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Writing a gameboy emulator

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A small project that I have been working on to familarize myself with TypeScript and my next step in writing emulators. This Readme will currently serve as a place for me to track my progress.

Fascination reading material. Did some minor error fixes with typing. Updated the module system to use ES modules. Since modules don't work via loading from the file-system, needed to add small static node server to serve up files. Got button handlers fixed up as well.

Started adding instructions to Z80 class. Wound up breaking them up into seperate files. I have created MemoryBank instances for most of the ram, but have realized that my scheme doesn't work well with the Imran Nazar's ramoffset.

Current thoughts are to read eram size from cartridge header and have the bank instance for eram take in the number of 8k banks to create. Then when reading and writing, we can pass in the rambank number for read and writes.

After looking over the MMU even more, I'm starting to think that I should be using the same scheme i use for handling ram banks with the roms banks as well. A: Better to use the same patter for similar operations and B: will probably help me debug.

Now I need to come up with a good implementation. You can now pass in the size of the memory you are trying to emulate, and it will create the appropriate number of banks based off the bank size. It will now allow you set the active bank as well, so writes can occur in the right bank. Skip to content. Dismiss Join GitHub today GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together.GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together.

If nothing happens, download GitHub Desktop and try again. If nothing happens, download Xcode and try again. If nothing happens, download the GitHub extension for Visual Studio and try again. A small project that I have been working on to familarize myself with TypeScript and my next step in writing emulators. This Readme will currently serve as a place for me to track my progress. Fascination reading material. Did some minor error fixes with typing.

Updated the module system to use ES modules. Since modules don't work via loading from the file-system, needed to add small static node server to serve up files. Got button handlers fixed up as well. Started adding instructions to Z80 class. Wound up breaking them up into seperate files. I have created MemoryBank instances for most of the ram, but have realized that my scheme doesn't work well with the Imran Nazar's ramoffset.

Current thoughts are to read eram size from cartridge header and have the bank instance for eram take in the number of 8k banks to create.

NEW GAMES for OLD Consoles - SNES / NES / Genesis / GBA

Then when reading and writing, we can pass in the rambank number for read and writes. After looking over the MMU even more, I'm starting to think that I should be using the same scheme i use for handling ram banks with the roms banks as well.

Why did I spend 1.5 months creating a Gameboy emulator?

A: Better to use the same patter for similar operations and B: will probably help me debug. Now I need to come up with a good implementation.

You can now pass in the size of the memory you are trying to emulate, and it will create the appropriate number of banks based off the bank size. It will now allow you set the active bank as well, so writes can occur in the right bank. Skip to content. Dismiss Join GitHub today GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together. Sign up. TypeScript Branch: master.

Find file. Sign in Sign up. Go back. Launching Xcode If nothing happens, download Xcode and try again. Latest commit. Robert Gibb fixing minor error is jsGB file due to moving of instructions out of … … …Z80 class. Latest commit df Apr 15, Gameboy Emulator A browser based GB emulator for the modern world A small project that I have been working on to familarize myself with TypeScript and my next step in writing emulators.

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Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. Do you have to emulate the processor of those machines by interpreting its particular assembly instructions? What else goes into it? How are they typically designed? Can you give any advice for someone interested in writing an emulator particularly a game system?

Writing a Game Boy emulator, Cinoop

Emulation is a multi-faceted area. Here are the basic ideas and functional components. I'm going to break it into pieces and then fill in the details via edits.

Many of the things I'm going to describe will require knowledge of the inner workings of processors -- assembly knowledge is necessary. If I'm a bit too vague on certain things, please ask questions so I can continue to improve this answer. Emulation works by handling the behavior of the processor and the individual components.

You build each individual piece of the system and then connect the pieces much like wires do in hardware. With all of these paths, you have the same overall goal: execute a piece of code to modify processor state and interact with 'hardware'.

writing a gameboy emulator

Processor state is a conglomeration of the processor registers, interrupt handlers, etc for a given processor target. For theyou'd have a number of 8-bit integers representing registers: AXYPand S ; you'd also have a bit PC register. With interpretation, you start at the IP instruction pointer -- also called PCprogram counter and read the instruction from memory. Your code parses this instruction and uses this information to alter processor state as specified by your processor. The core problem with interpretation is that it's very slow; each time you handle a given instruction, you have to decode it and perform the requisite operation.

With dynamic recompilation, you iterate over the code much like interpretation, but instead of just executing opcodes, you build up a list of operations. Once you reach a branch instruction, you compile this list of operations to machine code for your host platform, then you cache this compiled code and execute it.

