Apple unveiled next-generation software for its popular iPhone on Thursday which gives users the ability to run multiple programs at once and to quickly move between them.

"We weren't the first to this party but we're going to be the best," Apple chief executive Steve Jobs said of the multi-tasking feature in version 4.0 of the hot-selling smartphone's new operating system.

In addition to the much-called-for multi-tasking function, Jobs said iPhone OS 4.0 -- which will be available as a software update for iPhone and iPod Touch users this summer -- includes over 100 new features.

Among them is an iBooks reader that will allow users to download electronic books online and transfer them between their iPhones and iPads, the touchscreen tablet computer which went on sale in the United States last week.

"You can buy your books once and read" them on both devices, said Jobs, speaking at an event at company headquarters here to introduce the latest version of the iPhone operating system.

The iBooks function will wirelessly sync between the two devices so readers will not lose their place when shifting from one to the other.

A new iPhone Game Center includes a matchmaking feature that will hook users up with similarly skilled partners for multi-player games. Jobs said the Apple App Store now offers more than 50,000 game and entertainment titles.

Other new features include the ability to create playlists on the phone, a 5X digital zoom for the camera and folders to organize and access applications.

Improved mail functionality lets users funnel incoming mail from multiple e-mail accounts into a single inbox.

The new operating system also features a spell check function and will allow users to buy applications and gift them directly through their iPhones.

Explaining the delay in bringing multi-tasking to the iPhone, Jobs said Apple was busy developing a system that would not slow the phone's performance or quickly drain its battery.

"They've obviously addressed a lot of the criticism of the platform, with multi-tasking being the core feature of the concerns," said Ben Bajarin, consumer technology analyst at Creative Strategies, a technology industry analysis and strategic planning company.

"Switching between things quickly is the core part of it, and they did it in a smooth way," Bajarin said.

Among seven "tent pole" innovations introduced by Jobs was iAd, a mobile advertising system that is built into the phone.

Most of the existing mobile advertising "really sucks," Jobs said. "We thought we might be able to make some contributions."

The iAd system will allow developers and ad agencies to build advertising directly into their applications, compete with the emotional impact of television ads and earn money beyond selling their applications, Jobs said.

He demonstrated a sample advertisement for retail giant Target in which users can custom outfit their college dormitory rooms with sundry school paraphernalia from rugs to trash cans.

The new operating system will be fully functional on the iPhone 3GS and third generation of the iPod Touch. Many, but not all, of its features will work on earlier versions of the iPhone and iPod Touch.

Jobs said Apple has sold more than 50 million iPhones to date. That number jumps to 85 million if iPod Touches are included.

The Apple chief executive also released the latest sales figures for the iPad tablet computer.

He said that as of Thursday, Apple had sold 450,000 iPads and users had downloaded 600,000 iBooks.

"We're making them as fast as we can," Jobs said. "Evidently we can't make enough of them yet, so we are going to have to try harder."

Jobs said iPad owners have so far downloaded 3.5 million applications.

Apple sold over 300,000 iPads on Saturday, its first day of availability in the United States.