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Inside of a digital circuit, like a computer, what is acting as the switch that drives the inputs or internal values from 0 to 1, and how does this work? Like in instructions for a computer, how are the values of 0 and 1 set for the different 32 bit instructions, because obviously no one is flipping a switch to make them change.

winny
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Michael Bradley
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    At very low level, memory, switches, etc. are implemented using transistors. At a system level there are several components with their own jobs, connected together according to the architecture level design to make things work. Please refer to a basic computer architecture course/book to get detailed insights. – beta_me me_beta Apr 21 '22 at 06:53
  • logic gates and transistors mostly. Inside the logic gates are transistors so really, you could say transistors and keep the answer at that. Is that the sort of answer you're looking for? I don't want to write a full answer without really understanding what you're asking – MCG Apr 21 '22 at 06:54
  • Look up Ben Eater on YouTube. His 8 bit computer on breadboard series is really good for understanding what goes on at low level inside a computer. It's a good place to start – MCG Apr 21 '22 at 06:55
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    We sure did flip switches back in the day. In fact, if you want to understand how all this takes place perhaps the very best place to start is learning how to use a manual switch to set a 0 or 1, as an input to logic. You could spend some serious time on that topic alone. I first learned about programming using a paper computer by Bell Labs called a CARDIAC. Since ***I*** had to run every instruction myself using paper and pencil, I quickly learned machine language. It was only two more years before I designed and built a working 8 bit computer from 7400 series devices. – jonk Apr 21 '22 at 07:07
  • they are actually made of tiny mosfets etched onto silicon wafers. – dandavis Apr 21 '22 at 07:27
  • Complementary Metal Oxide Semiconductor Field Effect Transistors. Usually just referred to as "CMOS." The CMOS logic revolution was really the key enabler for the kind of digital computers we have today. P-channel FETs on the high side and N-channel FETs on the low side. This gives you logic gates, and logic gates give you flip flops. Once you have logic gates and flip flops, you have everything you need to design a digital computer. At least conceptually. – user57037 Apr 21 '22 at 07:40
  • Related: https://electronics.stackexchange.com/questions/53142/how-are-logic-gates-created-electronically – MarkU Apr 21 '22 at 07:40

1 Answers1

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What drives an input...?

An output.

There have been a multitude of different digital logic families over the years with differing levels of commercial success. What they all have in common is that they define the way in which the output from one logic gate signals a "0" state or a "1" state to the input or inputs of one or more other logic gates.

In virtually all computing machinery made since the 1960s, the switching elements are transistors, and in current-day computing machinery, it's almost entirely MOSFET transistors.

Without getting too technical, a transistor is a three-terminal device, in which the voltage between one pair of its terminals controls the ability of current to flow between a different pair of its terminals.

By far, the most commercially successful logic family ever is called CMOS in which there are two MOSFETs for each output, and only one of the two FETs is allowed to be "on" at the same time. One FET connects the output to the positive voltage supply rail (a.k.a., \$V_{+}\$,) and the other connects it to ground.

When the FET connecting the output to \$V_{+}\$ is "on," then we call that logic state "1" or "high," and when the other FET is "on," we call that "0" or "low."

In some, specialized logic gates there is a third possibility: If neither FET is "on," then the output is said to be "floating," or "Hi-Z," or "tri-stated." This does not signal either a "0" or a "1." An output of this type is used to drive a bus (i.e., a signal that can be driven by more than one gate.) At any given time, only one gate is allowed to drive the bus to a "0" or a "1," and all of the other connected gates must be put into the "floating" state to prevent them from interfering.

How any given gate decides whether to drive its output to "0" or "1" or "floating," is a topic for a different question.

Solomon Slow
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