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I am trying to gain a more holistic, general, and high-level understanding of programs and programming languages.
I would like to understand how they actually function. I understand at the lowest level is machine code which is 0 and 1s. Then you have assembly. Then you have another high level language where every instruction/function/method/call/routine whatever you want to call it maps to some instruction or group of instructions in assembly right? The higher level language cannot provide or do anything outside of what the lower level language assembly provides correct?
Similarly, since all code runs on an OS, that code can only do things that the OS provides. It is impossible for the code to do anything outside of what the OS actually provides correct?
The computer has an instruction set, machine code, which defines what can be done on the computer. Assembly code is essentially a more convenient representation of that, so assembly code can do anything the machine can do. A higher-level programming language has to run on the machine, so it can't do anything the machine can't do, though it may well be able to express it more conveniently (e.g. print "foo" rather than several dozen machine-code instructions). It is a matter of implementation choice whether the compiler for that programming language generates machine code directly, or assembly code, or any other form that might need further processing.
This brings us to the question of whether it's possible for a program (regardless of what it's written in) to do something the OS does not explicitly provide for. I find this an odd way to express it, since the point of writing a program is to give you some capability that you didn't have before, so in some sense you only write programs for things the OS does not explicitly provide you with. The problem lies in defining what the OS "provides". If it's a general-purpose OS, then its designers likely intend to "provide" the ability for you to write a wide range of programs. The OS may choose to provide some convenient feature (say, the ability to create files) but if it did not provide such a feature, you could perhaps do it yourself, given suitable motivation (and, for the file creation example, the ability to do disk I/O - possibly requiring you to write a disk driver too).
I am trying to gain a more holistic, general, and high-level
understanding of programs and programming languages.
I would like to understand how they actually function.
I recommend working through the understanding of modern hardware to get good performance and energy efficiency with the following as example:
With subword parallelism to improve matrix multiply by a factor of 4.
Doubling the performance by unrolling the loop to demonstrate the value of instruction level parallelism.
Doubling performance again by optimizing for caches using
blocking.
Finally, A speedup of 14 from 16 processors by using thread-level parallelism.
All four optimizations in total add just 24 lines of C code to a matrix multiply example you find in
Computer Organization and Design: The Hardware Software Interface (5th ed.)
or similar books.
To make a point, it really is worth it to dig deeper than just "learning python" even if it is a good start. So understanding low level really effects high level programming in many ways and thats what you are after according to your question.
There is actually not just Assembly - there are hardware description languages as VHDL thats handled in this topic f.e.:
https://electronics.stackexchange.com/questions/132611/whats-the-motivation-in-using-verilog-or-vhdl-over-c
Long put short: The teacher who taught me through out the last year has only recently left and has been replaced with a new one. This new teacher has given me an assignment that involves things (like this) that we were never previously taught. So this task has showed up on the assignment and I have no idea how to do it. I can't get hold of the teacher because he's poorly and not coming in for the next few days. And even when I do ask him to explain further, he gets into a right mood and makes me feel like I'm completely retarded.
Describe how the hardware platform impacts upon the choice for
the programming language
Looking at my activity here on SO, you can tell that I'm into programming, I'm into developing things, and I'm into learning, so I'm not just trying to get one of you guys to do my homework for me.
Could someone here please explain how I would answer a question like this.
Some considerations below, but not a full answer by any means.
If your hardware platform is a small embedded device of some kind, then your choice of programming language is going to be directed towards the lower level unmanaged languages - you probably won't be able to (or want to) load a managed language runtime like the Java JVM or .NET CLR. This is down to memory and storage requirements. Similarly, interpreted languages will be out of the question as you won't have space for the intepreter.
If you're on a larger machine, it's more a question of compatibility. A managed language must run on a platform where its runtime is supported. In the case of .NET, that's Windows, or other platforms if you substitute the Microsoft CLR with the Mono runtime. In the case of Java, that's a far wider range of platforms.
