Media Notes / Backwards Compatibility

Backwards Compatibility is the ability for a new product to have compatibility with an old product; while this refers to all sorts of technological devices, most people have probably heard about backwards compatibility in relation to video game systems; that is, the ability for your a video game console to play games developed for its predecessors.

What's so great about backwards compatibility? Well, imagine the headache it would be if your device's USB 2.0 plug couldn't be used with a USB 3.0 port? Or you had to download entirely new versions of every piece of software you own whenever you updated your computer's operating system? Backwards compatibility allows for technological progress without having to start from scratch upon every advancement, while also giving enough time for even the most technologically-inept or lazy among us to eventually upgrade to the improved product. Plus, it's just plain convenient and makes for a good value proposition; a Blu-Ray player is great because it not only plays Blu-Ray discs, but it can also read DVDs, so you don't need separate devices for both media.

With gaming hardware, being able to utilize even the physical discs or cartridges for previous generation games on your new system also means that it is easier to engage with older titles you would missed out on otherwise. For example, if you've stumbled upon a cheap physical copy of an Xbox 360 game you're interested in, but you don't own that old system, you can still buy it to play on one of the newer Xbox systems and it'll work just as well.

There are a few ways to make a device accomplish backwards compatibility

• Hardware based:
  • Architecture evolution: The new product is an evolution of the previous system's architecture. For example, the Wii and Nintendo GameCube have very similar chips to the point that the former can play the latter's games by dropping the CPU's clock speed and limiting RAM access. Similarly, the Game Boy Color was an evolution of the Game Boy and could play GameBoy games in the same fashion.
  • Integrating older hardware: The new product integrated hardware from the previous system into itself. For example, the Game Boy Advance can play Game Boy Color games because it has the latter's CPU in addition to its own, though that CPU is still used when playing GameBoy Advance games for other purposes.
  • Add-On: The device on its own can't pull it off, but you can attach an adapter that has the needed hardware to get the job done. For example, the Atari 5200 also had an add-on module for Atari 2600 games.
• Software based:
  • Emulation: A program mimics the hardware of the older system in order for the game to run. For example, some versions of the PlayStation 3 did this in order to play PS2 games.
  • Virtualization: Creates a virtual computer in the system, allowing other, compatible operating systems (typically older ones) to run alongside the system's operating system. This is commonly done with older computer games, such as those originally made for earlier versions of Windows. However, virtualization still uses the system's hardware, so it can't allow say an ARM based smartphone to run an x86 based OS.

As an aside, this page also covers forwards compatibility, which is the ability for older systems to run software created for newer systems.

See also Game Within a Game. May also involve Embedded Precursor.

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Examples:

Apple

• The Apple IIGS was backwards-compatible with its predecessor, the Apple II, even though it was a 16-bit computer. As with the OS/2 mentioned below, it's believed that the ability of the computer to run software for the older machines in the Apple II family discouraged native development. This caused a chicken-and-egg scenario where people didn't buy the new computer because there was little software that took advantage of its features, and people didn't develop for the new computer because it didn't sell well. It didn't help that it was hobbled by a slow processor. The IIGS was ultimately overshadowed by the more powerful Macintosh.
• macOS is based on Unix, and Apple released a version of Unix on the Mac called A/UX in the late '80s and early '90s, but the company also offered the Macintosh Application Environment for Sun and HP workstations in the mid-90s.
• Apple released an Apple IIe card for certain Mac models, mainly the LC series, designed to get Macs into elementary schools that were still mainly based on Apple II computers.
• The Apple Macintosh has endured changes requiring backwards compatibility.
  • With the move from 68k to PowerPC processors, macOS gained a 68k emulator. PowerPC Macs were able to run 68k programs, and programs that mix 68k and PowerPC code. The emulator was generally faster than a 68k Mac, but 68k code was slower than native PowerPC code.
  • Mac OS X, versions 10.0 through 10.4, used the "Classic" emulator to run Mac OS 9 programs at full speed (though this includes waiting for Mac OS 9 to boot inside Classic). To avoid this emulation, there were also "Carbonized" apps that run in both OS 9 and OS X (without Classic).
  • Intel Macs, running OS X 10.4 through 10.6, use the Rosetta emulator to run PowerPC programs. (Sorry, Rosetta didn't run Classic.) OS X Lion 10.7 can't run Rosetta.
  • During the era of Mac OS 7, Apple made a DOS Compatibility Card with a 486. Later, OrangePC made similar cards. These NuBus or PCI cards had a 486 or Pentium and booted DOS or Windows. These cards were so expensive, that a whole PC might be cheaper. Later, emulation became feasible (on faster processors like the PowerPC G3), and emulators like Virtual PC appeared.
  • Intel-based Macs can use Boot Camp to dual-boot macOS and Windows, similar to how PCs can dual-boot Windows and Linux with a bootloader.
  • ARM-based Macs, which debuted in 2020 alongside macOS Big Sur, can use the Rosetta 2 emulator to run software made for Intel-based Macs.
• iOS applications made for earlier versions starting from when Apple allowed third-parties to write native apps could be run on later versions, though only 64-bit applications could be run on iOS 11 onwards, with app developers expected to port their wares to ARM64 if they still want their apps and games listed on the App Store.

