I went to college at the University of Texas at Dallas.<p>Our online Student Information System (SIS) was one of the very rare online services with <i>operating hours</i>. It wasn&#x27;t available at night (and possibly on weekends too, but I graduated eight years ago, so my memory is fuzzy).<p>At one point, I started asking around, and I found out that it was because SIS ran on an old mainframe that couldn&#x27;t read and write the disk at the same time. During operating hours, it reads the disk and makes all changes in memory. When operating hours are over, it writes all its changes to disk.<p>This was in 2003-2007 and they still had that mainframe controlling all their class schedules and student data. UTD was considered the biggest tech-oriented university in the southwest, too.