This is by no means a definitive answer, but my first thought would be embedded systems. A task I perform on an embedded system, or other low powered battery operated computer, would need to be handled completely different to that performed on a computer which has access to mains electricity.
One simple impact.. would be battery life.
If I use wasteful algorithms on an embedded system, the battery life will be affected.
Hope that helps stir the brain juices!
Clearly, the speed and amount of memory of the device will impact the choice. The more primitive and weak the platform is, the harder it is to run code developed with very high level languages. Code written with them may just not work at all (e.g. when there isn't enough memory) or be too slow or it will require serious optimizations (i.e. incur more work), perhaps affecting negatively the feature set or quality.
Also, some languages and software may rely heavily on or benefit from the availability of page translation in the CPU. If the CPU doesn't have it, certain checks will have to be done in software instead of being done automatically in hardware, and that will affect the performance or the language/software choice.
I want to practice programming code for future hardware. What are these? The two main things that come to mind is 64bits and multicore. I also note that cache is important along and GPU have their own tech but right now i am not interested in any graphics programming.
What else should i know about?
-edit- i know a lot of these are in the present but pretty soon all cpus will be multicore and threading will be more important. I consider endians (big vs little) but found that not to be important and already have a big endian CPU to test on.
My recommendation for future :)
nVidia CUDA
nVidia Tegra
Or you can focusing on ray tracing.
If you'd like to dive into a "mainstream" OS that has full 64 bit support, I suggest you start coding against the beta of Mac OS X "Snow Leopard" (codename for 10.6). One of the big enhancements is Grand Central, which is a "facility" for developers to code for multicore systems. Grand Central should distribute workload not only between core, but also to the GPU.
Also very important is the explosion of smart devices such as the iPhone, Android, etc. I strongly believe that some upcoming so-called "netbooks" will rely on OS such as Android and iPhone OS, and as such knowing how to code against their SDK, and knowing how to optimize code for mobile devices is very important (e.g. optimizing performance graphic or otherwise, battery usage).
I can't foretell the future, but one aspect to look into is something like the CELL processor used in the PS3, where instead of many identical general purpose cores, there is only one (although capable of symmetric multithreading) plus many cores that are more specific purpose.
In a simple analysis, the Cell processor can be split into four components: external input and output structures, the main processor called the Power Processing Element (PPE) (a two-way simultaneous multithreaded Power ISA v.2.03 compliant core), eight fully-functional co-processors called the Synergistic Processing Elements, or SPEs, and a specialized high-bandwidth circular data bus connecting the PPE, input/output elements and the SPEs, called the Element Interconnect Bus or EIB.
CUDA and OpenCL are similar in that you separate your general purpose code and high performance computations into separate parts that may run on different hardware and language/api.
64 bits and multicore are the present not the future.
About the future:
Quantum computing or something like that?
How about learning OpenCL? It's a massively parallel processing language based on C. It's similar to nVidia's CUDA but is vendor agnostic. There are no major implementations yet, but expect to see some pretty soon.
As for 64 bit, don't really worry about. Programming will not really be any different unless you're doing really low level stuff (kernels). Higher level frameworks such as Java and .NET allow you to run code on 32 bit and 64 bit machines. Even C/C++ allows you to do this (but not quite so transparently).
I agree with Oli's answere (+1) and would add that in addition to 64-bit environments, you look at multi-core environments. The industry is getting pretty close to the end of the cycle of improvements in raw speed. But we're seeing more and more multi-core CPUs. So parallel or concurrent programming -- which is rilly rilly hard -- is quickly becoming very much in demand.
How can you prepare for this and practice it? I've been asking myself the same same question. So for, it seems to me like functional languages such as ML, Haskell, LISP, Arc, Scheme, etc. are a good place to begin, since truly functional languages are generally free of side effects and therefore very "parallelizable". Erlang is another interesting language.
Other interesting developments that I've found include
The Singularity Research OS
Transactional Memory and Software Isolated Processes
The many Software Engineering Podcast episodes on concurrency. (Here's the first one.)
