Creating an NES Emulator From Scratch Part 1: CPU Basics

My first task was to read up a little bit on how emulation actually works. In our case, Emulation is when you design a software program to act as a hardware component such as a Microprocessor.

Before going further, I needed to understand how a CPU works as well as some history. So here we go. I’ll put a TL;DR at the bottom if you don’t understand… it’s a lot of stuff to try to understand , though not all the history or small details are all that important.

A CPU is a type of Microprocessor, and a Microprocessor is a chip that contains a logic center, as well as small areas of memory, called Registers. Registers are used to store numbers, states, or memory addresses pointers temporarily, until a program interacts with them. Registers are used because it is much faster for a CPU to use a register value than to fetch the same value from RAM.

Programs executed by CPUs are written in Binary, aka Machine Code. This type of language is formed by a string of 1’s and 0’s. Each 1 or 0 is called a bit, and if you have 8 bits, you have 1 byte. If you have 1000 bytes, you have a kilobyte. If you have 1000 kilobytes, you have a megabyte, and so on. Just think, every photo you have ever taken or any digital music track you have ever listened to is actually millions of 1’s or 0’s.

You may now be thinking “Hmm, I understand how data could be stored in 1’s and 0’s, but how can programs possibly be made up of them, since they aren’t physical data in the same sense as a picture?”

The gritty details are out of the scope of this first post, but CPUs (and most microchips) assign various internal instructions to correspond with a pattern of 1’s and 0’s, fed to it by a stream of data. Imagine it like Morse Code, where letters are spelled using either long (written as dashes - ) or short (written as dots . ) bursts of Light, Sound, hand signals, etc. If you were in danger, you would signal SOS to a plane with . . . - - - . . .

SOS is an instruction, and it is relayed by morse code. Binary can be thought of in a similar manner, where each instruction corresponds to a different stream of 1’s or 0’s. Instructions are referred to as OpCodes.

Back to programming. If you are familiar with programming, you may not realize that although you write your program in Java or C++, when you compile, your program is converted into this machine code, to later be used by the processor. But before there were ‘high level’ languages such as those, programmers would of had to write all their programs in binary… had it not been for a few other “Inventions”.

If you look at a page full of 1’s and 0’s eventually you are going to get dizzy and the whole lot is just going to be unreadable. To make binary more human readable, we use a numbering system called Hexadecimal. By the time this is posted I would have posted an explanation of Hex, so I’m not going to cover it again here.

Okay, so at least now we aren’t looking at a page full of 1’s and 0’s but hexadecimal isn’t much better. That’s why the next step was to program in Assembly. Assembly was created to assign “words” and “syntax” to hexadecimal (and thus, binary) commands. I use quotation marks around those two words because as you will see, Assembly kinda sucks to read as well. However, things began to look much more structured.

However, apart from straight-up binary or Hex, Assembly is one of the most painful languages to program in especially for beginners (like me!), because it appears to be so abstract, yet readable at the same time. Interestingly, at this point the only reasons assembly is worth understanding are if you are really interested in how CPU’s work, have lots of spare time and want to learn a low level language, or if you are writing an Emulator, Compiler, among other niche programs. Don’t be afraid though, we wont write our emulator in assembly.

Note that assembly, though a human readable representation of Machine Code, couldn’t be fed to the processor itself. It has to be compiled into binary before that could happen.

Eventually people got tired of Assembly and invented higher level programming languages (like C), that are much easier to read and program in, but that could be compiled (aka translated) by a computer, [back into Assembly], then finally into Binary, where a computer was ready to run the program.
 

My fingers are hurting so TL;DR:

- Emulation is where you use one computer to run a program with instructions designed for another… translation needs to be done.
- A CPU is a chip that has a logic center and small areas of memory called Registers

- Registers are temporary storage areas used in programs

- Programs are a series of 1’s and 0’s that a CPU reads as instructions. Different variations of 1’s and 0’s mean different things.

- Instructions (defined streams of 0’s and 1’s) are called OpCodes

- Hexadecimal is used to make data easier to read than if it were in Binary

- Assembly is used to make program data easier to read than if it were in Hex.

- High Level programming languages are used to make program data easier to read (and write!) than if it were in Assembly.

How Hexidecimal Works

Below is a post I wrote a while back somewhere else about how Hexidecimal works. This will become important for when I post the next part of my NES emulator series.

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Decimals to Hex and Binary

The numbers we use everyday work on a base ten system. That is we count from 0 to 9, then add a 1 in front of the 9 then return the 9 back to 0 ( 09 becomes 10 ). At 99, a 1 gets added in front of the 99, and the 99 loops back to 00 ( 099 becomes 100 ). This way of looking at it is very important to understand other base systems.

