I have always had a soft spot for minicomputers... real minicomputers from the early 70's... machines that filled 19" racks with panels full of blinking lights.
I remember my father having a PDP8 at work before micros slowly took over...
and the Data General Nova I supported in one of my first jobs.... oh the Nova 3...
what I lovely machine. I wish I had the forthought to save it when it was decommissioned.
But back to now.... from time to time I have played with simh mainly OS8 on the PDP8 but had never really done anything serious. The lack of blicking lights didn't make it quite the same. One day I would build a PDP8 simulator to get a better experience but that is a big job and it stayed an idea until I wondered... could I compile simh (at least the simpler machines... definitely not the VAX or PDP11) and run this on a DLink DSL-502T router? (my NZ$1 universal hardware platform at the moment). If I could then was it possible to add some blinking lights?
Part One... Building simh for OpenWRT
This turned out to be surprisingly easy if you have an OpenWRT build environment. The steps are:
1. Download the simh sources from http://simh.trailing-edge.com/sources/simhv38-1.zip
2. Create a simh subdirectory in the package directory of your OpenWRT build environment.
3. Create a Makefile in the simh directory so the OpenWRT build process will find your new simh package.
4. Create a src directory in the simh directory.
5. Unzip the contents of the simh sources zip file into the src directory. If you check the src directory contents you will find a subdirectory for each emulator and some shared files.
6. Replace the standard simh Makefile in the src directory with one compatible with the OpenWRT build process.
The following example builds the PDP8 and Altair simulators and is based on the style of the standard simh makefiles. You can download it from here.
7. Use Make MenuConfig to select the OpenWRT packages. The simh package will appear in the Utilities group if you used the Makefile from step 3. Select to build simh as a Module.
8. Use Make to rebuild. All going well this will compile simh with the results in the build_dir/mipsel/SIMH folder in my environment. Copy the pdp8, altair, altairz80 and any other simulators you have compiled and upload them to your OpenWRT powered router.
It worked well for me. The DSL-502T is not particularly fast and is a bit limited in available storage but it happily runs OS8 on the PDP8, 4K and 8K Basic and AltDos on the Altair. I haven't tried CP/M yet and need to free up some space or add some additional flash memory.
In the mean time... on to the blinking lights. The DSL-502T does not have enough free GPIO pins to provide a display of a 12 or 16 bit address or data bus. It does support I2C via the GPIO-i2c driver so I have ordered a couple of MCP23016 I2C port extenders to try. Each chip can drive 16 LED's.... the trick will be to get enough blinking light effect without slowing the simulator. I am sure the bit bashed I2C bus will be too slow to update with ever address change in the simulator. Watch this space.