How does it know where my value is in memory?

Question

When I write a program and tell it int c=5, it puts the value 5 into a little bit of it's memory, but how does it remember which one? The only way I could think of would be to have another bit of memory to tell it, but then it would have to remember where it kept that as well, so how does it remember where everything is?

Answer 1

Your code gets compiled before execution, at that step your variable will be replaced by the actual reference of the space where the value will be stored.

This at least is the general principle. In reality it will be way more complecated, but still the same basic idea.

Answer 2

There are lots of good answers here, but they all seem to miss one important point that I think was the main thrust of the OP's question, so here goes. I'm talking about compiled languages like C++, interpreted ones are much more complex.

When compiling your program, the compiler examines your code to find all the variables. Some variables are going to be global (or static), and some are going to be local. For the static variables, it assigns them fixed memory addresses. These addresses are likely to be sequential, and they start at some specific value. Due to the segmentation of memory on most architectures (and the virtual memory mechanisms), every application can (potentially) use the same memory addresses. Thus, if we assume the memory space programs are allowed to use starts at 0 for our example, every program you compile will put the first global variable at location 0. If that variable was 4 bytes, the next one would be at location 4, etc. These won't conflict with other programs running on your system because they're actually being mapped to an arbitrary sequential section of memory at run time. This is why it can assign a fixed address at compile time without worrying about hitting other programs.

For local variables, instead of being assigned a fixed address, they're assigned a fixed address relative to the stack pointer (which is usually a register). When a function is called that allocates variables on the stack, the stack pointer is simply moved by the required number of bytes, creating a gap in the used bytes on the stack. All the local variables are assigned fixed offsets to the stack pointer that put them into that gap. Every time a local variable is used, the real memory address is calculated by adding the stack pointer and the offset (neglecting caching values in registers). When the function returns, the stack pointer is reset to the way it was before the function was called, thus the entire stack frame including local variables is free to be overwritten by the next function call.