Now imagine a system that works on base sixteen instead of base ten. You would essentially be counting up to 15, then adding a 1 in front and setting 15 to zero. So, like 8, 9, 10, 11, 12… you would have (0)(13), (0)(14), (0)(15), (1)(0), (1)(1)…. and so on. ( (0)(15) becomes (1)(0) )

I used brackets to separate each 'digit' but we need a way to represent 10-15 as a one digit character. Lets replace 10, 11, 12, 13, 14 and 15 with A,B,C,D,E,F. So 9 in decimal would still be 9, but 10 in decimal would be A. 15 in decimal would be F, and 16 in decimal would be 10, because if you go one higher, you set F to zero and add 1 at the beginning (0F becomes 10). Congratulations, you have learned the essentials of Hexadecimal.

Converting Dec to Hex

An easy way to convert to Hex is by looking at the number of digits and the values each represent. As an example, we will look a couple of examples in decimal. The number 300. It has three digits, and can be rewritten as 3x10^2. The number 50 can be rewritten as 5x10^1. Finally, 8 can be rewritten as 8x10^0. Because 300 + 50 + 8 = 358, you could say that (3x10^2)+(5x10^1)+(8x10^0) = 358.

That in mind, lets do the same for Hex. We will decode, or find the decimal value that is associated with the hex value 0x166. The "0x" will now be added to distinguish Hex from decimal. We can rewrite this number as (1x16^2)+(6x16^1)+(6x16^0) to give us 358.

Finally, lets encode a Hex value given a decimal number. Starting small, this number will be 39.
- To start, we want to see how many powers of 16 the number has. So grab a calculator, unless you are good with exponents. See how many times you can multiply 16 by its self without going over your decimal number. In this example, we can only go as far as 16^1, because 16^2 is 256, way over.
- Now we need to see how close we can get to our number by multiplying 16, again without going over. The most we can go is 2x16^1 = 32.
- We need to find what is left after this multiplication, so subtracting 32 from 39 leaves us with 7.
- Because this is inside the base 16, we leave this at 7.
- Therefore, the hex value for 39 is 0x27

Converting Hex to Binary

Binary is simply another system of numbering. Remember that while decimal is base-10 (0-9), and hexidecimal is base-16 (0-F), binary is base-2 (0-1)! 

To count up in binary, you start at zero. Then you count up until the number reaches 1. Then, on the next count, you set that digit back to zero and add a 1 in front of it, like so:

0 -> 1 -> 10 -> 11 -> 100 -> 101 -> 110 -> 111-> 1000 -> 1001 -> 1010 -> 1011 -> 1100 -> 1101 -> 1110 -> 1111

Now, here is where it gets interesting. Check this table out:

 

1111 corresponds to F... So, the last one digit hexidecimal number fits in four digits of binary. In computer science, each 1 or a 0 of binary is called a bit. Because one digit of Hex is 4 bits, you can fit 2 digits of Hex in a byte (which is 8 bits).

11111111 is equal to 0xFF, and equal to 255. In one byte, you can store a number up to 0xFF (255 in decimal).

Infographic

Here is the infographic that really helped me learn this. Credits to the original owner... I just found it on Google Images :)

Count Like a Computer Infographic

Creating an NES Emulator From Scratch Part 0: The Start of an Adventure!

My next project is quite an ambitious one. My goal is to build an NES (Nintendo Entertainment System) emulator in either Java or C++ from scratch.

The reason for this is three-fold. Firstly I've been interested in retro games and emulators for a long time now. Secondly, for my first term of first year university I'm taking a C++ course and previous experience is recommended. I'm quite comfortable with programming in Java and I have some experience with C++ but I think learning through a project will provide a significant boost to my knowledge. Finally, a side project like this is very valuable to tell employers when applying for a job, and writing an emulator demonstrates understanding of both software (the programming of the emulator) and hardware (the details of the machine you are trying to emulate).

There is a fourth reason as well, though not as selfish of one. I want to try and inspire and teach anyone that wants to do this project as well, in a very comprehensive way. We can learn about this together, in essence.

I'm going to start by saying that I can't guarantee much. I have no idea how far I'll get on this before I run out of time due to school, or before I drop the project due to other reasons.
I also can't guarantee this emulator will be a polished and playable application. My main goal is to make something that works and if I have the time afterwards, improve on it.
In addition, I will not guarantee that as of any revision of any post that the information contained within is correct. Because I am writing as I am learning this, I may have misunderstood the content. If you spot mistake please comment! Otherwise at some point I'll end up figuring it out. Probably.

I will be starting my research soon. I'll post my findings and my sources on this blog. Once I start real programming I'll post it on my GitHub page as well.

I think this will be an exciting adventure!

Nerf Mod Complete!

After a break to go places and do things that needed to be done, I finished the Neft build. I hot glued the wires, boards and switches, and added a battery holder for 9v at the bottom of the gun.

