Programming code tells the computer to do something, but what makes the code actually mean anything to the computer?
In: Engineering
The basic building blocks are gates, where you can have AND gate and an OR gate. Both take 2 signals and give a result.
A signal is just a voltage, so ‘1’ is more than 2 volts, and ’0’ is less than 2 volts, and 1 signal is called a bit.
Out of these basic building blocks you can make an addition / subtraction/multiplication/division operations for 2 Sets of 64 bits that is.
but when do you use which operation? You build a block that takes 2 extra bits, and you basically can decide that 00 is for addition, 01 for subtraction, and so on, meaning the 4 blocks all have access to whatever is the storage of the 2 64 bits, but you select which operation you want with the 2 bits.
So now you have a list of operations that work on 2 sets of 64 bit numbers that you can reference.
Instead of hardcoding the bits, you can create a compiler that converts certain words into certain operations so instead of ‘00’ for addition, you can give it the word ‘ADD’ and then the 2 sets of bits you want to add.
When you have this basic compiler, you can then extend it to make more complex things like ’if’ and ‘for’ loops, and you can save many commands together as a single command (functions).
So what gives the program meaning? People made the design decisions by making the building blocks, and listing the available blocks, and then designing a compiler that can select and extend those building blocks.
A lot of people are giving you metaphors, or just repeatedly using the phrase ‘the computer interprets’ as though no matter how deep you drill down there is some sort of ineffable, intelligent gremlin that performs the magic of understanding.
So, here is what’s done. It may be a bit harder to follow than ELI5 ought to be, but it should actually answer the question in as simple a form as I can make it.
The most basic kind of process that would resemble a computer is an arithmetic processing unit. But we’ll focus on an even more basic kind of processor than that – something that just lets us store numbers in memory, and add them together. We’ll do this with 4-bit numbers.
What you need to make this device is an adder circuit, and a bunch of memory circuits. You can look up how to make these from logic gates – but all you need to understand is their basic input-output behavior.
For an 4-bit adder, you’ll have 8 input lines which you can make high or low (first 4 represent first number, last 4 represent second number) and 4 output lines, which will represent their sum (with any carry-over truncated.) This circuit acts more or less instantly – whatever you assert on the input lines, the output will automatically match.
For an 4-bit memory circuit, you have 5 input lines. You have 4 input lines which you assert as high or low to represent your number. And then you typically have a one more input line which, when asserted, will ‘store’ each of these bits, so that the 4 output lines match the current value of each of the 4 input lines, and will continue to output them until they get another ‘store’ command. Without that ‘store’ command the memory slot will just ignore the current inputs.
To make our super-dumb calculator, you’ll arrange these kinds of circuits together as shown in the following diagram, with 4-wire buses (4 wires in parallel that link outputs of circuits to inputs of others). The horizontal and vertical lines represent these buses. The sideways arrows -> represent ‘gates’. A gate is just an electrically-controlled switch. Not drawn are a bunch of single wires connected to these gates. If that wire goes ‘high’ the gate is opened. If the wire is ‘low’ the gate is closed. I’ll mention what the ARB is for in a minute. We’ll attach the input to our gates at the memory-inputs to the memory’s ‘store’ function as well, so when the gate is opened it’ll store the value.
>|||| -> memory 0—Adder……|||| <- ARB 1
|||| ——————Adder –>|||| <- ARB 2
||||……………………………….|||| <- ARB 3
||||……………………………….|||| <- ARB 4
|||| <- memory 1 <———–||||
|||| <- memory 2 <———–||||
|||| <- memory 3 <———–||||
|||| <- memory 4 <———–||||
|||| <- memory 5 <———–||||
So how would we use this to do operations?
Well, let’s say we want to add the numbers 3 and 5 together. For now let’s just say that the value of 3 (0011) is stored in memory slot 1 and the value of 5 (0101) is stored in memory slot 2. So how do we do that?
Well, they have a single set of 4 wires along the left-hand side attached to the outputs of all the memory slots. If we ‘open the gate’ at the memory 1 output, then the value of 3 (0011) will spread across the four wires. It will run up against the input to the adder, and the input to the memory 0 slot. We can’t also open up the gate at the memory 2 output, because that would put both the values of 3 and 5 (0011 and 0101) on the bus at the same time, so you’d probably get a random result, or the superposition of the two (0111 = 7). So we can’t just open both gates and let them flow into the adder. And two 4-wire buses would be inefficient, since we’d need two gates for each memory circuit, one to connect it to each of the two buses.