This article from ACM Queue on "Real World Concurrency"
Of course this question is hard to answer because nobody knows what future hardware will look like (at least in long terms), but multi-threading/parallel programming are important and will be definitely even more important for some years.
I'd also suggest working with GPU computing like CUDA/Stream, but this could be a problem because it's very likely that this will change a lot the next years.
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I didn't find any useful information about programming languages for real time systems. All I found was Real Time Systems and Programming Languages: Ada 95, Real-Time Java and Real-Time C/POSIX (some pdf here), which seems to talk about extensions of Java and C for real times systems (I don't have the book to read). Also, the book was published in 2001, and the information may be obsolete now.
So, I'm dubious whether these languages are used in real world applications, or if real time systems in the real world are made in other languages, like DSLs.
If the second option is true for you, what are the outstanding characteristics of the language you use?
I am an avionics software engineer.
I was able to participate in several development projects.
The languages I used in those projects are: C, C++, and Real-time Java.
C is great.
C++ is not so bad but C/C++ require strict coding standards for the safety considerations such as DO-178B.
I think Real-time Java is the way to go but I don't see many avionics applications, yet.
Korean jet trainer T-50 will have a mission computer running RT Java application serving HUD and MFD displays, and all of the mission critical functions.
The Real-Time Specification for Java now has several commercial-grade implementations:
Sun's JavaRTS
IBM's WebSphere Real-Time
Aonix PERC
aicas JamaicaVM
Apogee Aphelion
These products span the continuum from compilation to native code (Aonix) to J2ME (aicas, apogee), to full J2SE (Sun, IBM). Most, if not all, have seen deployments in small numbers of safety- or mission-critical systems, but momentum is building. Examples include Eglin AFB's space surveillance radar modernization and the US Navy's use of RTSJ in the DDG-1000/Zumwalt destroyer. Sun also claims deployment in the financial transaction processing domain.
If you are interested in RTSJ, I suggest Peter Dibble's Real-Time Platform Programming, or Professor Wellings' Concurrent and Real-Time Programming in Java.
On a related note, there is also work underway to provide a Safety-Critical profile for the Java programming language, built as a subset of RTSJ. Also, an expert group has formed to explore a Distributed RTSJ DRTSJ, but the work is stalled.
The book covers use of Ada 95, the Java Real-Time System and realtime POSIX extensions (programmed in C). None of these is directly a domain specific language.
Ada 95 is a programming language commonly used in the late 90s and (AFAIK) still widely used for realtime programming in defence and aerospace industries. There is at least one DSL built on top of Ada - SparkAda - which is a system of annotations which describe system characteristics to a program verification tool.
This interview of April 6, 2006 indicates some of the classes and virtual machine changes which make up the Java Real-Time System. It doesn't mention any domain specific language extensions. I haven't come across use of Java in real-time systems, but I haven't been looking at the sorts of systems where I'd expect to find it (I work in aerospace simulation, where it's C++, Fortran and occasionally Ada for real-time in-the-loop systems).
Realtime POSIX is a set of extensions to the POSIX operating system facilities. As OS extensions, they don't require anything specific in the language. That said, I can think of one C based DSL for describing embedded systems - SystemC - but I've no idea if it's also used to generate the embedded systems.
Not mentioned in the book is Matlab, which in the last few years has gone from a simulation tool to a model driven development system for realtime systems.
Matlab/Simulink is, in effect, a DSL for linear programming, state machines and algorithms. Matlab can generate C or HDL for realtime and embedded systems. It's very rare to see an avionics, EW or other defence industry real-time job advertised which doesn't require some Matlab experience. (I don't work for Matlab, but it's hard to over emphasis how ubiquitous it really is in the industry)
Real time applications can be made in almost any language. The environment (operating system, runtime and runtime libraries) must however be compliant to real time constraints. In most cases real-time means that there's always a deterministic time in which something happens. Deterministic time being ussually a very low time value in the microseconds/milliseconds range.