Then when you hit a given instruction group again, you only have to execute the code from the cache. BTW, most people don't actually make a list of instructions but compile them to machine code on the fly -- this makes it more difficult to optimize, but that's out of the scope of this answer, unless enough people are interested. With static recompilation, you do the same as in dynamic recompilation, but you follow branches. You end up building a chunk of code that represents all of the code in the program, which can then be executed with no further interference.

This would be a great mechanism if it weren't for the following problems:.This is the article for the 8th day of qiita advent calendar. The register set is fairly simple. It has only 7 general purpose registers, a flag register fa program counter pc and a stack pointer sp.

There're about instructions supported by the CPU, which can be roughly divided into these 5 types:. This is the table of all the instructions. As the CPU has about instructions, writing all of the emulation code by hand sounds troublesome. So I generated the code.

What I did is as follows:. By scraping this websiteI retrieved the information needed to generate the emulation code. I used the crate, scraper for scraping, and visited each cell in the instruction table.

I used pest parser library to parse the string.

Programming a Gameboy Color emulator

With this library, once we define parsing expression grammars PEGwe can easily extract strings we need. This is the PEG I wrote. I converted the retrieved information to the yaml file. Later I found that some of the information in the original website was wrong. So I patched the generated yaml. I may report this to the owner of the website. I used terathe famous Rust template engine, which is similar to Jinja2. I basically wrote a template macro for each operator some operators need more than one template macros.

For example, for ld operator, I wrote this macro.Have you ever wanted to go back and replay that nostalgic video game from your past? As time goes on, it only gets more difficult and expensive to track down physical copies of these old systems and game cartridges from decades ago. If you kept looking for a way to play them despite this, you have likely discovered and used emulators.

Screenshots from my emulators, Gameboy Crust and original GB emu. An emulator is a piece of software that enables programs for one system the guest system to run on an entirely different system the host.

Have you ever wondered how they work? Many people who have used emulators my younger self included tend to write them off as some kind of black magic witchcraft. However, taking the time to understand how an emulator works under the hood is an incredibly rewarding and educational experience in many ways. This page will describe what I learned during the process of developing an emulator for the Nintendo Gameboy Color. Pseudocode will be presented in Rust syntax. There are many systems that can be emulated, but the choice was easy for me since the Gameboy was my first videogame console.

The Gameboy, along with all old video game consoles, are nothing more than simple versions of computers. We know that a computer is a device that can carry out logical tasks.

writing a gameboy emulator

An emulator works by virtualy representing the physical hardware of the guest machine. This means that in order to create an emulator for the Gameboy, we have to virtually simulate how the real hardware of the gameboy would function. Memory is one of the easiest things to implement in code. Regardless of what is actually being stored in each memory location, we represent it in the same way.

We can equate a block of memory in the hardware to simply allocating N bytes of memory in our program:. Then all we need are methods to read and write to our virtual memory. The Gameboy has a bit address space, so we can use an unsigned bit integer to index our memory:. And that is all that is needed to emulate any kind of physical memory in the Gameboy.

The only exception to this is the game cartridge which is read-only memory ROM. Since this memory is read-only, we do not implement a write method. The ROM is still a contiguous buffer of data, but we read it in from a file instead of initializing an empty vector:.This is part 1 of an article series on emulation development in JavaScript; ten parts are currently available, and others are expected to follow.

It's often stated that JavaScript is a special-purpose language, designed for use by web sites to enable dynamic interaction. The simplest way of checking whether this is viable is, of course, to write such an emulator in JavaScript. This article sets out to implement the basis for a GameBoy emulation, by laying the groundwork for emulating each part of the physical machine.

The starting point is the CPU. The traditional model of a computer is a processing unit, which gets told what to do by a program of instructions; the program might be accessed with its own special memory, or it might be sitting in the same area as normal memory, depending on the computer. Each instruction takes a short amount of time to run, and they're all run one by one. From the CPU's perspective, a loop starts up as soon as the computer is turned on, to fetch an instruction from memory, work out what it says, and execute it.

After an instruction is fetched from memory, the PC is advanced by however many bytes make up the instruction. Each of them is one byte, so each one can hold a value from 0 to Most of the instructions in the Z80 are used to handle values in these registers: loading a value from memory into a register, adding or subtracting values, and so forth.

If there are possible values in the first byte of an instruction, that makes for possible instructions in the basic table. That table is detailed in the Gameboy Z80 opcode map released on this site.

Each of these can be simulated by a JavaScript function, that operates on an internal model of the registers, and produces effects on an internal model of the memory interface.

