What Does R/390 Mean?
R/390 is an expansion card used in an IBM RS/6000 mainframe server. All the server's configurations, as well as its complete systems, are called R/390. The R/390 system is an IBM mainframe system designed to run on a personal computer.
The R/390 servers were marketed as development environments that provided an economical approach for companies moving their legacy applications from old mainframe systems to a new operating environment.
Techopedia Explains R/390
Up until the mid-1990s, mainframe computers were huge machines that filled entire rooms and required special air conditioning and power arrangements. Today, mainframe computers are much smaller and far sturdier than their predecessors.
The original R/390 had a 67 MHz POWER2 processor with 32 MB of RAM, or a 77 MHz processor with 512 MB of RAM. Several early PCI RS/6000s could have the PCI P/390 card installed. However, the MCA P/390 expansion card will work in any MCA RS/6000 system. All configurations are referred to as R/390, and the machines as R/390 servers, which require AIX version 2 for the operating system.