Assembly is the code the machine runs, tidied up just enough that it isn't completely unreadable to a human. On this site it'll look like this:
0: li r3, 20
4: blrTwo lines, two instructions — assembly is one instruction per line, and it runs top to bottom.
The first thing to notice: the numbers on the left don't count up 1, 2, 3. They jump by 4, and they're written in hex. That's because they aren't line numbers at all — each one is the address of its instruction, where that instruction sits in memory. It'll matter in later lessons; for now you can ignore the exact values and just know that's what they are.
Every instruction is a mnemonic followed by zero or more operands.
Take the first line, li r3, 20. The mnemonic is li and the operands are r3 and 20. li is short for load immediate — "immediate" just means a constant written straight into the instruction. By PowerPC convention the destination comes first and the value second, so this line reads as "load the literal 20 into r3."
That r3 is a register. Registers are the small, fast slots inside the processor where it keeps the values it's actively working with — there are 32 of them, named r0 through r31. So this single instruction drops the number 20 into register r3.
blr has no operands at all. It's the return: it ends the function and hands control back to whatever called it. You'll find a blr at the bottom of nearly every function you decompile.
There are a lot of mnemonics, and you'll meet them a few at a time — but you don't need to keep them all in your head. Hover any instruction in the diff and a tooltip explains what it does, with that line's own registers and values filled in. Lean on it as much as you like; the common ones will stick on their own soon enough.
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This is a simplification — there's far more to assembly than two instructions, and you'll pick the rest up a few at a time, exactly when a lesson needs them. But it's enough to read your first target. Let's go match your very first C function.