM chips. NES boards separated PRG ROM (program code read by the CPU) from CHR ROM (graphics read by the PPU). As NesHacker (2025) puts it, “Game boards have a chip inside called a program ROM, and it contains all the code and graphics.” A ROM file is a digital dump of those chip contents, or a dump of original optical media for CD and DVD systems. Disc images may use BIN/CUE, ISO, CHD, RVZ, or another format; they remain copies of the original media rather than native ports.
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What is inside a typical NES dump?
The common iNES layout, documented by bugzmanov from Marat Fayzullin’s format, has a 16-byte header, an optional 512-byte trainer block, PRG ROM in 16 kB units, and CHR ROM in 8 kB units. The header identifies the sizes, mapper number, and name-table mirroring mode (horizontal, vertical, or four-screen). NES 2.0 extends the backward-compatible header with more cartridge metadata and reduces guesswork for larger or unusual boards.
| Part | Purpose | Typical detail |
|---|---|---|
| Header | Tells the emulator how to wire the image | 16 bytes in iNES; NES 2.0 carries additional metadata |
| Trainer | Legacy copier data placed before the ROM | Optional 512 bytes; usually skipped by modern emulators |
| PRG ROM | CPU instructions and program data | Stored in 16 kB units in iNES |
| CHR ROM | Pattern tables and graphics data | Stored in 8 kB units in iNES |
Why is a ROM dump not enough by itself?
“Because NES cartridges contain custom hardware, ROM dumps alone are not enough to accurately describe how a game should be emulated,” notes Emulation General Wiki (2026). A mapper chip can swap different portions of a large ROM into the CPU or PPU address space, generate interrupts, or expose extra audio. The file’s header tells the emulator which mapper implementation to attach, but the implementation itself must exist in emulator code. An unheadered dump, or a newly dumped board with an unknown mapper, may therefore fail to boot even when every ROM byte is correct.
Super NES cartridges can add coprocessors, and Game Boy Advance cartridges can use different save-memory devices in the same address-map region. bsnes.org (2023) describes combining ROM images with headers, checksums, and high-level coprocessor emulation to decide how a title is wired. This is why two files with similar sizes can require different emulator settings.
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How did original cartridges save progress?
Save data is separate from read-only program ROM. The game writes only the fields its designers intended to persist: a few player records, inventory values, checksums, and other metadata. Depending on the cartridge, that writable memory is battery-backed SRAM, EEPROM, flash, or (for disc systems) a memory card.
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Static RAM is fast and freely writable but volatile: it loses contents when power disappears. A coin cell keeps it in low-power retention mode while the console is off. The familiar design uses the console’s 5-volt supply during play, a lithium cell during shutdown, a diode that prevents the battery from powering the whole cartridge, a second diode that prevents the console from charging the cell, and resistors that keep the chip deselected as voltage falls. The retention current is only a few microamps.
Nintendo Power described the consumer promise 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 1990s cartridges are now far beyond that design life. CR2032 cells with vertical through-hole tabs are common on NES, SNES, N64, some Sega boards, and GameCube clock circuits; Game Boy, Game Boy Color, and some GBA boards often use tabbed CR1616 or CR2025 cells. A bare store-bought coin cell lacks the welded tabs these boards require.
Games commonly store a checksum over SRAM. If corruption makes the checksum invalid, the game may erase the slot and present it as blank. Editing a .srm or .sav file in a hex editor therefore requires recomputing that title’s game-specific checksum.
Some SNES SxROM/MMC1 boards used an 8 kB RAM chip with an SL solder selection that determined whether a battery, diodes, and resistors were fitted. The same board family could consequently be battery-backed or batteryless.
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EEPROM and flash memory
EEPROM and flash retain data without a coin cell. Many Nintendo 64 cartridges used small EEPROM devices, while others used SRAM or flash, so the absence of a battery is normal for numerous titles. Documented official GBA save types 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 type cannot simply use another because the interfaces occupy the same GBA memory-map region.
Why GBA save detection can be wrong
The GBA internal header does not identify the save chip. Nintendo SDK libraries left strings such as EEPROM_V, SRAM_V, FLASH_V, FLASH512_V, and FLASH1M_V in the ROM. Emulators scan for those fingerprints, but developers sometimes altered strings, included multiple libraries, or triggered copy-protection behavior that caused the wrong device to be selected. VBA-M uses defaults plus exceptions, while MAME aims for a more exhaustive database; some emulators also expose a manual override. That override can be confusing because it may ask for flash-vendor details a player cannot know without opening the cartridge. Emulating EEPROM does not wear out the original physical chip; wear is a hardware-cartridge concern.
What is a save state?
A save state is an emulator-created snapshot, not a game save. The core serializes CPU registers, RAM, video memory, timers, controller state, sound state, and any other internal variables it tracks, then writes that blob to disk. Reloading restores the emulated machine at essentially the same frame. On a Game Boy, that includes the LCD position, scrolling registers, palettes, VRAM tiles, sprites, timers, joystick I/O, and audio state.
| Save method | What it contains | Portability | Main risk |
|---|---|---|---|
| In-game save / SRAM / .srm or .sav | Only data the game writes to its save device | Usually portable among compatible emulators and, with suitable hardware, real cartridges | Battery failure, corruption, or a game checksum rejecting the file |
| Save state / .state | Whole tracked emulator machine, including registers and RAM | Bound to a specific core and often a specific version; not usable on original hardware | Core updates, accidental overwrite, or capturing a broken state during a glitch |
Data Crystal summarizes the distinction: "SRAM differs from emulator savestates, which are unique to each emulator." It also explains that savestate files hold all data in the emulated system, while SRAM holds only what the game designers intended to save. A state can work at an arbitrary point—even during a jump or boss attack—and it supports games that never had a password or battery. It is nevertheless a convenience layer, not an archival format. Held Games’ practical rule is: "The golden rule: Use in-game saves as your primary save method. Use save states as a convenience layer on top."
