4) Returning a Value

Now that we have an empty function, we can start building some code! Let's learn what a register is.

A register is like a variable, except there are a small number of them - you have about eight general purpose 64-bit integers registers on amd64 (we won't talk about floating point or other special registers):

• rax
• rbx
• rcx
• rdx
• rdi
• rsi
• rbp
• rsp

All mathy stuff that a computer does (add, subtract, xor, etc) operates on registers, not directly on memory. So they're super important!

Specific registers have some implicit meaning, mostly by convention. For example, when a function returns, its return value is typically put in rax. Let's do that!

To move a value into a register, use the mov instruction; for example:

mov rdx, 1

In a higher-level language this would be equivalent to:

rdx = 1 —

For this level, can you return the number '1337' from your function?

That means that rax must equal 1337 when the function returns.

Hints:

• Unlike most languages, you don't return values with ret X; instead, you set rax to a value, then return
• To set a register, use mov REGNAME, value; for example, mov rcx, 123
• The return register is rax
• If something isn't working, don't forget to check the debugger that pops up when you click Execute! It'll show you the state of every register at every step