On Fri, 1 Sep 2023, Rick Bensene wrote:
The machine was able to be implemented with so few transistors because the microcode word was quite wide, and was designed so that it was sequentially interpreted as the bits streamed out of the delay line, so not all that many flip flops were needed. Working registers were stored in the other delay line, along with program steps (yes, the machine was programmable).
Well, the machine (Diehl Combitron etc.) itself is not microcoded. The machine instructions (5 bit code) are all hardwired. The program on the steel tape is the customer application, i.e. the implementation of the calculator. We had a student who developed a cross assembler for the Combitron, able to load a program or data into the delay line (hooked the reader input to a PC).
The small delay line contains the current 11 instructions (one 55 bit word), a double-wide accumulator and an MQ register, in total 4 words.
The basic instructions are the four arithmetical functions (yes, hardware multiply and divide) as well as some I/O (for the keyboard and printer) and fill/wait instructions to transfer words from the big delay line, i.e. data/program storage, into or from the small delay line containing the registers and the small block of the current instructions (there was room for 11 instructions). There are *no* jumps! OTOH, to jump to another "location", which means changing the 11-instruction word with another one from the main delay line, you issue a special wait (delay lines are like magnetic drums) and then a fill instruction to transfer a word from the big to the small memory. And, it must be explicitely noted, the CPU does *not* have a program counter!
A very ingenious but also very complex/braintwisting design. Christian
