Jumps

So far, all the code we have seen was linear: it executes top to bottom. But this doesn’t scale: sometimes, we need to perform certain actions depending on the result of others (“if the crêpes start sticking, grease the pan again”), and sometimes, we need to perform actions repeatedly (“If there is some batter left, repeat from step 5”).

Both of these imply reading the recipe non-linearly. In assembly, this is achieved using jumps.

The CPU has a special-purpose register called “PC”, for Program Counter. It contains the address of the instruction currently being executed¹, like how you’d keep in mind the number of the recipe step you’re currently doing. PC increases automatically as the CPU reads instructions, so “by default” they are read sequentially; however, jump instructions allow writing a different value to PC, effectively jumping to another piece of the program. Hence the name.

Okay, so, let’s talk about those jump instructions, shall we? There are four of them:

We will focus on jp for now. jp, such as the one line 5, simply sets PC to its argument, jumping execution there. In other words, after executing jp EntryPoint (line 5), the next instruction executed is the one below EntryPoint (line 16).

Conditional jumps

Now to the really interesting part. Let’s examine the loop responsible for copying tiles:

	; Copy the tile data
	ld de, Tiles
	ld hl, $9000
	ld bc, TilesEnd - Tiles
CopyTiles:
	ld a, [de]
	ld [hli], a
	inc de
	dec bc
	ld a, b
	or a, c
	jp nz, CopyTiles

First, we copy Tiles, the address of the first byte of tile data, into de. Then, we set hl to $9000, which is the address where we will start copying the tile data to. ld bc, TilesEnd - Tiles sets bc to the length of the tile data: TilesEnd is the address of the first byte after the tile data, so subtracting Tiles to that yields the length.

So, basically:

• de contains the address where data will be copied from;
• hl contains the address where data will be copied to;
• bc contains how many bytes we have to copy.

Then we arrive at the main loop. We read one byte from the source (line 29), and write it to the destination (line 30). We increment the destination (via the implicit inc hl done by ld [hli], a) and source pointers (line 31), so the following loop iteration processes the next byte.

Here’s the interesting part: since we’ve just copied one byte, that means we have one less to go, so we dec bc. (We have seen dec two lessons ago; as a refresher, it simply decreases the value stored in bc by one.) Since bc contains the amount of bytes that still need to be copied, it’s trivial to see that we should simply repeat the operation if bc != 0.

ld a, b and or a, c “bitwise OR” b and c together; it’s enough to know for now that it leaves 0 in a if and only if bc == 0. And or updates the Z flag! So, after line 34, the Z flag is set if and only if bc == 0, that is, if we should exit the loop.

And this is where conditional jumps come into the picture! See, it’s possible to conditionally “take” a jump depending on the state of the flags.

There are four “conditions”:

Thus, jp nz, CopyTiles can be read as “if the Z flag is not set, then jump to CopyTiles”. Since we’re jumping backwards, we will repeat the instructions again: we have just created a loop!

Okay, we’ve been talking about the code a lot, and we have seen it run, but we haven’t really seen how it runs. Let’s watch the magic unfold in slow-motion in the next lesson!