Atari

• Infamously, thanks to them being able to recreate the Atari 2600's hardware without infringing on any of the company's patents, all of Atari's competitors were able to have their systems also play 2600 games with no legal repercussions. The Colecovision's Expansion Module and the Intellivision II's System Changer are the most well-known of these peripherals, and Coleco would even go on to sell a standalone 2600 clone console called the Gemini. Coleco is the only one of these companies that Atari tried taking to court over this, with the situation ending in an out-of-court settlement where Coleco continued to sell their products as a licensee.
• The Atari 5200 had an add-on module for Atari 2600 games known as the CX-55 2600 Adapter. This rather clumsy add-on contained the entire 2600 hardware on-board, including 2 extra controller ports for the otherwise incompatible DB9 joysticks. Further complicating matters is the fact this adapter only worked on later 2-port units and not the original 4-port consoles without being modified. This caused Atari to start scraping the latter consoles until Gary Rubio, one of their service engineers, created a fix.
• The Atari 7800 has the 2600's Television Interface Adapter chip, which in conjunction with slowing down the system processor and limiting RAM access, allows it to play Atari 2600 games.
• Recently, the Atari VCS has stated that in addition to smart TV features and new games from this generation, Atari 2600 hardware is part of the VCS with almost all games already added into the console.

Microsoft

• Xbox 360 is backwards compatible with approximately half of the Xbox game library, though many of these games have game-specific speed hacks and glitches not in the original, as detailed here on The Other Wiki. This ends up making the emulator similar to ZSNES, an emulator infamous for its inaccuracy.
• By popular demand, Microsoft repeated the same formula with Xbox One. By the end of the eighth generation, about a quarter of the Xbox 360 library, and a small number of original Xbox games, could be played on the system. The mid-generation refresh console, the Xbox One X, also plays these games with increased graphical resolution without altering the game code. One reason why not all games were made backwards compatible is due to licensing complications, with tie-in games based on licensed properties and older sports games being largely out of the question.
• Xbox Series X and Series S is backwards compatible with the vast majority of the Xbox One library, and continues with the effort of making even more games from prior Xbox generations compatible, as games compatible with the Xbox One one are also compatible for the X|S. Many of these games also see improved graphical resolution and better performance. The only major exception are games that require the Kinect accessory.
• And while Microsoft barred unofficial emulators from being marketed on their app stores, the Xbox One and its successors do have a dev mode accessible through a one-time paywall which allows said emulators to be sideloaded, turning these consoles into a viable emulation box.
• In a What Could Have Been example, there were conversations between Sega and Microsoft about having Dreamcast games be compatible with the Xbox. The idea was abandoned when the two companies disagreed on how the Dreamcast's online features would be implemented, but the companies retained a good enough relationship that Sega would develop some exclusive games for Xbox, like OutRun 2, Shenmue II and Jet Set Radio Future.
• Microsoft Windows:
  • All 32-bit editions support 16-bit Windows applications. MS-DOS applications are supported to a varying degree.
    • The Windows 9x family are 16/32-bit hybrid operating systems, with a 32-bit kernel and user space, but with parts of it still in 16-bit as well as MS-DOS being used to both boot the OS and as a compatibility layer. Since 9x still depended on DOS to a degree, 16-bit applications for Windows 3.x and DOS should work, though some apps and games may present issues when launched from Windows; this is why the "Restart to MS-DOS Mode" option exists in 95 and 98.
    • Windows editions based off the NT kernel do not depend on MS-DOS at all, delegating all MS-DOS applications to a virtual DOS machine (NTVDM), whose compatibility with DOS applications being spotty at best. With Windows XP being the first consumer intended version of Windows NT to come out, NTVDM had great MS-DOS support, at least for most of the popular games of the 90s. Afterwards, MS-DOS support was spotty again. With Windows 11 dropping support for 32-bit processors, NTVDM has been discontinued.
  • 32-bit applications are supported on 64-bit versions by way of a compatibility layer. 16-bit applications and MS-DOS programs won't work because of the lack of a 16-bit compatibility layer (which was explained that the way OS tracks resources can't fit in how 16-bit applications track them) and the lack of an MS-DOS emulator of some sort. Those who still need DOS and/or Windows 3.x support on modern systems may turn to third-party solutions like DOS Box or [https://github.com/otya128/winevdm winevdm], a compatibility layer based on both WINE and MAME's 80386 core.
  • Windows heavily relies on the so-called "Side-by-side" system and shims for application compatibility as well. The "Side-by-side" system selects a version of a library from multiple versions depending on what the so-called application manifest says. Shims intercept calls to a library, accepting the application's input, then generates an output the application is expecting. This includes behavior caused by bugs in the originally supported version.
  • Windows has had an extensive history of backwards compatibility especially since the Windows 9x days, to the point that at least some Windows applications from the 90s could still run unmodified on Windows 11, which was in many ways purposely done for business users still dependent on legacy hardware and software. This however came at the cost of a messy and inconsistent user interface especially in Windows 10 and 11, where elements from previous operating systems are still somehow present. Windows 11 even has two context menus, with the legacy flyout accessible from the "Show more options" option!

