SDL Things:
https://github.com/tiehuis/zig-gameboy/blob/master/src/window_sdl.zig
It's been awhile since Day 1 and I'll pretend that a year did not pass since.
For today's note, I want to describe an ever important debug line:
A: F5 F: 10 B: 01 C: 10 D: C1 E: 00 H: 41 L: 00 SP: FFFE PC: 00:020C (0D 20 F7 78)
Without worrying about the particulars, this is the defacto standard log line when debugging the CPU. In this line, we display the values of all registers and read memory at the upcoming PC, as well as the following three.
There are a variety of logs from working emulators which output in this format; these come from running blargg test roms.
With this knowledge combined, I can take the output of my emulator and do a line-by-line review between my emulator and the working one. Of course, doing this manually is not ideal, so a simple script can help:
#!/bin/bash
file1=$1
file2=$2
# Compare the files line by line
i=0;
while IFS= read -r line1 && IFS= read -r line2 <&3; do
i=$((i+1))
if [ "$line1" != "$line2" ]; then
echo "${i} Line differs:"
echo "> $line1"
echo "< $line2"
fi
done <"$file1" 3<"$file2"
# Handle error cases
if [ $? -ne 0 ]; then
echo "Error: Either $file1 or $file2 does not exist or is empty"
fi
This approach helped me pinpoint where errors existed within my CPU
instructions. Once I identified the line, I would observe the first value in
the brackets (e.g. (0D ...)) and review the instruction I've written for it.
In all cases when debugging my CPU, my issue was that the instruction was
incorrectly implemented.
At this point, I have an emulator which matches the log output of #6 (blargg test rom). Next step is to get the actual output of the blargg rom showing up, my emulator doesn't ever seem to write to the serial port (0xff01)
About a year ago I had begun dabbling in emulation. The space was new to me and the itch was there through a desire to be reconnected to the video games of my childhood. I could just as easily play those games, but that didn't feel rewarding. I had this desire to see them come to life through a technically difficult project that I could accomplish within a meaninful time frame.
Roll back to September 2020. I'm living in Halifax, have a new born, and am actively on the job hunt. In order to flex my programming muscles I spend the extremely limited time I have attempting to program a Gameboy emulator, mostly in the evenings after the kids fall asleep. For the most part, that time is spent wrapping my head around the concepts of emulation:
- Understanding that you're writing software which is "pretending" to be hardware
- The concept of clocks, cycles, ticks, how a CPU works, memory maps, etc.
For many evenings, because my work was so sporadic (newborns don't sleep, in case you weren't aware), I ran in circles around the concepts. Eventually, I took a break to breathe.
A few rested months later, I decided to revisit emulation and programmed a simple CHIP-8 Emulator using Zig. This proved to be a great opportunity to learn Zig, a fun language that feels as if it's here to replace C, but also an opportunity to take those concepts -- of clocks, cycles, cpu, etc. -- and apply them to a simpler project.
Writing the CHIP-8 emulator validated my understanding and provided me confidence, which ultimately can lead to an initial piece of advice: Start with Hello World!, and in emulation, that's a CHIP-8 emulator!
And now we're here in the present. Today, I revisit emulating the Game Boy. Because I enjoyed it so much the first time, I'll be using Zig (I write Go during my day job and I love the idea of learning a new language each year, so I'll flex more Zig muscles for this project!)
- Pandocs, a comprehensive technical reference for the Game Boy