That’s why memory slot 0 is there. We open the gate from mem1, which will deliver its value (0011=3) to the adder and the input of memory 0. Then we open the gate on memory zero input (which is also tied to the ‘store’ line) so the value in mem1 gets delivered to and stored in mem0. Now the value that was in memory slot 1 is continually being input to the adder via the memory 0 output.
So, then we close the gate leading from memory 1 to the left bus, and we close the gate leading into memory 0 so it won’t get overwritten with a new value. Then we open the gate on the output of memory 2, so that the value there (0101=5) flows directly into the Adder’s second input. With this gate open connecting memory 2 to the adder, and memory 0 deliverying memory 1’s value to the adder, the output of the adder will be the sum of the values in memory 1 and memory 2 (1000 = 8). This value will be output constantly at the output of the adder.
If we open the gate on the output of the adder, and open the gate to the input of memory slot 3, then the sum will get stored in memory slot 3. Thus we will have added two values and stored the result.
Going back to the ARB thingy – we need a way to place our desired values into memory in the first place. How do we get the values of 3 and 5 into memory slots 1 and 2 to begin with? We have to put them there. So ARB_IN is actually just a source of high voltage connected to the 4-wire bus with 4 individual gates. We open or close those gates to place a total value on the bus, and then open the input of a single memory slot to store the value. So to store the value 3 in memory 1, we open the memory 1 input gate, and then open ARB_In gates 3 and 4 while leaving 1 and 2 closed (0011 = 3). We do a similar thing to place the value 5 (0101) in memory 2. Thus, we can input ARBitrary values into arbitrary memory by opening the right combination of gates. This is also why we need a gate on the adder output – so we can block off the result constantly flowing out of it if we want to put an arbitrary value on the memory input line instead.
So this really just breaks down into 4 steps. Leaving all other gates closed:
>1) Open gate mem1_in, ARB gate 3, and ARB gate 4
2) Open gate mem2_in, ARB gate 2, and ARB gate 4
3) Open gate mem1_out and mem0_in
4) Open gate mem2_out, adder_out, and mem3_in
This will:
>1) store the value 3 (0011) into memory slot 1
2) store the value 5 (0101) into memory slot 2
3) store the value in mem1 into mem0
4) deliver the value in mem2 to the adder, adding it with mem0 and storing it in mem3
This is our program. *This* is what we need to tell the computer to do, at the most basic, fundamental level. Build it so that we can make it do different, useful things by opening and closing gates, and then tell it what gates to open and close in what order to make something happen. So what does this *program* look like when written for the computer? Well, we have 16 total gates. We’ll label them 0-9 and the remaining six A-F.
>0) ARB 4
1) ARB 3
2) ARB 2
3) ARB 1
>4) mem0_in
5) mem1_in
6) mem2_in
7) mem3_in
8) mem4_in
9) mem5_in
>A) mem1_out
B) mem2_out
C) mem3_out
D) mem4_out
E) mem5_out
>F) adder_out
So our 4 instructions above become:
>1) open gates 5, 1, and 0
2) open gates 6, 2, and 0
3) open gates A and 4
4) open gates B, F, and 7
These gates are all blocking (no signal gets through) if they receive a low voltage (0) and are connecting if they receive a high voltage (1). So that’s exactly what our instruction looks like. Our instruction is formatted as 16 high/low signals delivered to the gates in the format of:
>FEDC BA98 7654 3210
(spaces added to make it easy to identify individual bits)
So our 4 instructions, finally, in fundamental ‘machine code’ are:
>1) 0000 0000 0010 0011
2) 0000 0000 0100 0101
3) 0000 0100 0001 0000
4) 1000 1000 1000 0000
That’s it. That’s the machine code. There’s no interpretation going on. There’s no Gremlin. These are the physical instructions that make the machine go. You could deliver these instructions, say, in the form of a punch-card. Have a cardboard card with lines of 16 perforated holes that can be punched out. Place it between a piece of metal with high voltage, and a line of 16 spring-loaded contacts each attached to a gate. Any holes that are punched out will let the contacts connect to the plate, delivering voltage to their individual gate. Perform each instruction by pushing the card through, to the next line so all 4 lines occur in-sequence (with all gates closed in between to prevent any instruction mixing).