Nintendo

• Super Nintendo Entertainment System:
  • Nintendo initially planned for the SNES to be backwards compatible with the Nintendo Entertainment System, but the plan was scrapped due to time and cost; the system was already late to market, coming out two years after the rival Sega Genesis, and adding backwards compatibility would have increased the time needed to develop the system and also increased the price. Nintendo then considered an add-on for this purpose as well, but this was presumably scrapped due to cost as well, as the differences between the NES and SNES meant that the add-on would have needed the development of its own sound chip. What they ultimately do is advise NES owners in the SNES manual to connect both systems' RF switches together so that players can use both, which isn't true backwards compatibility at all, but it's the closest the SNES will ever get.
  • What the SNES did get was what we'll generously call "sideways compatibility" via the Super Game Boy adapter cartridge, which allowed one to play Game Boy cartridges on the TV. Several games were "Super Game Boy enhanced" (i.e., had custom color palettes, higher quality music, and even entire games that were only accessed when used with the peripheral). A Super Game Boy 2 was released in 1998 exclusively for Japan, adding a link port for multiplayer and also fixing a technical issue with the SGB's clock speed.
• Nintendo 64:
  • The Nintendo 64 itself had no backwards compatibility with the Super Nintendo, but it's interesting to note that the manual actually suggested ways to use both on one system, which included connecting the N64 and SNES to the TV with an AV cable and RF switch respectively, connecting the N64 to a VCR via an AV cable, and then connecting the VCR's RF out to the input of an SNES's RF switch, or just daisy-chaining the two RF switches together to connect both systems.
  • The WideBoy 64 was a non-commercial successor to the Super Game Boy, meant for use with the Nintendo 64. It was made exclusively for game developers to test games on a TV and make it easier for journalists to take screenshots for publications. The first version supported Game Boy and Game Boy Color titles, followed by a revision that added GBA support. Since it wasn't meant for the general public, finding one nowadays for a reasonable price is a game in and of itself.
  • An unlicensed accessory similar to the WideBoy 64 was the GB Hunter. It however relied on software emulation and could not play audio at all, instead subjecting players to an endlessly looping theme tune.
  • The Transfer Pak, an accessory that transferred data between N64 games and GB/GBC games, could be used to play handheld games on the television, but the only games that made use of this ability were the first two Pokémon Stadium entries, which let you play the Gen I and Gen II mainline games.
• The Game Boy Color was basically a Game Boy with a higher-clocked CPU, and thus was able to play old games like Tetris (or play Tetris DX in color). More interesting is that a third of the system's library was actually forwards compatible, thus could be played on the original Game Boy hardware (albeit stuck in monochrome) and the Super Game Boy (with some even being "SGB enhanced"); these cartridges resemble typical Game Boy cartridges, but usually black instead of gray. Game Boy Color-exclusive carts, meanwhile, were translucent and rounder in shape.
• The Game Boy Advance included the Game Boy Color's Z80 processor alongside its own ARM7 processor, thus allowing it to play both Game Boy and Game Boy Color games in addition to GBA ones. Some GBA games also used the Z80 as an additional sound processor. Games were stretched to the GBA's screen ratio by default, but pressing the shoulder buttons during gameplay can change it to the GB/GBC's. Multiplayer also remained intact, but required the use of the old link cables rather than the new ones. While the GBA SP would retain backwards compatibility, the Game Boy Micro would drop the feature.
• Nintendo GameCube:
  • The next handheld-to-TV adapter was the Game Boy Player for the GameCube. Containing all the hardware of GBA, it allowed one to play games from across the entire Game Boy family of systems, and much like its predecessor, several GBA games had enhanced features when used on the Game Boy Player (such as more vibrant color palettes and rumble support). The only GBA games that refuse to run were the Game Boy Advance Video series of cartridges, due to concerns that people would record the high quality videos with a VHS or DVD recorder.
  • Datel (the manufacturer of the GameShark and the Action Replay series of cheat devices) released their own third-party GBA adapter known as the Advance Game Port. This would plug into the Memory Card slot of a GameCube and use a disc containing a software emulator to run GBA cartridges plugged into the Memory Card slot adapter. It didn't support Game Boy or Game Boy Color cartridges, but this peculiar setup also meant that — unlike the Game Boy Player — it was forward compatible with the Wii. Though it must be said that the quality of the emulator was questionable at best, being prone to sound issues and slowdown for more graphically-intense games.
• The original and Lite variants of the Nintendo DS had a GBA cartridge slot, and all the GBA hardware to go with it (minus the Z80 CPU). One was also limited to playing single-player GBA games, as the Nintendo DS lacked the GBA's link cable port for multiplayer. Beyond backwards compatibility, various DS games would use the GBA slot to allow for things such as data transfer (namely the Pokémon games) and as a peripheral slot (such as the "Guitar Grip" used for the Guitar Hero: On Tour games). The slot and hardware of the GBA were removed in the DSi and the DSi XL revisions.
• Wii:
  • Given that the Wii was not inaccurately described as "two GameCubes duct-taped together", it was fairly trivial for Nintendo to implement full backwards compatibility. This was achieved with a compatibility layer known as "MIOS" which lowered the Wii's CPU and GPU clock speeds down to the GameCube's original speeds, limited the available amount of RAM, and patched any problematic games before launching them. The Wii has four GameCube controller slots and two memory cards located on top of the console, and these were required — GameCube games on Wii did not support Wii Remotes/Classic Controllers or saving games onto the Wii itself. On the plus side, GameCube controllers could be used with a multitude of Wii games - but not the actual Wii menu itself, forcing the use of a Wii Remote just to start the game. Despite this annoyance and lacking support for the Game Boy Player and GameCube broadband adapter, the Wii is generally the preferred option for playing GameCube games for most these days. Primarily, this is due to the Wii being much cheaper to find secondhand, component cables for GameCube being very rare and expensive, and it being much easier to pirate games on Wii compared to GameCube. The last two years of the Wii's lifespan introduced the Wii Family Edition (RVL-101) and the Wii Mini (RVL-201) revisions of the Wii hardware; these officially removed GameCube backwards compatibility, but it can accessed through homebrew. (Homebrew also enables the use of non-GameCube and even non-Nintendo controllers, saving game data onto the SD card, broadband adapter emulation, and the ability to set up a GameCube-controller compatible loader to launch on boot.)
  • The Wii also introduced Virtual Console, which lets players download and emulate games from older Nintendo and non-Nintendo consoles. The Nintendo 3DS and Wii U would have their own versions of Virtual Console, each adding a few more systems into the mix, before the brand was discontinued and replaced with the on-demand library available as part of Nintendo Switch Online. As a technical note, rather than having emulators for each gaming system, each game released through Virtual Console had their own individual, specialized emulator. Nintendo would go the standard route for emulation starting with the Switch.
• The Nintendo 3DS plays Nintendo DS games (and DSiWare) with no problem; no need to worry about the 3DS being Nintendo's only region-locked handheld somehow affecting your ability to play European DS games on a North American 3DS system. Similar to the Game Boy Advance when it played Game Boy and Game Boy Color games, the 3DS resizes DS and DSi games to fit its larger screen; holding down the Start or Select button when opening the game launches it in the original screen ratio, surrounded by black borders. Speaking of the GBA, every 3DS also has the ability to run GBA games through a hidden software layer known as AGB_FIRM, which is used for the exclusive Ambassador Titles (and custom injections on homebrewed systems).
• The Wii U keeps Wii compatibility using a similar method as its predecessor, but ditches GameCube compatibility. Everything for the Wii — game discs, controllers, peripherals, etc. — still work, but you can no longer use GameCube controllers in Wii games because the controller ports no longer existed. Nintendo would later release an adapter that allowed for the use of GameCube controllers, but it only worked for Super Smash Bros for Wii U. Similarly to the later Wii revisions, it is possible to access GameCube backwards compatibility regardless with hacking, though it will still be incapable of playing the actual discs. The Wii U also has a separate 512MB NAND memory chip for Wii save data and downloadable titles.

