Joonas Javanainen

Errata for "Reverse engineering fine details of Game Boy hardware"

I held a presentation about Game Boy reverse engineering at Disobey 2018, a security/hacking-oriented conference held in Helsinki. It was a really nice chance to show off some of my tools and research results, and at least from my point of view the presentation went really well! Here’s a YouTube video of the presentation:

Unfortunately, even with a lot of preparation, some errors managed to creep into the last section of the talk, because I happened to misread some of my own notes about my very recent findings. Looks like I foreshadowed this in the talk when I said “I’m not sure about all of this yet” ;)

Data sampling

In the data sampling part (~31:00) I claim that by looking at the cartridge bus signals, we can see that the data bus is sampled on the falling edge of the machine cycle, so the sampling point is basically in the middle of the cycle. However, there’s an off-by-one error here, and the data sampling actually happens one clock cycle later.

Slightly later (~33:35) I explain that by abusing serial IO, we can see that the CPU actually samples data internally at a different time (rising machine clock edge). This is still correct, so the main points of this part are still valid.

Interrupt handling

In the interrupt handling part (~40:00) I explain how there are two different clock edges involved in interrupt handling: “should we dispatch” and “where do we jump to”. The first edge (= “should we dispatch”) is correct, but the second clock edge is early by an entire machine cycle. So, the “where do we jump to” edge is still on a falling machine clock edge, but it’s the last one in the interrupt dispatching phase and just 4 clock edges before the first interrupt handler instruction clock edge!