Real time systems depend solely on this criteria, as the specificiations usually say something like 'Every x (period of time) (do something | check something)'. Usually this happens if the system interfaces with external sensors and controls life-saving or life-threatening systems.
I was working on an in-car navigation and infotainment system developed mostly in C/C++ with an operating system configured specifically to meet the real-time constraints to provide real-time navigation and media playback.
But this is not all to real-time systems: Usually the selection of algorithms in the entire system is made to have deterministic runtimes according to the Big-O notation, mostly using linear or constant time. Everything else is considered non-deterministic and thus not useable for real-time systems.
All of the real-time systems I have worked with were predominantly written in C with some bits of assembler, or written mostly in assembler with little bits of C. (Depending on whether we're talking the 90s and beyond, or the 80s, respectively.) However, some of the real-time systems I've worked with have used -- not exactly DSLs -- special homegrown code generators.
Real-time oriented language?
What is real-time
First we have to define what real-time mean.
Of course depending on how your tool will work against the physical environment pure real-time couldn't be effectively done, mostly because there will be a lot of third party dependencies.
If you are building embed stuff by using microcontrollers like arduino, the language to use will be limited by the hardware, but with more complex stuff like Raspberry Pi, the language choice is very wide.
Granularity
This is depending on what you are measuring, if you're working with:
weather temperatures, one read each 10 minute could be enough
people height or weight, one or maybe four read by day
server status, between 1 second for fine debugging to approx 1 hour for quiet unimportant secondary server.
atomic collision count: something finer...
Event based reading
The right (better) way for collecting data is based on value change event... whenever the device do permit it.
Your tool have to not poll values from device, but the device have to send values to your tool, when they change.
This could be done by using an hardware interrupt trigger or by using port protocole like RS-232 staying listening on some serial port, for sample.
Monitoring environment
The last thing to be warned is how legitimate user will interact with.
If you're building embed standalone device, like robot, you may use graphic libraries to interact with touch screen.
If you're building web based monitor, you may have to keep in mind that the client could be an old 800x600 monochrome screen, using poor internet connection and small processor... But depending on final goal if you may interact with clients, you could ensure strong hardware and strong internet connections. Anyway you have to watch for connexion loosing and event for communication delay between server and client. There is mostly third party dependencies.
Which programming language?
From there, the language choice is wide and clearly depend on
your knowledge.
granularity requested (by using event-based too, of course)
the amount of time you have to build the tool (money;)
delay, co-workers...
kind of device
kind of monitoring
some other political reasons
You could build real-time monitoring engine by using bash and sql only, I've seen sophisticated engines that was built under postgresql only... I've personally built a web based, solar energy monitor by using perl, mysql and javascript.
I cannot believe no one has mentioned LabVIEW programming language which is widely used for Real-time safety-critical systems. It has extensive libraries and well-known design patterns for architecturing and implementing for RT systems.
Also National Instruments makes various hardware (cRIO, PXI and etc) which are designed for real-time applications.
We use LabVIEW for Fracking (Hydraulic Fracturing) which is used in safety-critical environments.
PLCs run ladder and fbd code which is really a real-time dsl in the sense that your options are so limited that it is difficult to program in a way that would result in unpredictable runtime performance
A really purposeful application of the C language to real-time programming - and all related issues (such as parallel programming) - is offered by my Kickstarter
http://www.kickstarter.com/projects/767046121/crawl-space-computing-with-connel
It is called "Wide Programming" and I've been doing it most of my life. The rewards include a software library and a book - designed to be useful.
the company I've been working for since 2003 has been developing and deploying a Scada/Mes platform. Original implementation started in 1993, used Modula2 on OS/2. Later (1998) it was ported to Ada95 and Windows. Currently (2019) we use Ada compiler by AdaCore. Our system was ported and has been deployed to 32/64 Windows, HPUX, OpenVMS (and lately even to Raspberry). We have multiple installation in central Europe (gas industry, refineries, factories, power plants).
We feel Ada's features give our system a high degree of reliability and prevents a lot of errors that would easily occour if we used languages like C.