Why save states stop working after an update
There is no single industry-wide state format. A Beetle PSX state is not expected to load in another PlayStation core, and a state made by PPSSPP 1.16 may fail in 1.17. Before changing cores, updating an emulator, or installing handheld firmware, copy both the in-game save directory and the state directory. Keep named slots instead of relying on one Quick Save that is continually overwritten. Handheld firmware can place .srm files and states in the same tree—for example, KNULLI uses /userdata/saves/ organized by system—so identify files by extension before deleting anything.
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Why did my Zelda or Pokémon save disappear?
- Dead battery: Battery-backed SRAM loses its contents when the coin cell can no longer supply retention power.
- Battery replacement without backup: Removing the old cell while the SRAM is unpowered usually erases the save. Dump the SRAM first or keep the chip powered during the replacement.
- Wrong save type: A GBA image configured as EEPROM instead of SRAM (or the reverse) can appear to save and then load blank.
- Checksum failure: Corrupted or edited bytes can cause the game to wipe the slot.
- Wrong file path or name: The emulator may be looking for a different basename, extension, or system folder.
- State confusion: Loading an old state can make it appear that a newer in-game save vanished, because the state restores older RAM over the current session.
How do you dump a cartridge you own?
A cartridge dump is made by electrically reading its ROM and save chips with a dumper; it is not downloaded from an emulator. Open-source Sanni Cart Reader builds use an Arduino Mega 2560 Pro, custom PCBs, and firmware that writes ROM and save dumps to an SD card. The documented hardware path can also handle unusual media such as Satellaview 8M packs and Nintendo Power SF Memory cartridges. Older devices such as the Retrode existed but became expensive and largely discontinued.
- Inspect the cartridge and identify the system, board, mapper or coprocessor, and save technology.
- Connect the cartridge to a compatible reader and install the reader’s firmware as documented by its project.
- Dump the ROM to an SD card or computer, then make a separate SRAM/EEPROM/flash dump if the reader supports it.
- Verify the dump with the reader’s checksum or a known-good database entry; retain the original untouched files.
- Load the ROM in an emulator that supports the reported mapper and copy the save file using the emulator’s expected basename and extension.
Some older floppy-based copiers placed a 512-byte prefix in SRAM dumps; removing that prefix may be necessary before an emulator recognizes the file. Linus Tech Tips’ Save The Hero coverage also warns that 16-bit-era battery-backed SRAM is well past its intended life, making a pre-replacement dump especially important.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Are emulators legal, and are ROMs legal?
The legal answer depends on jurisdiction and exactly what is copied or distributed. Independently written emulator software that does not include copyrighted ROMs, console BIOS, or encryption keys is widely treated as the lawful core. Gizmodo (2024) states, "These hardware-to-software environments are fair game so long as they don’t contain any ROMs themselves."
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Personal dumping of a cartridge you own is argued as format shifting or archival use in some jurisdictions, but there is no current U.S. court holding establishing that every personal dump is categorically legal, especially where copy-protection circumvention is involved. Forum discussions sometimes cite 17 U.S.C. § 117 for copies necessary to use a lawfully owned computer program, yet games are also copyrighted audiovisual works and that interpretation is not a court ruling. Nintendo has challenged projects it views as circumvention; the Dolphin Steam dispute concerned the Wii Common Key, while Dolphin’s team said the project was not primarily designed to bypass protection.
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Official re-releases, remasters, and services such as Nintendo Switch Online are the rights-cleared route. Availability is incomplete because older games can involve separate music, character, or publisher licenses.
Which saving and playback method should you choose?
- For the most portable archive: keep the original ROM dump plus the game’s .srm/.sav or memory-card image. Those files represent the console’s intended save layer.
- For difficult sections: make a named save state, but also trigger the game’s own save immediately afterward.
- For switching emulators: move the in-game save first; expect to recreate states in the new core.
- For a physical collection: dump SRAM before replacing a CR2032, use a tabbed cell and a 3.8 mm Gamebit screwdriver, and desolder properly rather than taping a bare coin cell to the pads.
- For convenience without files: rewind keeps recent states in memory. One documented implementation stores 360 states—six seconds at 60 frames per second—but that is an example, not a universal specification; other emulators use compressed differences or different intervals.
- For maximum authenticity: original hardware with a repaired battery or flash cartridge, an FPGA recreation, or an official re-release avoids some emulator-version dependencies, but each option has its own hardware, licensing, and availability limits.
Keep at least two backups of the in-game save and the original dump. Treat save states as disposable checkpoints: useful for experimentation and recovery, but not the copy you expect to survive every core, firmware update, or hardware change.
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Frequently Asked Questions
Can a save state be converted into an .srm file?
Usually not directly. A state contains emulator-only registers, timers, and RAM, while an .srm file contains the game’s own save structure; the reliable route is to load the state, use the game’s save menu, then copy the newly written .srm or .sav.
Will replacing a cartridge battery always erase the save?
It can, because SRAM needs continuous power for retention. Dump the save first or keep the SRAM powered during the change, and install a properly tabbed cell with the correct polarity.
Why does the same ROM need different emulator settings?
The image may rely on a mapper, coprocessor, or save chip that is not obvious from its raw bytes. Header data, checksum databases, and emulator overrides tell the core which hardware model to attach.
Are save states safe to share with another player?
Only when both players use the same emulator core and a compatible version, game image, region, and settings. Sharing the game’s .srm or memory-card file is generally more portable, subject to the title’s checksum and save format.
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