PC

• The "PC" as we tend to know it (at least as of 2021) was built off of and based around the IBM 5150 Personal Computer. A driving point for clones and updates of the IBM PC was that backwards compatibility was paramount. If the computer you built wasn't compatible with many of the software used on the IBM PC, your chances of success were slim. And most of this standard is still being followed today. For example, the CPUs you put in your PC still boot up in a legacy mode called Real Mode, which the CPU behaves like the original Intel 8086/8088, including its now paltry 1 megabyte of addressable memory limit. It's to the point with little effort you can install and run MS-DOS 6.22 on a computer made some 25 years after its release.
  • The distinction of being based on the original IBM PC is important, as there are some computer systems that use x86 processors like PCs do, but cannot use PC operating systems without modification. The PlayStation 4 is an example of this. Even around the time, there were Intel 8086/8088 processors used for some computer-like systems, like a Kodak "computer" that was simply a slideshow creator/presenter (the predecessor to presentation software like Microsoft PowerPoint).
  • In 2023, Intel made headlines in that it's thinking about removing the legacy features. However, this means that, should they make CPUs that follow this, older 16-bit and 32-bit operating systems won't work anymore... which hasn't seen prominent use anyway since the early 2010s.
• Thanks to the enormous processing power of modern computers, emulators for PC exist for all consoles up to the sixth generation, albeit with varying degrees of compatibility and requirements. For more information, see our Emulation page, and Emulation on The Other Wiki. If that's not enough, you can run multiple operating systems on the same machine through multi-booting and virtualization.
• OS/2 could run Windows applications, though this backfired spectacularly after IBM and Microsoft split in the early '90s. Because software developers could target Windows and OS/2 users just by writing a Windows app, very few people developed OS/2 programs.
• Wine (WINE Is Not an Emulator) allows Unix-like systems to run Windows programs. Its status in the Linux community is controversial because some users believe it discourages native Linux app development as OS/2's Windows compatibility did, but its proponents argue that WINE can attract more users to the platform.
  Valve Software has taken this further by incorporating a WINE-based subsystem right into Steam on Linux. Called Proton internally and "SteamPlay" in marketing, it allows a ridiculous number of Windows games to run with minimal issues and minimal user involvement. A growing number of games are even "whitelisted" as being guaranteed to work out of the box.
• Going to the Windows side, Windows Subsystem for Linux (WSL) allows developers to run native Linux binaries on Windows by translating Linux system calls into equivalent Windows system calls. WSL 2 even uses an actual Linux kernel. It's possible to launch Linux and Windows programs from each other's command lines. Before that, the open source Cygwin implemented an API to allow Unix-like programs to be compiled into Windows binaries. Microsoft's development of WSL also allowed them to develop an Android compatibility layer for Windows 11, though Microsoft eventually discontinued it. The company did ship a Wayland/X server with WSL on Windows 11 to enable it to run Linux GUI apps as well as terminal programs.
• One of the reasons that Linux vaulted ahead of other free Unix clones in the '90s has been the Linux community's emphasis on interoperability. It's always been very easy to install Linux and keep using Windows through dual-booting. This meant that Linux users could still use mainstream applications along with the powerful Unix toolset.
• When using Wayland (a new display server protocol for Linux and other Unix-like systems that aims to replace X), apps still using X can run using the Xwayland compatibility layer.
• Similar to Wayland, the XQuartz project allows macOS to run X11 applications. Apple previously developed a dedicated X11 app for this, but stopped development in favor of the open source XQuartz. As most graphical Unix applications have native macOS ports, it's more useful for accessing X programs on a remote server using SSH.
• There are several X servers available for Microsoft Windows, though like on the Mac they're more useful for running X11 apps over SSH. It's possible to use them with Cygwin or Windows Subsystem for Linux.
• Versions of Windows NT through Windows 2000 had the ability to run OS/2 applications, though only in text mode without some additional software.
• The ARM version of Windows 10 and 11 can run x86 applications through the use of an emulator.
• At the moment, 64-bit versions of Windows 10 and 11 could still run older 32-bit applications from as far back as Windows 95 and Win32S programs written for windows 3.1 and Windows NT 3.1 natively. By installing a compatibility layer called WineVDM (a fork of Wine), even 16-bit software from as far back as Windows 1.0 can also be run.
• Applications written for middleware like Java or Flash Player are generally guaranteed to run on any platform that has a working copy of that middleware installed. This is how Minecraft was able to have working Mac and Linux ports from day one. You could even play Flash games on the Wii through its built-in web browser (and, presumably, still can, provided they were made for Flash Player 7 or older and are still online).