It still fires like it should, not interfering with any wires. I've documented this whole process and will be releasing an Instructable tutorial in the near future!

Check back soon for my next project. Maybe I'll do the quadbot project now!

BTW I had some pictures but since rooting my phone and installing CyanogenMod I cant post them using the Blogger app... So I'll edit those up soon.

Quick Break

Sorry everyone, I meant to finish my build of the Nerf gun but its been a busy week so it will be a few more days.

Almost finished Nerf Mod

So after a bit of a break, I continued with my Nerf build.

Today I drilled the holes for the power LED and Switch, and soldered wires on to both of those.

I had to start thinking about how I was going to connect all the wires together. I don't have perf board so I cant solder them that way, and I really don't want to just leave components and wires directly soldered to eachother unless I really have to.

So I had to figure out another solution.

I dug through my box of circuit boards until I found 2 pretty small through-hole ones. I desoldered all the components, leaving me with empty boards. I'm going to solder my components on to these, using the premade traces to my advantage. It worked out well!

Everything is all set to solder! But that will have to wait until tomorrow.

Link to Pacman 3000!

Edit: Media Hogs Studios has closed. Thank you all. I'm still deciding what the future holds for Pacman 3000. Stay tuned.

Again, still recovering so no progress on any of the builds, though I am going to plug a video game that I made in grade 9 for a business trade show project. It's called Pacman 3000, and you play as Pacman, running around a fully 3D world trying to eat coins while avoiding ghosts. Try it out!

Since grade 9 I've added a high score system, and plan to add more features and maps, as well.

Without further ado, here is the game!
http://mediahogsstudios.com/games/pacman3000/


With the exception of that page, the website on the whole is rather unfinished. I haven't had time to work on it much.


Also hello HackTheNorth people :)

Quick Update

So my Wisdom Teeth surgury went very well, I could even say it was a good experience! They had me on IV sedation which basically made me feel really sleepy and relaxed through the whole thing. What felt like 5 minutes was actually an hour and a half, and eight needles! I'm still swollen and my teeth hurt a bit but not as much as I originally expected.

In the mean time I've been working on small projects. I built and painted two plastic models, and today I built the hovercraft kit you see in the picture. I've had it for a long time and never got around to building it.

The assembly was obviously easy, but when I turned it on, the thing barely moved, with the back end scraping on the ground. The 4 fresh AAA batteries I had installed just didnt provide enough power.
Solution? Stick a 9V in there. Man did it fly, even over carpet which suprised me for a kit of this type.

As you could imagine though, the motor with the AAA's demanded a lot of current, and with the 9V, it sucks it like no tomorrow. Between the low resistance of the motor and the gradually increasing heat of all components, I'd say you could run this thing for 10, maybe 15 minutes before it would not have enough power. And 9V batterys are expensive!

That's all for today. Ill get back into my other projects soon.

Posting Break

Im getting my Wisdom Teeth removed today.... And I'm not sure sure what state I will be in and probably won't be able to do much work on any projects for a day or two. So stay tuned :D

Some Coding Stuff

Just a quick post to plug my GitHub profile... I have two repos, one models the Infinite Monkey Theorum (where a monkey typing away into infinity will eventually produce a well known literary text), and another one that I'm currently working on, a resistor color calculator. I'm mostly learning the basics of GUI in Java with this one... the programming of the back end isnt too hard, but I'm struggling with GUI, mostly because I've never done it before.

Anyway, here it is! GitHub Link

First Stages of Nerf build

The first thing I decided to do was to create an in-gun flashlight and laser sight. I would place 3 white LEDs and a modified dollar store laser into the front panel of the gun, where there was just enough clearance for them. It is a pretty tight spot to work in but I managed to make it work. I drilled the holes and they all fit perfectly. Next, I soldered the LEDs together in parallel and then to a wire. The dollar store laser was a little difficult to extract.... To get to the PCB I had to literally destroy the thin metal case for it. After unsoldering the battery spring and pushbutton and soldering positive and negative leads to it I installed both the laser and the LED.

I'm far from done... The is still the power supply system as well as the power switches for both the laser and the flashlight. Not to mention I still have to design and make the ammo capacity display. But in the pictures attached you will find my progress so far.

DVD Player Fun

I finished dismantling a broken DVD player. My main goal for most of my (one way) dismantals is usually to salvage as many parts as possible, but today I was after the laser diode in particular. With some effort I was able to safely extract it, as well as about 20+ components. I still need to test out the diode, as I've never hooked one up before.

I also played around with using the lenses in the laser reader module as Macro lenses for my cell phone camera, and it worked really well. As you can see in the photos, I took a picture of the laser diode. The true size of that is quite a bit smaller than my pinky fingernail. In another picture you see Her Majesty The Queen's eye on the front of a Canadian $20 bill.

