ROM images are also used when developing for embedded computers. Software which is being developed for embedded computers is often written to ROM files for testing on a standard computer before it is written to a ROM chip for use in the embedded system. At present, this article deals mainly with the use of ROM in relation to emulation.
Terminology:
ROM chips, while still in use, have been replaced in many instances by optical media such as CD-ROMs and DVD-ROMs, magnetic media such as hard disks and magnetic tapes and, more recently, Flash Memory chips. However, the term ROM is commonly also used to cover many of these newer media so, for instance, a computer game copied from a magnetic tape may also be incorrectly referred to as a ROM. The correct names for tape and disk 'roms' are Tape image and Disk image respectively. Images copied from optical media are also called ISO images, after one of the standard file systems for optical media, ISO 9660. Many ROMs used by emulators, and particularly console emulators, are not true images of the ROM chips in the cartridge. They are often modified to allow easier functionality in emulators through methods such as combining the images from multiple ROM chips.Usage:
ROMs can be copied from the read-only memory chips found in cartridge-based games and many arcade machines using a dedicated device in a process known as dumping. For most common home video game systems, these devices are widely available. Dumping ROMs from arcade machines, which in fact are highly customized PCBs, often requires individual setups for each machine along with a large amount of expertise.
Creating images from other media is often considerably easier and can often be performed with off-the-shelf hardware. For example, the creation of tape images from games stored on magnetic tapes (from, for example, the Sinclair ZX80 computer) generally involves simply playing the magnetic tape using a standard audio tape player connected to the line-in of a PC sound card. This is then recorded to an audio file and transformed into a tape image file using another program. Likewise, many CD and DVD games may be copied using a standard PC CD/DVD drive.
Digital preservationThe lifespan of digital media is rarely great. While black-and-white photographs may survive for a century or more, many digital media can become unreadable after only 10 years. This is beginning to become a problem as early computer systems may be presently fifty or sixty years old while early home video consoles may be almost thirty years old. Due to this aging, there is a significant worry that many early computer and video games may not survive without being transferred to new media. So, those with an interest in preservation are actively seeking older arcade and video games and attempting to dump them to ROMs. When stored on standardized media such as CD-ROMs and DVD-ROMs, they can be copied to future media with significantly reduced effort.
The trend towards mass digital distribution of ROMs images, while potentially damaging to copyright holders, may also have a positive effect on preservation. While over time many original copies of older games may deteriorate, be broken or thrown away, a copy in ROM or Image form may be distributed throughout the world, allowing games which would otherwise have been lost a greater chance of survival.
Copy prevention mechanisms:
While ROM images are often used as a means of preserving the history of computer games, they are also often used to facilitate the unauthorized copying and redistribution of modern games. Seeing this as potentially reducing sales of their products, many game companies have incorporated features into newer games which are designed to prevent copying, while still allowing the original game to be played. For instance, the Nintendo GameCube used non-standard 8 cm DVD-like optical media which for a long time prevented games from being copied to PCs. It was not until a security hole was found in Phantasy Star Online Episode I & II that GameCube games could be successfully copied to a PC, using the Gamecube itself to read the discs.
SNK also employed a protection on their Neo Geo games starting with The King of Fighters in 1999 which used an encryption algorithm on the graphics ROMs which prevented them from being played in an emulator. Many thought that this would mark the end of NeoGeo emulation. However, as early as 2000, crackers found a way to decrypt and dump the ROMs successfully, making them playable once again in any NeoGeo emulator.
Another company which used to protect their arcade games was Capcom which is known for its Capcom Play System II arcade board. This contained a heavy copy protection algorithm which was not broken until 7 years after the system's release in 1993. The original crack by the CPS2Shock Team was not a true emulation of the protection because it used XOR tables to trick the game into decrypting and play in an emulator. Their stated intent was to wait until CPS-2 games were no longer profitable to release the decryption method (three years after the last game release).[4] The full decryption algorithm was cracked in 2007 by Nicola Salmoria, Andreas Naive and Charles MacDonald of the MAME development team.
Another copy protection technique used in cartridge-games was to have the game attempt to write to ROM. On an authentic cartridge this would fail or cause an exception, however, emulators would often allow the write to succeed. Pirate cartridges also often used writable chips instead of ROM. By reading the value back to see whether the write succeeded, the game could tell whether it was running from an authentic cartridge. Alternatively, the game may simply attempt to overwrite critical program instructions, which if successful renders it unplayable.
Some games, such as Game Boy games, also had other hardware such as memory bank controllers connected to the cartridge bus. The game would send data to this hardware by attempting to write it to specific areas of ROM; thus, if the ROM were writable, this process would corrupt data.
Capcom's latest arcade board is the CPS-3. This was resistant to emulation attempts until June 2007, when the encryption method was reverse-engineered by Andreas Naive. It is currently implemented by MAME and a variant of the CPS-2 emulator Nebula.
Hobbyist collectors:
Like many other items such as stamps and coins, ROMs are also collected by many people. The motives for doing this vary from a desire to preserve the history of computer and video games to obsessive collectors. Those who desire to collect all ROMs have been derided by the MAME developers as PokéROMs, in a reference to the obsolete Pokémon catchphrase "gotta catch 'em all." PokéROM can also refer to "Pocket ROMs" as Pokémon refers to "Pocket Monsters"; since the advent of the GP2X, PSP, DS and other portable handheld gaming machines capable of emulation and even with some Cellphones, people can now have an entire library of old games in their "pocket".
Given the desire by many people to collect ROMs, there are many projects on the internet which dump ROMs, catalogue them or provide tools to verify the correctness and completeness of ROM collections. However, it is illegal to download copyrighted ROMs from the internet. Most old games are copyrighted.
Internet distribution:
The trading of ROMs over the Internet is widespread. Many methods are used for such distribution, including:
- HTTP (hypertext transfer protocol) and FTP (File Transfer Protocol)
- BitTorrent
- Direct Connect hubs
- eMule
- IRC
- DCC
- Usenet binaries
Although the large size of games for recent consoles makes the distribution of more than one game at a time impractical, it is often the case for older consoles that many thousands of games can be distributed together as a collection. Larger games are often distributed one by one.
No comments:
Post a Comment