Assembly basics

Alright, now that we know what the tools do, let’s see what language RGBASM speaks. I will take a short slice of the beginning of hello-world.asm, so that we agree on the line numbers, and you can get some syntax highlighting even if your editor doesn’t support it.

INCLUDE "hardware.inc"

SECTION "Header", ROM0[$100]

	jp EntryPoint

	ds $150 - @, 0 ; Make room for the header

EntryPoint:
	; Shut down audio circuitry
	ld a, 0
	ld [rNR52], a

Let’s analyze it. Note that I will be ignoring a lot of RGBASM’s functionality; if you’re curious to know more, you should wait until parts II and III, or read the docs.

Comments

We’ll start with line 10, which should appear gray above. Semicolons ; denote comments. Everything from a semicolon to the end of the line is ignored by RGBASM. As you can see on line 7, comments need not be on an otherwise empty line.

Comments are a staple of every good programming language; they are useful to give context as to what code is doing. They’re the difference between “Pre-heat the oven at 180 °C” and “Pre-heat the oven at 180 °C, any higher and the cake would burn”, basically. In any language, good comments are very useful; in assembly, they play an even more important role, as many common semantic facilities are not available.

Instructions

Assembly is a very line-based language. Each line can contain a directive, which instructs RGBASM to do something, or an instruction1, which is copied directly into the ROM. We will talk about directives later, for now let’s focus on instructions: for example, in the snippet above, we will ignore lines 1 (INCLUDE), 7 (ds), and 3 (SECTION).

To continue the cake-baking analogy even further, instructions are like steps in a recipe. The console’s processor (CPU) executes instructions one at a time, and that... eventually does something! Like baking a cake, drawing a “Hello World” image, or displaying a Game Boy programming tutorial! *wink* *wink*

Instructions have a mnemonic, which is a name they are given, and operands, which indicate what they should act upon. For example, in “melt the chocolate and butter in a saucepan”, the whole sentence would be the instruction, the verb “melt” would be the mnemonic, and “chocolate”, “butter”, and “saucepan” the operands, i.e. some kind of parameters to the operation.

Let’s discuss the most fundamental instruction, ld. ld stands for “LoaD”, and its purpose is simply to copy data from its right operand (RHS) into its left operand (LHS). For example, take line 11’s ld a, 0: it copies (”loads”) the value 0 into the 8-bit register a2. If you look further in the file, line 33 has ld a, b, which causes the value in register b to be copied into register a.

InstructionMnemonicEffect
LoadldCopies values around

ℹ️

Due to CPU limitations, not all operand combinations are valid for ld and many other instructions; we will talk about this when writing our own code later.

🤔

RGBDS has an instruction reference worth bookmarking, and you can also consult it locally with man 7 gbz80 if RGBDS is installed on your machine (except Windows...). The descriptions there are more succinct, since they’re intended as reminders, not as tutorials.


1

Technically, instructions in RGBASM are implemented as directives, basically writing their encoded form to the ROM; but the distinction between the instructions in the source code and those in the final ROM is not worth bringing up right now.

2

The curious reader may ask where the value is copied from. The answer is simply that the “immediate“ byte ($00 in this example) is stored in ROM just after the instruction’s opcode byte, and it’s what gets copied to a. We will come back to this when we talk about how instructions are encoded later on.