• This is subverted with applications on Linux. While Linux has a huge array of compatibility out of the box with regards to what hardware it can run on, it's the software where you may run into trouble. Most Linux distributions use a package manager system to distribute software. However, this software is usually only delivered as-is, requiring any dependencies the software needs to also be hosted on the package manager system. If the dependency isn't there (which is rare) or the version that's hosted is incompatible with what you're trying to run, you'll run into problems. The best you can do is hope a compatible version is uploaded somewhere, requiring you to find it, download it, and install it or build it. Windows and macOS get around this by having the application installer ship with the dependencies it needs and either runs them in an encapsulated manner or installs those dependencies. Though there are certain app distribution systems on Linux such as Snap, Flatpak, and AppImage that aim to resolve this.
• macOS has switched which CPU architecture it runs three times: the Motorola 68k family to the PowerPC family, from PowerPC to x86, and finally from x86 to ARM. For the first transition, app developers usually shipped with two versions on the same distribution media (with CDs at the time, this wasn't a problem). For the second transition, they had an application called Rosetta to emulate PowerPC systems, even running an instance of Mac OS 9 for older apps. For the third transition, they created Rosetta 2 to emulate x86 on ARM.
• A lot of interface standards over the years have at least tried to keep backwards compatibility in some form or fashion whenever an update to it gets released.
  • Modems that run over a phone line over the decades they were used simply evolved from the Bell 103/V.21 300 bps standard. In fact, all modems basically talk in this 300 bps mode first, doing some handshaking before agreeing on which speed to actually talk over. The distinctive sound of a dial-up modem connecting is basically the handshaking going on. The last version of the 56 kbps standard had a special message to skip much of the handshaking when this got to be too much.
  • The ISA expansion slot had two expansions to it that allowed the use of older ISA cards. The first one, a 16-bit extension, simply added more pins with a key where the old ISA slot ended. The second one, Extended ISA (EISA), had something of a two-layer-of-pins setup, where ISA cards can work in the upper layer while EISA cards uses both the upper and lower layers. Technically, the VESA Local Bus (VLB) slot used ISA, but it was considered a separate thing.
  • PCI, which succeeded ISA, also had an extension called PCI-X (not to be confused with PCI Express), which basically added onto the existing PCI slot. In addition, PCI slots and cards were done such that they would be installed upside down relative to ISA cards, which allowed one expansion slot to fit either an ISA or PCI card. There were also 5V and 3.3V versions of the card, with cards eventually coming out with a universal standard.
  • Storage drive interfaces like IDE and SATA retained the same connectors and simply updated the electrical bits over time.
  • Today, CPU sockets tend to be good for around two or three generations of CPUs and only for CPUs from a particular company (Intel or AMD). However, Socket 3 from Intel was the sole exception to this. Not only was it compatible with a wide range of CPU families from Intel, it was also compatible with CPU families from AMD and Cyrix. It was also compatible with CPUs with fewer pins than the socket had.
  • USB's wide range of compatibility stems from using standard device profiles that manufacturers must adhere to and using physical connectors that extend previous versions. However, this only allowed older devices to plug into newer receptacles. USB 3.0 Type B connectors can't fit in a USB 2.0 Type B receptacle, which led to devices using 2.0's Type B receptacles for longer than anticipated.
    • USB Type-C flips this on its head unfortunately. What it actually supports depends on what the device manufacturer decided. So it's best to not treat every USB-C port as equal.
  • The PCIe standard that replaced PCI, ISA, and other expansion card slots is by far the most flexible expansion interface so far.
    • Devices/receptacles forwards and backwards compatible, meaning a PCIe 1.0 device can plug in a PCIe 4.0 receptacle and work and vice versa. At worst this incurs a performance hit since the revisions mostly pertain to how much data the slot/card can transfer per second.
    • For the PCIe slot and cards themselves, the physical size of the connector doesn't matter. A PCIe x1 card will fit in a x16 slot and vice versa. Though x1 slots tend to be keyed to prevent larger cards from being installed.
    • The physical interface itself doesn't matter as long as it can carry the signal. Thus, PCIe has been found in different form factors such as M.2, Thunderbolt (which used a Mini DisplayPort plug), and USB-C (which uses the Thunderbolt standard to carry the signal). Enterprising people have been able to connect a graphics card in an external enclosure by tapping into Thunderbolt or for more Awesome, but Impractical goodness, the M.2 slot.

