Since the rise of the personal computer in the 1980s, IBM and other vendors have created PC-based IBM-compatible mainframes which are compatible with the larger IBM mainframes. They are also referred to as plug-compatible mainframes, a term used for the original System/360 and 370 compatible clones. The original advantage being that the PC-based mainfarmes had a much smaller footprint, required less power, and cost less money.
Background
Up until the late 1980s, mainframes were very large machines that often occupied entire rooms. The rooms were often air conditioned and had special power arrangements to accomodate three-phase power required by the machines.
System/370
- Details about the System/370 based PC mainframes from the 1980s.
System/390
S/390 Processor Card
An important goal in the design of the S/390 Processor Card was complete compatibility with existing mainframe operating systems and software. The processor implements all of the ESA/390 and XA instructions which prevents the need for instruction translation. The S/390 Processor Card also incorporated 32MB of dedicated memory with an optional 96MB daughter card for a combined total of 128MB of RAM. The processor was officially rated at 4.5 MIPS.
R/390
R/390 was the designation used for the expansion card used in an IBM RS/6000 server. The R/390 featured a 67 MHz or 77 MHz POWER 2 processor and 32MB to 512MB of RAM, depending on the configuration. R/390 servers required AIX to be run as the host operating system.
P/390
P/390 was the designation used for the expansion card used in an IBM PC Server and was less expensive than the R/390. The original P/390 server was housed in an IBM PC Server 500 and featured a 90 MHz Intel Pentium processor for running OS/2. The model was revised in mid-1996 and rebranded as the PC Server 520, which featured a 133 MHz Intel Pentium processor. Both models came standard with 32MB of RAM and were expandable to 256MB. The PC Server 500 featured eight Micro Channel expansion slots while the PC Server 520 added two PCI expansion slots and removed two Micro Channel slots.
S/390 Integrated Server
- Details on the S/390 Integrated Server.
z/Architecture and Today
Since the late 1990s, PC processors have become fast enough to perform mainframe emulation without the need for a peripheral card. One of the most popular PC-based IBM-compatible mainframe products as of 2006 is Fundamental Software's FLEX-ES product. FLEX-ES emulates both System/390 (ESA/390) and z/Architecture. While FLEX-ES is capable of running on most PC hardware, the licensing agreement prohibits it. FLEX-ES must run on approved IBM xSeries servers or ThinkPad laptops.
Another popular emulator is the open source Hercules emulator, which has been in development since 1999 and emulates the System/370, System/390, and z/Architecture instruction sets. While Hercules cannot legally run modern IBM operating systems, earlier System/370 operating systems are considered to be public ___domain and can be legally run on Hercules.