The basic model of the Z80 emulation would therefore require the following components:. The flags register F is important to the functioning of the processor: it automatically calculates certain bits, or flags, based on the result of the last operation. There are four flags in the Gameboy Z Since the basic calculation registers are 8-bits, the carry flag allows for the software to work out what happened to a value if the result of a calculation overflowed the register.

With these flag handling issues in mind, a few examples of instruction simulations are shown below. These examples are simplified, and don't calculate the half-carry flag. A processor that can manipulate registers within itself is all well and good, but it must be able to put results into memory to be useful. In the same way, the above CPU emulation requires an interface to emulated memory; this can be provided by a memory management unit MMU.

Since the Gameboy itself doesn't contain a complicated MMU, the emulated unit can be quite simple. At this point, the CPU only needs to know that an interface is present; the details of how the Gameboy maps banks of memory and hardware onto the address bus are inconsequential to the processor's operation. Four operations are required by the CPU:. With these in place, the rest of the CPU instructions can be simulated.

Writing gameboy emulator in Rust

Another few examples are shown below:. With the instructions in place, the remaining pieces of the puzzle for the CPU are to reset the CPU when it starts up, and to feed instructions to the emulation routines. Having a reset routine allows for the CPU to be stopped and "rewound" to the start of execution; an example is shown below. In order for the emulation to run, it has to emulate the fetch-decode-execute sequence detailed earlier.

This loop takes each instruction, decodes where it must be sent for execution, and dispatches it to the function in question. Implementing a Z80 emulation core is useless without an emulator to run it. In the next part of this series, the work of emulating the Gameboy begins: I'll be looking at the Gameboy's memory map, and how a game image can be loaded into the emulator over the Web. The model The traditional model of a computer is a processing unit, which gets told what to do by a program of instructions; the program might be accessed with its own special memory, or it might be sitting in the same area as normal memory, depending on the computer.

Figure 1: The fetch-decode-execute loop. NOP, ZFor me, the most favorite type of a computer program is an emulator. Being able to run code from a completely different hardware architecture always seemed like a magic. The old computers are great on their own, so this kind of connection between historic machines and the modern computing environment feels almost like a time travel.

As a developer I often think about the internal design of an emulator. I imagined this big switch construct that chooses the right operation for the current CPU opcode and the array modelling the memory.

It was also very addicting - every time I had a few minutes during the day and basically every evening I felt an irresistable urge to move the emulation a bit forward - fix this strange GPU bug, pass one more compatibility test or implement one more missing feature. I learnt a lot during the process:. For all of these points I had a general idea before, but actually implementing them required a deeper understanding.

However, when the original Gameboy starts, it executes a simple bytes program, a kind of firmware displaying the Nintendo logo and self-testing the system. That was exactly the thing I was looking for - after just 3 days I had an application running the Gameboy code!

At this point I believed that the CPU does what it should do. Now I had to make sure that the work is being done in a timely manner.

The common approach adopted by the emulators is to hard-code these cycle values. Instead, I decided to model the CPU and GPU operations in a way that is close to the original hardware, so the timing will be preserved implicitly. For example, rather the defining the LD A, a16 opcode which loads a memory byte into the internal register as follows:.

I decided to split into some kind of micro-operations, as the real CPU does. In general, each micro-operation that accesses the memory takes 4 cycles:. This also allowed to split the execution of a single opcode into a few phases, so the GPU and other parts of the Gameboy system takes chance to run their own logic between the load " a16 " and store "A"etc. The micro-operations themselves are atomic. Also, having this DSL is place made defining all the opcodes much easier, as we can group them together:.

The next step was implementing the basic graphics support. Gameboy displays two kinds of graphics: background which is usually more static and sprites moving objects, like Mario or falling block in Tetris.

writing a gameboy emulator

For the GPU, I also wanted to have it emulated closely to the original. The aforementioned Ultimate GameBoy talk contains a pretty good explanation on how the Gameboy copies video memory into screen, using the fetcher. I implemented the fetcher, pixel FIFO, the background support and bang!

My next goal was running a simple game - like Tetris or Dr Mario. I still missed a few features to have it working, but they are rather simple, assuming that we already have the emulator stub working:. Joypad and timer are really simple subsystems. They can be accessed using just a few bytes in the memory and their protocol is unambiguous.

The GPU implementation was already started and I had the placeholders to put the appropriate logic. DMA is a mechanism that allows to copy a memory block into the sprite video memory without involving the CPU. Having all these things implemented, probably in a buggy and incomplete way, I was able to run Tetris.


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