The IBM mainframe line that superseded System/360. Introduced in 1970, the System/370 added virtual memory and other enhancements. In the early 1980s, IBM added Extended Architecture (XA) to the series, featuring I/O enhancements and increased addressing from 24 to 31 bits. In the late 1980s, it debuted the Enterprise System Architecture (ESA), which added more registers and address modes. The 370 architecture was brought down to the PC level in 1983 with the PC XT/370, and then again in 1989 with the VM/SP Technical Workstation. See System/360, System/390 and IBM mainframes.