Mac clone maker Psystar has accused Apple of abusing copyright laws and locking Mac OS X to its hardware with code that prevents non-Apple machines from booting properly.

In a change of tactics, Psystar has revamped its countersuit, first filed in August, to make allegations about Apple's "brazen misuse" of federal copyright laws, including the Digital Millennium Copyright Act (DMCA).

US District Court Judge William Alsup, who last month tossed out Psystar's antitrust charges, at the time left the door open to an amended complaint. Psystar took advantage of the opportunity, and filed a revised lawsuit Monday.

"Apple is attempting to use its copyrights in the Mac OS, not to prevent unauthorised production of any copyrightable elements, but to prevent competitors from developing competing hardware systems interoperable with the Mac OS," said Psystar.

"Through the use of anti-circumvention and the DMCA, Apple is attempting to leverage its copyright limited monopoly in reproduction of the Mac OS into a broader monopoly in a separate hardware market," the lawsuit continued. "This is the exact behavior that is prohibited by the copyright misuse doctrine."

Late last month, Apple amended its original complaint to include a new charge that Psystar violated the DMCA by dodging copy-protection technologies Apple uses to protect Mac OS X.

This week, Psystar essentially said that Apple's claim is bogus. "Psystar is further informed and believes and thereon alleges that Apple does not actually employ a technological copyright protection measure that controls access to the Mac OS," the company said in its revised suit.

Instead, Psystar continued, Apple includes code that checks the hardware at boot time, and induces a "kernel panic" or an "infinite loop" if it recognizes a non-Apple system. "There is no specific reason as to why this infinite loop is present in the code as the kernel is capable of restating/rebooting on a much broader range of hardware ... thus the restart/reboot infinite loop exists for no functional reason," said Psystar.

Previously, Psystar had spelled out its kernel panic claim, but the infinite loop charge is new to the revamped lawsuit, as is the company's reasoning that the two are linked to Apple's claim that its operating system was protected by the DCMA.

Specifically, Psystar charged that Apple had stuck code in Mac OS X that checked for the type of processor on board, and if it found "any x86 processor not sold by Apple," the operating system crashed or locked up. "The code that causes kernel panic and/or infinite loop does not constitute a technological copyright protection measure," Psystar said as it asked Judge Alsup to rule that Apple's claim DMCA protection was unenforceable.

Psystar has sold Intel-based computers preconfigured with Apple's Mac OS X since April, when it began marketing machines for as little as $555 (£340), substantially less than comparably-equipped Mac desktop system. In early July, Apple struck the first legal blow by accusing Psystar of breaking multiple copyright and software-licensing laws.

A month later, Psystar fired back with a countersuit that argued Apple violated multiple anti-trust laws by tying Mac OS X to its own hardware.