See also my blog
https://www.ipesoft.com/en/blog/what-language-is-the-d2000-written
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How do emulators work? When I see NES/SNES or C64 emulators, it astounds me.
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)?
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.
Basic idea:
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.
Processor emulation:
There are three ways of handling processor emulation:
Interpretation
Dynamic recompilation
Static recompilation
With all of these paths, you have the same overall goal: execute a piece of code to modify processor state and interact with 'hardware'. Processor state is a conglomeration of the processor registers, interrupt handlers, etc for a given processor target. For the 6502, you'd have a number of 8-bit integers representing registers: A, X, Y, P, and S; you'd also have a 16-bit PC register.
With interpretation, you start at the IP (instruction pointer -- also called PC, program 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:
Code that isn't in the program to begin with (e.g. compressed, encrypted, generated/modified at runtime, etc) won't be recompiled, so it won't run
It's been proven that finding all the code in a given binary is equivalent to the Halting problem
These combine to make static recompilation completely infeasible in 99% of cases. For more information, Michael Steil has done some great research into static recompilation -- the best I've seen.
The other side to processor emulation is the way in which you interact with hardware. This really has two sides:
Processor timing
Interrupt handling
Processor timing:
Certain platforms -- especially older consoles like the NES, SNES, etc -- require your emulator to have strict timing to be completely compatible. With the NES, you have the PPU (pixel processing unit) which requires that the CPU put pixels into its memory at precise moments. If you use interpretation, you can easily count cycles and emulate proper timing; with dynamic/static recompilation, things are a /lot/ more complex.
Interrupt handling:
Interrupts are the primary mechanism that the CPU communicates with hardware. Generally, your hardware components will tell the CPU what interrupts it cares about. This is pretty straightforward -- when your code throws a given interrupt, you look at the interrupt handler table and call the proper callback.
Hardware emulation:
There are two sides to emulating a given hardware device:
Emulating the functionality of the device
Emulating the actual device interfaces
Take the case of a hard-drive. The functionality is emulated by creating the backing storage, read/write/format routines, etc. This part is generally very straightforward.
The actual interface of the device is a bit more complex. This is generally some combination of memory mapped registers (e.g. parts of memory that the device watches for changes to do signaling) and interrupts. For a hard-drive, you may have a memory mapped area where you place read commands, writes, etc, then read this data back.
I'd go into more detail, but there are a million ways you can go with it. If you have any specific questions here, feel free to ask and I'll add the info.
Resources:
I think I've given a pretty good intro here, but there are a ton of additional areas. I'm more than happy to help with any questions; I've been very vague in most of this simply due to the immense complexity.
Obligatory Wikipedia links:
Emulator
Dynamic recompilation
General emulation resources:
Zophar -- This is where I got my start with emulation, first downloading emulators and eventually plundering their immense archives of documentation. This is the absolute best resource you can possibly have.
NGEmu -- Not many direct resources, but their forums are unbeatable.
RomHacking.net -- The documents section contains resources regarding machine architecture for popular consoles
Emulator projects to reference:
IronBabel -- This is an emulation platform for .NET, written in Nemerle and recompiles code to C# on the fly. Disclaimer: This is my project, so pardon the shameless plug.
BSnes -- An awesome SNES emulator with the goal of cycle-perfect accuracy.
MAME -- The arcade emulator. Great reference.
6502asm.com -- This is a JavaScript 6502 emulator with a cool little forum.
dynarec'd 6502asm -- This is a little hack I did over a day or two. I took the existing emulator from 6502asm.com and changed it to dynamically recompile the code to JavaScript for massive speed increases.
Processor recompilation references:
The research into static recompilation done by Michael Steil (referenced above) culminated in this paper and you can find source and such here.
Addendum:
It's been well over a year since this answer was submitted and with all the attention it's been getting, I figured it's time to update some things.
