The copyright symbol for a mask work is Ⓜ (M enclosed in a circle; Ⓜ) or *M*, not (M).
According to 17 USC chapter 9 (http://www4.law.cornell.edu/uscode/17/ch9.html), semiconductor mask work copyrights last only ten years (in contrast to perpetual copyright on most other works) and are not subject to the typical backup exemptions that 17 USC 117 provides for computer software.
A video game normally is granted two distinct copyrights: a Form TX copyright on the program (registered by sending in source code) or a Form PA copyright on the visual displays generated by the work (depending on whether code or art dominates a program; see Chessmaster vs. Myst), and a mask work copyright on the ROM that contains the binary. The lack of backup exemption means that Nintendo can go after sites that carry ROM dumps copyright 1992 or later (Super NES, Nintendo 64, Game Boy), but ROM sites hosting mostly software for older consoles may fall under normal copyright law's fair use, library (17 USC 108), and backup exemptions. Of course, if you have developed your own software for NES and have released it under a free software license, this doesn't apply.
Could this explain why Nintendo waited until the DMCA and foreign counterparts were passed before releasing a game console that used media other than cartridges?
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A more complete version of this article is available at http://www.wikipedia.com/wiki/Mask_work
"mask work@Everything2.com" by Damian Yerrick in 2002, available at http://everything2.com/?node_id=1108338
"Mask work: encyclopedia article from Wikipedia" by Damian Yerrick in 2002, available at http://www.wikipedia.com/wiki/Mask_work
"Mask work copyright: encyclopedia article from Wikipedia" by Damian Yerrick and an anonymous author in 2002, available at http://www.wikipedia.com/wiki/Mask_work_copyright&action=history
"mask work@Everything2.com" by Damian Yerrick in 2001, formerly available at http://everything2.com/?node_id=1108338