Sega

• The Sega Genesis — also known as the Mega Drive — had the Master System's Z80 chip as a co-processor (among other things), but wasn't able to natively have backwards compatibility due to the differing cartridge slot. As such, the Power Base Converter was made to serve as a passive adapter that allowed for compatibility with practically all of the Master System's library and its peripherals.^note Seven games require the use of a Master System controller, which could have been a problem considering they weren't sold in stores at the time and thus had to be ordered directly from Sega. That said, only one game (F-16 Fighting Falcon) doesn't work with the add-on at all, due to requiring a special video setting that the Genesis lacks. It should be noted that the peripheral only fit the Genesis I, though. It requires modding to work on a Genesis 2/Mega Drive Mark II, and while there was a "Master System Converter 2" made for the revision, it was only released in Europe and couldn't play any card-only Master System games due to lacking a card slot. Meanwhile, either version works at all with the Genesis 3. In any case, the peripheral gave Sega bragging rights over Nintendo's SNES, which had earned the ire of parents who were displeased by the fact that they had to buy a new console couldn't run games from the previous one at all.
• The Sega Game Gear, which hardware-wise was pretty much a handheld Master System, had a "Master Gear Converter" that allowed its owners to play Master System games on their Game Gear, but without FM sound. Of course, its existence was made somewhat redundant when the Game Gear's library ended up consisting of several ports of Master System games anyway.
• A minor example, but one is still able to play Genesis games when the 32X add-on is installed.
• A notable example is Bleem!, a commercial PlayStation emulator released by The Bleem Company in 1999 for IBM-compatible PCs and Dreamcast. The latter, most known as "Bleemcast!", was allowed to play games from PS1 into a Dreamcast, having some public releases as Metal Gear Solid and Gran Turismo 2 on the SEGA console. This was expanded when hackers leaked a Bleemcast! beta, so more games were adapted to Dreamcast, even years after the console was discontinued and The Bleem Company ended.

