Released on January 11, 2005 during the Macworld expo, the first-generation iPod Shuffle weighed 0. 78 ounces (22 g), and was designed to be easily loaded with a selection of songs and to play them in random order. According to Apple, owners of existing iPods had often left the music selection to "shuffle", and the new iPod Shuffle was a way of implementing that in a much more cost-effective fashion. It relied on the use of an "autofill" feature in iTunes, which selected songs at random from a user's music library (or from a specific playlist) and copied as many as would fit into the iPod Shuffle's storage. The first generation could hold up to 240 songs (1 GB model, based on Apple's estimate, of four minutes per song and 128 kbit/s AAC encoding). It used the SigmaTel STMP35xx system on a chip (SOC) and its software development kit (SDK) v2. 6, a flash memory IC, and USB rechargeable lithium cell. The STMP35xx SOC and its software was the most fully integrated portable MP3 playback system at release time and SigmaTel was Austin's largest IPO (2003) capturing over 60% of flash based MP3 player world market share in 2004. In 2005, peak iPod first-generation Shuffle production occurred at a hundred thousand units per day, at the Asus factory.