Meet Assembly

The Z80 runs instructions, reads and writes memory, and communicates with the Spectrum’s hardware. We will give it small jobs whose results we can predict and inspect.

Begin with a stock 48K ZX Spectrum, either emulated or real. You do not need to have studied BASIC or completed another programming course. Each lesson provides a complete source file and an editable browser experiment. For local work, Asm198x is the recommended assembler and Emu198x the recommended emulator; the same source also builds with Pasmo. Getting started covers installing both.

Eight experiments

1. Change the border: assemble a program and send a value to the hardware.
2. A byte becomes pixels: follow eight bits from a labelled address to the screen.
3. Build an eight-row shape: choose the patterns and addresses that form a small picture.
4. Draw with a counted loop: keep the picture, store its patterns as data, and replay the loop’s actual writes.
5. Call a drawing routine: draw in two places, follow each return, and check a routine’s contract.
6. Move your character: read keys, keep a position and inspect the erase and redraw.
7. Give movement a clock: hold a direction and change how many frames pass between updates.
8. Find the first wrong step: stop at a decision and compare a prediction with registers, flags and RAM.

Make a prediction before each run. When the result differs, follow the value and its destination: understanding the first unexpected change is more useful than guessing at several edits at once.

After the introduction

You can now follow instructions through registers and RAM, call a drawing routine, read input, pace movement and investigate a fault. Meteor Storm applies that foundation to a timed dodging game, beginning with small pixel and clock experiments.