Sun Microsystems has fulfilled its pledge to make its newest version of the Solaris operating system available for free.

On Monday evening, the server and software company began offering Solaris 10 as a free download for those who register. Anyone may use Solaris for commercial or noncommercial use, and Sun will supply security fixes as they're released, but those wanting bug fixes and support must sign a support contract with Sun.

Solaris 10 is a crucial part of the company's attempt to fend off Linux and Windows and to maintain leadership against its two main Unix rivals, IBM's AIX and Hewlett-Packard's HP-UX. Elements of the attempt include major technological upgrades, the no-charge licensing, and an effort to build an open-source community that helps Sun develop the software.

Sun sells support on an annual subscription basis for servers with up to four processors, charging US$120 per computer processor per year for basic support, US$240 for standard and US$360 for premium. Automatic retrieval of security fixes for all users through a service called the Sun Update Connection will be available mid-2005, Sun said.

Sun believes making the software free and open-source will draw new customers, developers and business partners to the Solaris ecosystem. The OpenSolaris project will provide full source code in the second quarter, Sun said in January.

But Solaris faces major challenges, in particular from Linux, a cooperative development project supported by dozens of computing companies. Meta Group predicts that the use of Linux in the database market will expand from its current share of about 8 percent or 9 percent to 25 percent in 2007, chiefly by replacing Unix systems.

Among the improvements for Solaris:
• Support for 64-bit x86 processors such as Intel's Xeon and Advanced Micro Devices' Opteron. The Linux packages from Red Hat and Novell support these processors, but Windows support was delayed until the first half of 2005, and the Unix versions from IBM and HP don't run on any x86 processors. Sun itself almost cancelled its x86 support but now is an enthusiastic proponent.

• Solaris Containers, formerly called zones and N1 Grid Containers, which let a single copy of the operating system be subdivided so it appears as if separate users each have their own machine. The feature lets a single system juggle multiple tasks better and be used more efficiently.

• DTrace, or Dynamic Tracing, which lets technically savvy users peer into running systems to find bottlenecks and other performance issues. This is the only component that so far has been released as open-source software.

• ZFS, or Zettabyte File System, which brings new levels of reliability and scale to how information is stored on hard drives. ZFS also smoothes over a format difference so that the same storage system can be used by Solaris running on x86 chips and Sun's UltraSparc chips. ZFS will be available in a later update.

• The Linux Application Environment, code-named Janus, which lets Red Hat Linux programs run unmodified on Solaris x86 servers. Janus will be available in a later update and also will be expanded to support other versions of Linux.

• Project FireEngine, revamped networking software intended to boost performance and help Solaris shuck its disparaging "Slowlaris" nickname.

• Predictive self-healing, which lets Solaris diagnose and recover from some types of hardware or software problems.

• Process Rights Management, technology that uses more detailed privilege levels to make it harder for a computer attacker to exploit a vulnerability.