The processor hardware design was led by Colin Chapman. I joined in January 1971 and was set to work alongside Colin, checking over some of his work, to learn the structure, and doing detail design work too, particularly on the ALU board. We worked very closely together on the prototype debugging phase. Due to some of the criticisms I had made of some of the original detail design I was given, for my sins, the task of the Mk II redesign, with, as you are aware, some additional instructions.Although in the job I had had before BCL I had been involved with digital design I had only limited experience and had never been involved with computers. You probably know about the overflow bit, or was it the carry? It did not work logically correctly in the Mk I. I fixed it in the Mk II, but because the operating system and some other software had been written catering for the logically incorrect operation I had to undo my fix and revert to the original incorrect logic.
The instruction set was most closely related to that of the Data General Nova of that era. The hardware design was all pure BCL. The core memory subsystems came initially from Plessey Memory Systems, a site in Poole, and later, with higher density, from Fabritek, being made in High Blantyre, Scotland. The processor design was all random logic. The original processor had something like 10 PCBs each carrying something like 80 integrated circuits. Now I know that real professionals, even then, would have used microprogramming.....and oh how much easier that would have made our lives!
I am quite proud of the fact that a machine structure we, I will include myself in this, designed in 1971/72 was still in use about 20 years later. I survived the CWT ( Computer World Trade ) takeover of mid 1974 but by late 1974 realised that it was unlikely that there would be interesting work of any kind and so sought alternative employment. This was delayed somewhat by breaking my leg in my office at BCL in November 1974, performing a minor celebratory leap on some papers we had just piled up after clearing out the desks and filing cabinets of our redundant, former fellow office occupants. I moved on in late March 1975.
Of the 4 places I have worked I have the fondest memories of the bunch of cowboys and rogues that made up BCL in the early 1970s. There was some very hard work, but they were a good bunch, and I liked living in the Brighton area, leaving it when I left BCL