More thoughts on a "Computer Demonstration Model"

Updated 18 Feb 26

Recently I took another look at the book Computer Models, by A. Wilkinson, published in 1968 and mentioned towards the end of this item on my RM website. On page 107, in a section titled Suggestions for Further Development, the author notes the limitations of his model, which is essentially a very simple calculator, and says a programme of instructions to be arranged and carried out ... is essential if the model is to do a complete programming job. 

Indeed and this makes me wonder what the simplest programmable transistor-based computer would comprise? What instruction set would it need? The author's machine can already be told to add and subtract. There is a shift register operation too under the covers, so let's make that available. A compare instruction for logic, is needed of course. And ones to move data (numbers) between storage areas. No doubt others too - a look at the Diehl calculators is needed.

Let's number those instructions. The simplest form of programming would be a sequence of individual steps on a sheet of paper for an operator to carry out, each comprising an instruction number (selected by a rotary switch perhaps) and a piece of data, according to an agreed syntax, followed by the operator pressing a button to run that step. To use logic (repeated addition for a multiplication program, say), the operator would need to jump to another location on the sheet. By default we could put, say, '+1' at the end of each instruction to tell the operator to move to the next step. If we need to jump, then it would be plus or minus some other number.

Or write the program on a punched card, pushed into a reader, either by the operator or automatically (I'm thinking of Computacar from ~1970!) one low-level instruction step at a time. This reader could be opto-electronic or electro-mechanical. Handling jump instructions would be tricky though.

This could be a very interesting project!