WALTHAM, Mass. - Microsoft Corp., expanding on an alliance with Novell Inc., has agreed to buy as much as $100 million more for subscription certificates for Novell's Linux products, Novell said Wednesday.

That comes on top of the $240 million in upfront purchases Microsoft agreed to make when the rival sofware companies announced their alliance in late 2006.

The deal is designed to help Microsoft's market-leading Windows operating system and the increasingly popular Linux system work together.

Under the deal, Microsoft may sell, reuse or distribute those certificates, which holders can redeem for expanded technical support from Novell, primarily for the company's SUSE Linux Enterprise Server.

Shares in Microsoft rose 19 cents to $27.51 in midday trading Wednesday, while Novell shares gained 16 cents, or 2.8 percent, to $5.90.