How To Write a Computer Emulator

by Marat Fayzullin

I wrote this document after receiving large amount of email from people who would like to write an emulator of one or another computer, but do not know where to start. Any opinions and advices contained in the following text are mine alone and should not be taken for an absolute truth. The document mainly covers so-called "interpreting" emulators, as opposed to "compiling" ones, because I do not have much experience with recompilation techniques. It does have a pointer or two to the places where you can find information on these techniques.

If you feel that this document is missing something or want to make a correction, feel free to email me your comments. I do not answer to flames, idiocy, and requests for ROM images though. I'm badly missing some important FTP/WWW addresses in the end of this document, so if you know any worth putting there, tell me about it. Same goes for any frequently asked questions you may have, that are not in this document.


So, you decided to write a software emulator? Very well, then this document may be of some help to you. It covers some common technical questions people ask about writing emulators.

What can be emulated?

Basically, anything which has a microprocessor inside. Of course, only devices running a more or less flexible program are interesting to emulate. Those include:

It is necessary to note that you can emulate any computer system, even if it is very complex (such as Commodore Amiga computer, for example). The perfomance of such an emulation may be very low though.

What is "emulation" and how does it differ from "simulation"?

Emulation is an attempt to imitate the internal design of a device. Simulation is an attempt to imitate functions of a device. For example, a program imitating the Pacman arcade hardware and running real Pacman ROM on it is an emulator. A Pacman game written for your computer but using graphics similar to a real arcade is a simulator.

Is it legal to emulate the proprietary hardware?

Although the matter lies in the "gray" area, it appears to be legal to emulate proprietary hardware, as long as the information on it hasn't been obtained by illegal means. You should also be aware of the fact that it is illegal to distribute the system ROMs (BIOS, etc.) with the emulator if the are copyrighted.

What is "interpreting emulator" and how does it differ from "recompiling emulator"?

There are three basic schemes which can be used for an emulator. They can be combined for the best result.

I want to write an emulator. Where should I start?

In order to write an emulator, you must have a good general knowledge of computer programming and digital electronics. Experience in assembly programming comes very handy too.

  1. Select a programming language to use.
  2. Find all available information about the emulated hardware.
  3. Write CPU emulation or get existing code for the CPU emulation.
  4. Write some draft code to emulate the rest of the hardware, at least partially.
  5. At this point, it is useful to write a little built-in debugger which allows to stop emulation and see what the program is doing. You may also need a disassembler of the emulated system assembly language. Write your own if none exist.
  6. Try running programs on your emulator.
  7. Use disassembler and debugger to see how programs use the hardware and adjust your code appropriately.

Which programming language should I use?

The most obvious alternatives are C and Assembly. Here are pros and cons of each of them:

Good knowledge of the chosen language is an absolute necessity for writing a working emulator, as it is quite complex project, and your code should be optimized to run as fast as possible. Computer emulation is definitely not one of the projects on which you learn a programming language.

Where do I get information on the emulated hardware?

Following is a list of places where you may want to look.



Console and Game Programming site in Oulu, Finland
Arcade Videogame Hardware archive at
Computer History and Emulation archive at KOMKON


comp.emulators.misc FAQ
My Homepage
Arcade Emulation Programming Repository

How do I emulate a CPU?

First of all, if you only need to emulate a standard Z80 or 6502 CPU, you can use one of the CPU emulators I wrote. Certain conditions apply to their usage though.

For those who want to write their own CPU emulation core or interested to know how it works, I provide a skeleton of a typical CPU emulator in C below. In the real emulator, you may want to skip some parts of it and add some others on your own.



    case OpCode1:
    case OpCode2:

  if(Counter<=0) { /* check for interrupts and do other hardware emulation here */ ... counter+="InterruptPeriod;" if(exitrequired) break; } } 
First, we assign initial values to the CPU cycle counter (Counter), and the program counter (PC):

The Counter contains the number of CPU cycles left to the
next suspected interrupt. Note that interrupt should not necessarily
occur when this counter expires: you can use it for many other purposes,
such as synchronizing timers, or updating scanlines on the screen. More on
this later. The PC contains the memory address from which our
emulated CPU will read its next opcode. 

