Stands for Application System/400, a midrange IBM server computing platform for business computing. AS/400 uses 64-bit RISC technology, supports Systems Network Architecture (SNA), and runs the OS/400 operating system. Traditional SNA user interfaces to AS/400 systems use either text-based 5250 terminals or desktop PCs running 5250 emulation software. But IBM has released a version of OS/400 that turns the AS/400 into a Web server and allows AS/400 applications to be interfaced using TCP/IP from a standard Web browser such as Microsoft Internet Explorer.
The AS/400 was originally based on a custom IBM CISC CPU which had an instruction set architecture, known as Internal MicroProgrammed Interface (IMPI), similar to that of the IBM System/370. It was later migrated to a POWER-based RISC CPU family eventually known as RS64.