Sony

• The PlayStation 2 was able to play PlayStation games thanks to including several of its system's processors; namely the main R3000 CPU which was used as the I/O processor and the SPU which was used for sound processing. The GPU was emulated on the PS2's Emotion Engine. You could even use PS1 controllers and memory cards without issue, although the original memory cards are needed for saving game data as the PS2 memory cards were incompatible (PS1 games can't write to them.) Later Slim models replaced the IOP processor with a custom PowerPC based chip known as Deckard.
  • Sony had also planned downloadable PS1 games in Japan through the PlayStation Broadband Navigator; a complete overhaul of the system's OS and the predecessor to the Play Station Network including web portal browsing, emails, and enhanced save data and title management for the optional internal HDD or an external USB 1.0 HDD. Only one game was released by Konami under the ID of SLBB-00001 but included a full software emulator of the PS1 that was feature complete. It was speculated to have been removed from the service in 2004 due to the emergence of various HDD loader's like HD Loader and the growing homebrew scene, coupled with the oncoming Slim revision lacking hard drive support. Said homebrew community discovered this emulator years later and modified it into POPStarter to install PS1 games to a hard drive. Meanwhile, Sony themselves would recycle this emulator for the PSP and its PS One Classic's series.
• Early SKUs of the PlayStation 3 came with the Emotion Engine (CPU) and the Graphic Synthesizer (GPU) from the PlayStation 2, allowing people to play PS2 games on the console; the Emotion Engine was shortly removed in later units, leaving the Graphics Synthesizer, and replaced with partial software emulation of the PS2 hardware in which the CELL Broadband Engine emulated the Emotion Engine, a similar approach to how the PS2 handled PS1 support, but also an approach that led to issues running some games. Eventually, all PS2 compatibility was left out for later models. Strangely enough, PS1 software emulation was supported across all models. Later on PS2 and PSP software emulation was added for use in the downloadable PS2 Classics and PSP Minis.
• Though the Playstation Portable can't fit PS1 discs, it can run PSOne Classic games from a built-in emulator known as POPS.
• The Play Station Vita contains the PSP's R4000 CPU for playing downloadable PSP games. The other aspects of PSP hardware (the Media Engine GPU, DSP and security co-processors) are all done via software emulation.
• The PlayStation 5 is fully backwards compatible with most of the PS4 gaming library and many of its peripherals such as the VR headset. This is accomplished by the CPU using special "game logic" to replicate the clocks of the PS4. Many games see improved graphics and performance when updated to take advantage of PS5 hardware as well. It should also be noted that Sony additionally allows developers to make their PS4 games incompatible with the PS5 if they so wish, which Konami did with P.T..
• Curiously while the PS4 and PS5 contain hardware that is found in PCs, they are not a PC in a strict technical sense. PCs that follow its lineage from the original IBM 5150 Personal Computer have a set of features that must be implemented. Due to backwards compatibility being a king in the IBM PC standard over the years, this means that the system must support features that are more or less outdated, such as the CPU starting in so-called real mode (the CPU must behave like the original 8088 in the IBM PC) to supporting the features found in the support chips (such as timers, interrupt controllers, etc.). The hacker group that jailbroke the PS4 found out that they couldn't run a bog-standard version of x86-64 Linux because it was trying to initialize things that didn't exist. They had to make a special PS4 version of said Linux in order for it to work. But once the PS4 booted up Linux, it was more or less happy with regular x86/x64 apps such that they could run Steam and Linux compatible games.