All computers are just [much] more complicated combinations of this fundamental mechanism. No matter how many layers of abstraction there are, at some point the system just turns into ‘telling which gates to be open and closed for each step to occur”. And the computer doesn’t have to ‘interpret’ anything. At the fundamental level, the instruction *is* the set of signals that electro-mechanically open and close the gates.
If you read through all this, well done. Let me know if it was helpful, or if any part was unclear.
There is a free course available called “From Nand to Tetris” and it teaches you how to build a Tetris game from scratch, and by scratch I mean Nand gates, which are (one of) the most elemental electrical circuits. It teaches you how to assemble these gates into building blocks of a cpu, and then machine code, assembly, a compiler and an operating system right up to a working Tetris game. It is fascinating and makes you really understand what is going on.
You can think of the computer as consisting of TWO parts. The Central Processing Unit and the Memory. Programmers put their code into the memory and you can look at it like a list that goes top to bottom. The CPU then gets fed that information and writes new stuff to that memory. How does that happen? With the pins! When you look at your CPU it has all these connections to the surface it gets placed on and now all you have to do is send some electricity to those pins. The CPU is constructed in such a way that if you send electricity to certain pins they will then redirect that electricity to other pins which you then can send back to the memory.
Computer’s processor (CPU) has a built in instruction set. It performs relatively simple things like “add two numbers together” or “reserve x amount of space in memory”. A programming language allows to input something closer to human language, though still quite cryptic to the uninitiated, and a compiler then takes this input and converts it into instruction sets, written in binary (0s and 1s) that the CPU can understand.
Programs then operate on data held in memory to make things happen.
The actual answer is [arithmetic logic unit](https://en.wikipedia.org/wiki/Arithmetic_logic_unit) (ALU). That’s the thing that follows the instructions, making them meaningful.
It’s a circuit that maps code numbers (instructions) to simple operations like sum, subtract, compare, store on memory, load from memory. With a sequence of enough of these simple operations, the computer can do all the other complex operations it does.
Edit: you can actually make a computer with very few instructions (like [one](https://en.wikipedia.org/wiki/One_instruction_set_computer)), but that’s hard to program in general. Modern computers have many instructions that make it easier on the programmers. And then you can also make programming languages on top of that with commands that translate to multiple instructions.
So there are basically lot of layers
1. programming code is converted to binary code which is series of 1s and 0s. these represent instructions to processor. for simplicity every continuos 32 0s or 1s is an instruction in a series of binary code.
2. instructions are executed by an always running program called kernel/operating system. it will feed this to processor.
3. the processor is an electronic circuit that when given different 32 bits gives respective output. it is designed like that.
4. the electronic circuit is made up of gates which perform boolean logic like – inverting a bit(not gate), checking if two bits are both 1(and gate) and so on.
4.1 the gates are made up of transistors.
4.2 the transistors are made up of diodes and materials.
5. basically any algorithm can be reprrsented using these gates and the output is used accordingly by operating system/kernel.
A high level programming language such as C++ needs to be compiled into hardware specific language aka assembly. Assembly is just a bunch of basic commands the cpu understands which is basically a string of 0s and 1s. The 0s and 1s tells the hardware what to do.
To understand how 0s and 1s tells a computer what to do, imagine you have a train that can end up in 128 destinations and depending on how you program the switch tracks determine where the train ends up, and each destination is a different instruction, such as add, subtract, multiply, divide, etc.
People explained how the language (Java, C, Python, Ruby, etc.) Become Machine languages that are 0s and 1s. To make this go even lower without getting deep it’s just electrical impulses in a specific order that the computer understands doing this trillions of times in a few seconds
There is a process called compiling that turns people code into machine code. Machine code is very basic commands stored in binary. The computer interprets the 0’s and 1’s of binary as high and low voltages which act as inputs to an electrical circuit that operates the device.
Source: am a computer engineering student and Minecraft redstone engineer
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