I.B.M. has aggressively promoted Linux, a variant of the Unix operating system that is distributed free.
The Unix operating system has similarly had a rich and enduring impact.
Linux is the popular free variant of the Unix operating system developed by the Finnish programmer Linus Torvalds.
The letters, dated Friday, assert that Linux, a variant of the Unix operating system, violates an SCO license and copyright.
These chips were originally designed for big computers using I.B.M.'s proprietary version of the Unix operating system, AIX.
The company argues that I.B.M., which is licensed by SCO to use its Unix operating system source code, improperly transferred some of that proprietary code into Linux.
Dell computers run mainly Windows, but the company is also aggressively promoting server computers running GNU Linux, a variant of the Unix operating system that is distributed free.
So it has invested in companies developing Linux, an increasingly popular version of the Unix operating system, which competes with Windows.
Created by University of Illinois students Marc Andreessen and Eric Bina, it was first released for computers using the Unix operating system on 23 January 1993.
Sun, which makes network computers based on the Unix operating system, is scheduled to deliver full results for the first quarter on Oct. 16.
SCO, which owns the license rights to the Unix operating system, contends that Linux, a variant of Unix that is distributed free, violates its license and copyright.
