gistfile1.txt

I'm going to write this with a made-up assembly language just to keep things simple. Suppose we have some registers named a, b, and c. Suppose we have a command "set" that sets a register's contents:

  set a, 3

Now a holds the value 3. And we have a command "add" that adds two things together and stores the result in the first one, and "mul", which does the same thing with multiplication:

  add a, 2

Now a holds the value 5.

We have a stack, which you can push things onto and pop things off of. If we do this:

  push a

The stack has 5 on it. If we do this:

  pop b

b now contains 5, and the stack is empty.

One more thing, we have an instruction "jump" to jump to another location in the program. I'll put labels for the memory locations of each instruction in the left margin:

0 set a, 0
1 add a, 1
2 jump 1

This program never stops, incrementing a 1 at a time forever. Now we have the tools to make things that look like functions. To call a function, you push an argument and the current instruction address onto the stack and jump to it, and for it to return, it pushes its return value onto the stack and jumps back to that address.

0 push 3 ; the return address
1 push 3 ; the argument to the function
2 jump 5 ; call the function
3        ; after the function returns we'll continue execution here
4
5 pop a ; (the start of the function) the argument is in a
6 pop b ; the return address is in b
7 mul a 2
8 push a
9 jump b