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Er, I was eavesdropping and just have to ask how this:

subtract memory location being pointed-at from the accumulator; if the accumulator is negative, skip the next location, else execute the next location

could be one instruction in machine code?

I can't even see this as a single instruction in interpretive language code.

Easy. It depends on how much circuitry you want to devote to an instruction. This is not even as complicated as IEEE floating point arithmetic that most Intel processors have. There have even been fused multiply-add units for years.

The Perkin-Elmer (if they still exist) computers had instrucitons that did things such as (add, update index register, jump on condition) as a single instruction. The old IBM650 had the address of the next instruction to be executed in the instruction itself. (This was a drum memory machine and the relative location of instructions on the drum contributed to program speed. You didn't want to drop a revolution.)

It's only a curiosity but it is amusing that a single-instruction computer can emulate a universal Turing machine.

I can't even see this as a single instruction in interpretive language code.

Universal computers have a "finite control function", that defines the nature and granularity of manipulations of the state. Depending on the type of machine, the execution of a single "instruction" (which is an abstract rather than literal construct) can have either very simple or very complex consequences to the state. The folding of a protein is an extremely complex behavior, but it can be triggered by the execution of a single "instruction" within that computational system. You are having problems with this because you are thinking of things like machine code, which is a very narrow instance of all possible control functions.

One of the mental hazards of computational theory is that most people view computers as being solely like the kinds of computers we build with silicon. They way we build computers in practicing is a consequence of history and practical engineering concerns, and doesn't even scratch the surface of the entire space of things that constitute "universal computers". This is a case where limited experience leads to conceptual prejudices that aren't justified.