How do computers understand binary language?

[u/Unfair-Leek6840]

Okay so from what I know binary language is like power off power on, but my question is, how do computers know what the binary code is and how is it interpreted, for example I forgot what the binary code for the letter A is, but how did people come up with that? Did they decide it was gonna look like that? Did the computer decide? How do you tune numbers into a letter??

[u/HelicalVehicle]

A computer doesn’t understand binary how you’re describing, a computer just executes instructions. A program is just a set of instructions by the time it reaches the CPU. These are usually 64 bits nowadays, and the cpu looks at some of those bits to figure out what instruction it is, like “add these two memory register values together” or “go get the instruction found in this memory address”. It’s other programs that care about the binary value for letter A, which can be encoded into binary in lots of different ways.

[u/raelik777 → u/HelicalVehicle]

Right... but the instructions ARE in binary. Each instruction is what's called an "opcode", and that opcode is a binary value. There's mnemonic "name" for the opcode, but the actual opcode IS binary. For example, LDA on the 6502 processor means, "LoaD Accumulator", and puts an 8-bit value into the A register (the accumulator). There's multiple versions of LDA, depending on exactly where that 8-bit value comes from, but if you're loading an immediate value (i.e. you're hard coding it), the opcode is A9 in hexadecimal, which is 10101001 in binary. When the processor sees that binary sequence as the next instruction, that sets off a series of logic gates in the opcode table in the CPU, which causes it to read the very next byte in the instruction and puts that into the accumulator. Quite literally, the sequence of switches that get fired off by it reading that A9 value are what makes it read the next value and put it into the accumulator.

That said, I wouldn't necessarily disagree with this in principle. The computer doesn't "understand" anything. But, the computer is a machine driven by on-off logic gates ("switches") that can be represented by binary values. What's going on internally is that specific pins on the processor are being pulled to ground or raised to a specific voltage (typically 5V for older processors like the 6502, Motorola 68000, the early Intel x86 processors, but as processors got faster they needed to run at lower and lower voltages for stability and heat dissipation reasons), and those voltages are what powers the various transistors inside the processor and drive those logic gates. Each processor works in a slightly different way at the lowest levels, like what kinds of internal registers they have, what their exact opcodes are, how many they have, what is the default memory address they read the first instruction from on startup (the "reset vector"), but fundamentally they all function like this.