Design Spark – a Raspberry Pi ($39) can be used to emulate a mainframe which would have filled a large computer room, and to run the same software which it would have run. Of course, the only reason you would do this is for fun, learning or perhaps as part of computer conservation efforts, e.g. in providing continued access to old computer software and/or data. A modern mainframe would massively outperform a Raspberry Pi and offer many benefits beyond simple processing power.
Having configured a mainframe on a Raspberry Pi, it was time to try out a Raspberry Pi on a mainframe! The image below shows the Pi sat on top (centre) of the CPU from an IBM 4381.
Hardware IBM 4381 vs Raspberry Pi
* 2-2.7 MIPS CPU
* Optional maths co-processor
* 4-32MB RAM
* Up to 4x I/O channels running at 3MB/second
Pocked-sized Raspberry Pi, which manages to pack:
* 965 MIPS CPU
* FPU + 24GFLOP GPU
* 256MB RAM
* 60MB/second I/O (USB2)
Emulation is possible using open source software called Hercules. Run this under Debian Linux on a Raspberry Pi.
First we install the hercules package from the Debian repository, and then create a configuration file which tells it to emulate a model 4381 machine with 16MB RAM and terminals and storage etc. attached. Hercules is then executed from the command line, and we run up an IBM 3270 terminal emulator from another machine on the network, providing it with the IP address of our Raspberry Pi and the port number specified in the Hercules configuration.
The MicroVAX 3900 hardware being emulated this time is a little more modern and somewhat smaller than the IBM 4381 processor, but the VAX architecture and OpenVMS operating system are no less impressive. On introduction in 1989 an entry level MicroVAX 3900 would have set you back over $120,000 and, as with IBM’s VM operating system, you’d be mistaken if you thought that OpenVMS was dead and buried as it runs many mission critical workloads today.
Emulation of the VAX hardware has been made possible by a pretty amazing piece of software called SimH. In order to be able to run OpenVMS on this a licence is required, but fortunately these are available free of charge via the OpenVMS Hobbyist programme.
The SimH software is configured to emulate a MicroVAX 3900 with 64Mb of memory, 1.5Gb disk and a CD-ROM drive.