Pointers and Memory Simulation

Try this

1. Each box is four bytes of memory. The variable name is above, the address below, and the current value in the middle.
2. Press Next line to run one line of code. When int *p = &a; runs, the p box receives the address of a and a yellow arrow connects p to a.
3. Run *p = 30;. The p box stays the same, but the value of a at the end of the arrow changes. The asterisk means "go to that address".
4. Switch the example to Arrays and pointer arithmetic. p++ adds 4 to the address, not 1, because it steps over one int.
5. In By value and by address, new boxes appear when a function is called. Passing a value changes only the copy. Passing an address changes the original.
6. Run Writing past the array to the end. The loop runs once too often and overwrites the variable right after the array. C does not stop you.

How to read it

A variable sits somewhere in memory, and the number of that place is its address. A pointer is a variable whose value is that number. Reading p gives the address, and reading *p gives the value at that address.

A function's local variables are created on the stack when it is called and disappear when it returns. The function boxes appearing and vanishing in this lab show exactly that.

The addresses imitate the start of RAM on an STM32 (0x20000000) as an example. The real layout depends on the compiler and optimisation.