Perhaps the most exciting thing in emulation right now is libcpu, started by the aforementioned Michael Steil. It's a library intended to support a large number of CPU cores, which use LLVM for recompilation (static and dynamic!). It's got huge potential, and I think it'll do great things for emulation.
emu-docs has also been brought to my attention, which houses a great repository of system documentation, which is very useful for emulation purposes. I haven't spent much time there, but it looks like they have a lot of great resources.
I'm glad this post has been helpful, and I'm hoping I can get off my arse and finish up my book on the subject by the end of the year/early next year.
A guy named Victor Moya del Barrio wrote his thesis on this topic. A lot of good information on 152 pages. You can download the PDF here.
If you don't want to register with scribd, you can google for the PDF title, "Study of the techniques for emulation programming". There are a couple of different sources for the PDF.
Emulation may seem daunting but is actually quite easier than simulating.
Any processor typically has a well-written specification that describes states, interactions, etc.
If you did not care about performance at all, then you could easily emulate most older processors using very elegant object oriented programs. For example, an X86 processor would need something to maintain the state of registers (easy), something to maintain the state of memory (easy), and something that would take each incoming command and apply it to the current state of the machine. If you really wanted accuracy, you would also emulate memory translations, caching, etc., but that is doable.
In fact, many microchip and CPU manufacturers test programs against an emulator of the chip and then against the chip itself, which helps them find out if there are issues in the specifications of the chip, or in the actual implementation of the chip in hardware. For example, it is possible to write a chip specification that would result in deadlocks, and when a deadline occurs in the hardware it's important to see if it could be reproduced in the specification since that indicates a greater problem than something in the chip implementation.
Of course, emulators for video games usually care about performance so they don't use naive implementations, and they also include code that interfaces with the host system's OS, for example to use drawing and sound.
Considering the very slow performance of old video games (NES/SNES, etc.), emulation is quite easy on modern systems. In fact, it's even more amazing that you could just download a set of every SNES game ever or any Atari 2600 game ever, considering that when these systems were popular having free access to every cartridge would have been a dream come true.
I know that this question is a bit old, but I would like to add something to the discussion. Most of the answers here center around emulators interpreting the machine instructions of the systems they emulate.
However, there is a very well-known exception to this called "UltraHLE" (WIKIpedia article). UltraHLE, one of the most famous emulators ever created, emulated commercial Nintendo 64 games (with decent performance on home computers) at a time when it was widely considered impossible to do so. As a matter of fact, Nintendo was still producing new titles for the Nintendo 64 when UltraHLE was created!
For the first time, I saw articles about emulators in print magazines where before, I had only seen them discussed on the web.
The concept of UltraHLE was to make possible the impossible by emulating C library calls instead of machine level calls.
Something worth taking a look at is Imran Nazar's attempt at writing a Gameboy emulator in JavaScript.
Having created my own emulator of the BBC Microcomputer of the 80s (type VBeeb into Google), there are a number of things to know.
You're not emulating the real thing as such, that would be a replica. Instead, you're emulating State. A good example is a calculator, the real thing has buttons, screen, case etc. But to emulate a calculator you only need to emulate whether buttons are up or down, which segments of LCD are on, etc. Basically, a set of numbers representing all the possible combinations of things that can change in a calculator.
You only need the interface of the emulator to appear and behave like the real thing. The more convincing this is the closer the emulation is. What goes on behind the scenes can be anything you like. But, for ease of writing an emulator, there is a mental mapping that happens between the real system, i.e. chips, displays, keyboards, circuit boards, and the abstract computer code.
To emulate a computer system, it's easiest to break it up into smaller chunks and emulate those chunks individually. Then string the whole lot together for the finished product. Much like a set of black boxes with inputs and outputs, which lends itself beautifully to object oriented programming. You can further subdivide these chunks to make life easier.
Practically speaking, you're generally looking to write for speed and fidelity of emulation. This is because software on the target system will (may) run more slowly than the original hardware on the source system. That may constrain the choice of programming language, compilers, target system etc.
Further to that you have to circumscribe what you're prepared to emulate, for example its not necessary to emulate the voltage state of transistors in a microprocessor, but its probably necessary to emulate the state of the register set of the microprocessor.
