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Booting PDP-11 BASIC

PDPjs can now simulate a PDP-11/20. It was one of the first PDP-11 models, and since it had no MMU, it was limited to a maximum of 56Kb of RAM (or as DEC would say, 28K words), since the top 8Kb (or 4K words) of its 16-bit address space was reserved for UNIBUS devices.

The first PDPjs test of a PDP-11/20 machine was running PDP-11 BASIC, by loading it directly into memory from the original paper tape image. This required changes to the RAM component, including a new loadImage() interface that understands DEC’s paper tape format.

To provide a more “authentic” experience, there is also a new PC11 Paper Tape component, and machines can now be configured to include a virtual paper tape reader, with an assortment of paper tapes ready to be loaded.

In the real world, before you could load any paper tape, you had to first enter a Bootstrap Loader. PDPjs has simplified that process by including the “Bootstrap Loader” as just another paper tape image that you can read directly into memory, using the Read button. Once that has been done, you can then load any other paper tape image using the Bootstrap Loader, once you Load it into the paper tape reader.

Jeff Parsons
Oct 21, 2016