Continuing the discussion from So many bit-serial CPUs, ancient and modern (Pilot ACE, LGP, 74 series, homebrew, …):
Looking into how the WITCH works (and it does still work, on display at TNMoC) I see that it’s a curious mix of serial and parallel. Showing that any taxonomy has limitations and outliers.
Each WITCH dekatron has ten stable states, holding one decimal digit, and individually is incremented, as the main thing it can do. As it is incremented, it can produce counted pulses, which means the natural operation is for each digit to recirculate and to have its value be added to another digit. (Equally naturally it can output a 9’s complement value, and cause a subtraction.) So the basic operation is to accumulate into the destination, all digits in parallel, but using 10 counts to get it done. And then there’s a series of additional pulses to resolve all the carries, running until no more carries remain. I think that could be up to one carry per digit in the result.
To move a value from one register to another, the destination must first be cleared, and then an accumulation performed. Any register, then, operates as an accumulator, and registers are all the storage there is - it’s much more like a programmable calculator in some ways, than a computer.
But, as far as I can tell, transfers must always be from one bank of 10 words to another, so the registers are in clumps.
Notably, the machine offers single-instruction divide and multiply operations.
Being a kind of bit-serial but digit-parallel, it’s a kind of opposite of many early electronic calculators, which have 4 bit ALUs and operate digit-serially.
Much information here at the CCS, from a 1951 paper:
An Electronic Digital Computor Using Cold Cathode Counting Tubes for Storage
A small sequence-controlled computor is described; it has storage capacity for up to 90 numbers each of eight decimal digits. Cold cathode counting tubes (Dekatrons) are used as storage elements. Parallel operation is employed, and the speed of operation is comparable with that of a desk calculating machine. Apart from speed, however, the computor provides most of the facilities found in larger, faster digital computors. Information is fed into the machine from perforated paper tapes and a teleprinter or tape perforator records the results.
I took a movie of the machine as seen at TNMoC, it has quite a nice sound to it.