I haven't forgotten about my RoboQuad project by the way. More on that to come soon!

Today in DIY

I did a lot today... Er... Yesterday... Its 12:40am...

I finished painting the controller, adding some red accents to the top. I'm terrible at acrylic hand painting as you can see but I think for a first effort ever it's not too bad. Maybe someday I will come back to it and make it better.

I also completed my first Nerf Gun mod. I recently got a Nerf disk blaster pistol, and today I increased the tension in the spring, causing the disks to fly faster, further and straighter. After I was done that I made a little device that would use the momentum of the disks to essentially launch ping pong balls from the gun (the gun would fire the disk as usual into a pingpong ball resting on the stand in front of it). I literally made it out of the first 3 things I saw laying around; duct tape, a plastic tape roll and a dental floss box.
Yes, a dental floss box.
Maybe Ill add some more mods to that gun... I'm thinking lights, and an LED display to show how many disks are left in the Magazine! I'll draw that up and post it sometime :)

Painting

Today after realising I had spray paint, I decided to customize my Datel XBox 360 controller. Another thrift store score ($5), this controller is a rapid fire trigger controller. This means you can set how many times the trigger button actives per second while you hold it down.

How useful! Though too bad I don't have an XBox and it doesn't connect to my PC, which was the real reason I bought it.

Anyway, because its relatively easy to disassemble and reassemble and I don't really mind if I mess it up, I decided to paint it. I have never painted anything before so it is definitely a learning experience.

The color scheme I wanted just so happened to be the paint colors I had around, which for now is a simple gunmetal body with reflective(ish) silver triggers and buttons. The body is solid gray for now but ill leave it as another exercise to hand paint on it later.

It was such a nice day out so I decided to do it outside and ended up eating lunch and writing this post while waiting for the paint to dry.

Ill post a picture when it has finished drying and is back together. But for now here are some things I did wrong!
- I Sanded too hard. There are some pretty bad scrapes I thought the paint would fill, but didn't.
- I didn't tape off the sides of the button holes. These will have to be sanded later so the buttons can fit and move freely.

But its all a learning experience like I said!

Initial Roboquad Research

So after researching a bit, I found the easiest way to interface an Arduino is to issue it commands through infrared, similar to how the remote works. With a carrier frequency of about 39.5kHz, you would send a 12 bit signal (with 1200 bits per second) through an IR LED to be received by the robot. Alternatively I would imagine you could bypass the infrared all together and hardwire a serial connection between the Arduino and the receiving ports of the CPU. (IE soldering a direct connection from the Serial Out to the negative pole of the LED, I assume. To make sure this works I'll try on some old DVD player or something first just in case I'm wrong and mess something up by accident)

The remote for the Roboquad has 19 buttons, however it has 4 shift levels, Unshifted, Green shift, Yellow Shift, and Red Shift. When a shift mode is active, it will change what each button does. Thus this allows for 72 different operations. (18 keys not including shift, times 4 functions per key)


The structure of the signal works like this:
- A 4 bit prefix denoting the model of the target robot is sent (If you have one remote to distinguish between different Wowwee robots). For the Roboquad its 0110 (0x600 in hex).
- An 8 bit instruction that contains information about what shift is active as well as what key was pressed. For example, the walk forward command is 0x601 (Prefix [0x600] + D-PAD-FORWARD UNSHIFT Instruction [0x001]).
- No end bit, so there needs to be about 500ms between commands.

If I wanted to send a walk forward command through my Arduino, here is what I think I'll have to do:
- Hook an IR LED to serial out
- In setup() use Serial.begin(1200); //to denote the Bits per Second as 1200
- Call Serial.write(0x601); or Serial.write(011000000001); or Serial.write(1537);. Not sure which will work, though all of them are equivalent.

This will be my first test. That is... after I salvage a new IR LED after blowing out my only one a while back :)


A really nice table of the commands (and my source for this information) is available here.

It is 1am and I have to be up again in 5 hours so I'm out for now.

Time Flies + First Modding Project

It certainly hasn't felt like a year and a half since my last post... I was shocked to see "November 2012" at the top of my other posts.

So I went to Goodwill today (thrift store) and managed to pick up a Wowwee Roboquad for $8... a steal when Amazon lists it as $180 new, and >$40 used. What a steal!
There were two there, and I was so glad to see the other one being picked up by this little girl who seemed like the happiest person in the world after finding it :)
Anyway, as I let it walk around my room as I worked on my computer and it ended up exploring the whole room; you could see its footprint trails in the carpet. It was really fun to watch.
I'm going to at some point open it up and attempt to interface my Arduino with it, while being as non-destructive to the original functionality and cosmetics as possible. That is to say I can't cut any wires without soldering them back later for example.

I figure I'll keep a record on this blog if I do end up doing anything!