Other

• Due to Google's lenient submission policies, along with the platform's open nature, more than a dozen emulators for popular consoles and computers are available on the Play Store. Though only a few of these emulators are either completely original works or directly ported from their desktop equivalents without any adware thrown in.
  • Which is in stark contrast to Apple's restrictive policies for its App Store, which basically kept unofficial emulators from even showing up at all. This didn't keep savvy people from resorting to sideloading or using Cydia on jailbroken devices though. And it's pretty much the same case with the Microsoft Store too. Some did manage to smuggle an emulator by masquerading the application as an unrelated program, but we all know what happened afterwards.
• In Minecraft, it is entirely possible to build your own working computer. However, such computers are far too primitive and slow to play games from any existing system.
• The FC3 Plus plays NES, SNES and Sega Genesis games all with the same console. Also the Retro-Bit company, known for their consoles as the RetroDuo and RetroTrio.
• Decades after these consoles were released, retrogamers discovered Dendy and similar famiclones have their game cartidges compatible with Famicom (aka Japanese NES) and vice versa. So, you can play Famicom games in a Dendy or other famiclone, as well you can play Dendy's pirated games on a Famicom (or a NES with a FC-to-NES adapter.)
  • One of the most famous famiclones, the "Creation" (seen mostly in Latin America and in Spain under the name of "Yess") is compatible with NES cartridges and controllers, being known as the "poor version" of the NES.
• Because Unix-like systems are easy to port to new hardware and many of them are open source, they've been ported to almost anything that's Turing-complete. The portability champion is the NetBSD project, via a strict separation of machine-dependent and machine-independent code. Their slogan is "Of course it runs NetBSD." NetBSD's license allows developers to keep their changes private, in contrast to the GPL. This makes it popular with embedded systems developers. It's even been ported to a toaster.
• Newer Chromebooks have the ability to run Android apps, and some models are able to run standard Linux applications.
• The Commodore 128 was compatible not only with the Commodore 64, but also with CP/M, though it was well past its prime when the 128 was released.
• Intellivision announced that with their new console in 30+ years, the Intellivision Amico, will include the complete library of the original Intellivision console apart of 20 reimagined games incorporated to the console.
• The Sega SG-1000 and ColecoVision both share the same off-the-shelf hardware, and while neither console could run games made for the other system under normal circumstances, a clone console called the Dina 2-in-1 can run SG-1000 and ColecoVision games, though compatibility with the latter is spotty at best due to the lack of a second numeric keypad; the sole keypad is integrated to the console itself making keypad-intensive games a chore to play. It is clear that Bit Corporation made the Dina with SG-1000 compatibility in mind due to it being a popular console in Taiwan, with ColecoVision support being more of an afterthought. A company called Telegames marketed the Dina in the U.S. as the Personal Arcade, only advertising the Colecovision capability and ignoring the SG-1000 functionality, as that system was never released in America.
• HTML hasn't really changed much over the years that for mostly static pages, i.e., websites that don't rely a lot on JavaScript and requests to the server outside of HTTP, that you could fire up an old version of a web browser and still view the web page. It may not look as pretty, but for a static website like Wikipedia, you can at least read through it and click on the links.
• Even before the digital age, several upgrades to analog systems were backwards compatible either deliberately or unintentionally.
  • Arguably the most (in)famous one is NTSC for color TV. By the time it was standardized, black and white TVs were already a staple in American homes and asking people to replace their sets would've been the death knell for the new standard. So the color signal was embedded in the normal black and white signal and the TV would have circuitry to decode it.
  • In a similar vein to color TV, FM radio stations that wanted to go digital adopted a hybrid approach. They still broadcast an analog signal for older/cheaper radios to pick up, but have a small amount of bandwidth on either side of their broadcast frequency to send digital signals. This standard is known as HD Radio.
  • CRT displays work in such a way that as long as the signal you feed it does not exceed its minimum/maximum timing specs, it will display almost anything you throw at it.
  • Component standards that separate brightness and color signals, like S-Video and YPbPr (what's usually called Component Video), can work in a composite input in a pinch, but only using the brightness signal and with 480i resolution.
  • If you were really desperate, you could play back a cassette recorded using Dolby Noise Reduction (NR) in a player without it, but the audio would sound muffled. You could also use newer Dolby NR standards in a player that only supports an older one, but it wouldn't work as well.
  • Dolby Pro Logic mixes four channel audio into two. You can still play this back on a two-channel (stereo) system without decoding the audio channels and it'll sound just fine. It even sounds quasi-surround sound, in the same way "virtual surround sound" sounds.
  • Stereo phonographs can play mono records easily, but older mono cartridges tracked heavier than stereo, which can prematurely wear stereo records played on a mono record player. Stereo-compatible mono cartridges became available later for people who couldn't afford to upgrade their players. Stereo cartridges that were simply wired to sum both channels to mono were also common on later mono players, typically portable models.
  • Some older turnables also had multiple stylus selection, typically between a larger one designed for 78 rpm records, while the other was designed for vinyl singles and LPs.

Fictional examples

• In Player Two Start, the Ultra Nintendo is backwards compatible with the SNES-CD. Then the Nintendo Wave is compatible with Ultra Nintendo and SNES-CD games. The Nintendo Sapphire is compatible with SNES-CD, Ultra Nintendo and Wave titles, allowing SNES-CD titles bought over a decade prior to the Sapphire's existence to officially be played with an HDMI cord.
  • Also, the Game Boy Nova can play Game Boy and Game Boy Color games.

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