After initial values are assigned, we start the main loop:

Note that this loop can also be implemented as

where CPUIsRunning is a boolean variable. This has certain
advantages, as you can terminate the loop at any moment by setting
CPUIsRunning=0. Unfortunately, checking this variable on
every pass takes quite a lot of CPU time, and should be avoided if
possible. Also, do not implement this loop as

because in this case, some compilers will generate code checking whether 
1 is true or not. You certainly don't want the compiler to 
do this unnecessary work on every pass of a loop.

Now, when we are in the loop, the first thing is to read the next opcode, and modify the program counter:

While this is the simplest and fastest way to read from the emulated
memory, it is not always possible for following reasons:
In these cases, we can read the emulated memory via
ReadMemory() function: 
There should also be a WriteMemory() function to write into
emulated memory. Besides handling memory-mapped I/O and pages,
WriteMemory() should also do the following:

The ReadMemory()/WriteMemory() functions usually put a lot of overhead on the emulation, and must be made as efficient as possible, because they get called very frequently. Here is an example of these functions:

static inline byte ReadMemory(register word Address)

static inline void WriteMemory(register word Address,register byte Value)
Notice the inline keyword. It will tell compiler to
embed the function into the code, instead of making calls to it. If your
compiler does not support inline or _inline, try
making function static: some compilers (WatcomC, for example)
will optimize short static functions by inlining them. 

Also, keep in mind that in most cases the ReadMemory() is called several times more frequently than WriteMemory(). Therefore, it is worth to implement most of the code in WriteMemory(), keeping ReadMemory() as short and simple as possible.

After the opcode is fetched, we decrease the CPU cycle counter by a number of cycles required for this opcode:

The Cycles[] table should contain the number of CPU cycles
for each opcode. Beware that some opcodes (such as conditional
jumps or subroutine calls) may take different number of cycles depending
on their arguments. This can be adjusted later in the code though. 

Now comes the time to interpret the opcode and execute it:

It is a common misconception that the switch() construct is
inefficient, as it compiles into a chain of if() ... else if()
... statements. While this is true for constructs with a small
number of cases, the large constructs (100-200 and more cases) always
appear to compile into a jump table, which makes them quite efficient. 

There are two alternative ways to interpret the opcodes. The first is to make a table of functions and call an appropriate one. This method appears to be less efficient than a switch(), as you get the overhead from function calls. The second method would be to make a table of labels, and use the goto statement. While this method is slightly faster than a switch(), it will only work on compilers supporting "precomputed labels". Other compilers will not allow you to create an array of label addresses.

After we successfully interpreted and executed an opcode, the comes a time to check whether we need any interrupts. At this moment, you can also perform any tasks which need to be synchronized with the system clock:

if(Counter<=0) { /* check for interrupts and do other hardware emulation here */ ... counter+="InterruptPeriod;" if(exitrequired) break; } 
Following is a short list of things which you may want to do in this if() statement:

Carefully calculate the number of CPU cycles needed for each task, then use the smallest number for InterruptPeriod, and tie all other tasks to it (they should not necessarily execute on every expiration of the Counter).

Note that we do not simply assign Counter=InterruptPeriod, but do a Counter+=InterruptPeriod: this makes cycle counting more precise, as there may be some negative number of cycles in the Counter.

Also, look at the

if(ExitRequired) break;
line. As it is too costly to check for an exit on every pass of the loop,
we do it only when the Counter expires: this will still exit
the emulation when you set ExitRequired=1, but it won't take
as much CPU time. 

This is about all I have to say about CPU emulation in C. You should be able to figure the rest on your own.

How do I optimize my C code?

Maintained by Marat Fayzullin