Generally speaking the smaller the level of detail of emulation, the more fidelity you'll get to the original system.
Finally, information for older systems may be incomplete or non-existent. So getting hold of original equipment is essential, or at least prising apart another good emulator that someone else has written!
Yes, you have to interpret the whole binary machine code mess "by hand". Not only that, most of the time you also have to simulate some exotic hardware that doesn't have an equivalent on the target machine.
The simple approach is to interpret the instructions one-by-one. That works well, but it's slow. A faster approach is recompilation - translating the source machine code to target machine code. This is more complicated, as most instructions will not map one-to-one. Instead you will have to make elaborate work-arounds that involve additional code. But in the end it's much faster. Most modern emulators do this.
When you develop an emulator you are interpreting the processor assembly that the system is working on (Z80, 8080, PS CPU, etc.).
You also need to emulate all peripherals that the system has (video output, controller).
You should start writing emulators for the simpe systems like the good old Game Boy (that use a Z80 processor, am I not not mistaking) OR for C64.
Emulator are very hard to create since there are many hacks (as in unusual
effects), timing issues, etc that you need to simulate.
For an example of this, see http://queue.acm.org/detail.cfm?id=1755886.
That will also show you why you ‘need’ a multi-GHz CPU for emulating a 1MHz one.
Also check out Darek Mihocka's Emulators.com for great advice on instruction-level optimization for JITs, and many other goodies on building efficient emulators.
I've never done anything so fancy as to emulate a game console but I did take a course once where the assignment was to write an emulator for the machine described in Andrew Tanenbaums Structured Computer Organization. That was fun an gave me a lot of aha moments. You might want to pick that book up before diving in to writing a real emulator.
Advice on emulating a real system or your own thing?
I can say that emulators work by emulating the ENTIRE hardware. Maybe not down to the circuit (as moving bits around like the HW would do. Moving the byte is the end result so copying the byte is fine). Emulator are very hard to create since there are many hacks (as in unusual effects), timing issues, etc that you need to simulate. If one (input) piece is wrong the entire system can do down or at best have a bug/glitch.
The Shared Source Device Emulator contains buildable source code to a PocketPC/Smartphone emulator (Requires Visual Studio, runs on Windows). I worked on V1 and V2 of the binary release.
It tackles many emulation issues:
- efficient address translation from guest virtual to guest physical to host virtual
- JIT compilation of guest code
- simulation of peripheral devices such as network adapters, touchscreen and audio
- UI integration, for host keyboard and mouse
- save/restore of state, for simulation of resume from low-power mode
To add the answer provided by #Cody Brocious
In the context of virtualization where you are emulating a new system(CPU , I/O etc ) to a virtual machine we can see the following categories of emulators.
Interpretation: bochs is an example of interpreter , it is a x86 PC emulator,it takes each instruction from guest system translates it in another set of instruction( of the host ISA) to produce the intended effect.Yes it is very slow , it doesn't cache anything so every instruction goes through the same cycle.
Dynamic emalator: Qemu is a dynamic emulator. It does on the fly translation of guest instruction also caches results.The best part is that executes as many instructions as possible directly on the host system so that emulation is faster. Also as mentioned by Cody, it divides the code into blocks ( 1 single flow of execution).
Static emulator: As far I know there are no static emulator that can be helpful in virtualization.
How I would start emulation.
1.Get books based around low level programming, you'll need it for the "pretend" operating system of the Nintendo...game boy...
2.Get books on emulation specifically, and maybe os development. (you won't be making an os, but the closest to it.
3.look at some open source emulators, especially ones of the system you want to make an emulator for.
4.copy snippets of the more complex code into your IDE/compliler. This will save you writing out long code. This is what I do for os development, use a district of linux
I wrote an article about emulating the Chip-8 system in JavaScript.
It's a great place to start as the system isn't very complicated, but you still learn how opcodes, the stack, registers, etc work.
I will be writing a longer guide soon for the NES.