A genuine emulation of the hardware? Not too tricky:

- get a list of all the logic bearing components in the GBA
- consider their inputs and output, and how they connect to one another
- find out how the outputs of each chip depend upon the inputs
- write code to reproduce all of the above behaviour

If the thing you want to emulate is quite fast compared to the thing you want to emulate on, you'll need to start cheating. The common technique is to identify the 'prime' component of a system and focus your emulation around that. The prime component of a GBA is the ARM CPU.

You'd then write code which fetched, decoded and executed ARM instructions. If you really had very little regard for accuracy whatsoever you might then do something like perform ARM instructions for the number of cycles it takes for one iteration of the display to be drawn, then take a break to update the display, then return to doing the CPU stuff.

A better implementation might do a similar thing but run the CPU for as long as it takes one scanline to appear, then draw the scanline. And so on.

If you are lucky you'll get a system whereby the components external to the prime component have only fixed effects upon it (i.e. interrupts always at the same time, timing always affected in the same way...), in which case cheats like the above can be made to be very close to the way the real hardware is working.

Be aware that often you will not derive a fully accurate model from just registers, opcodes and opcode cycle length information - on almost all computers other factors affect timing and therefore other implicit elements of interaction.

For example, on a GBA (and most things), the timing for memory which describes the display is different than that elsewhere. It may also be dependant upon exactly what the display chip is doing at the time (although it may not be, I know nothing of GBA specifics). Furthermore, well written software might exploit these differences to achieve effects that might not otherwise be possible - especially on the messed up 80s style 'sprite lists' that Nintendo still cling to.

An example from another platform is a particular ZX Spectrum game which exploits the way that RAM refresh is internal to the CPU in order to starve the video display circuits of memory access and selectively create a noise effect on screen.

osunds tricky...

The GBA is actually one of the easier to emulate. It had only one CPU and simple timing rules and non-CPU hardware. Additionally the CPU is quite slow and best of all is a proper RISC design, meaning that everything about it is simple.

If you wanted to make life hard, you might have a bash at dynamic recompilation! I keep meaning to do this, but my primary project continues to take up the majority of my time.

Bobby: Its a concept thats an order of magnitude harder to comprehend than "Copy & Paste".

You compile it to native code on whatever platform you're running on.

i have -25 experience in it. i thought you were getting at that. i wonder .. which would be easier-arm instruction table or recompilation.
are gba games written in c or c++? you could decompile and recompile and run the resulting exe! damn, i know it can;t be that easy:(

ot: how do you decompile an exe into c/c++ code?

Bobby, don't you think you should code something simple before starting with more complex concepts might be a good idea?

You still have problems with the C/C++ syntax. So, try to finish a few small programs. Then get some good books on the advanced topics (I'd suggest the Dragon book and "Operating Systems") work through them, and then start again.
Alternativly, you could read (silently) the corresponding emulation coding newsgroups until you actually understand what it's going on there.

This kind of stuff is complex. Right now you're not ready for it. First, you need to become a Jedi, then you can face Darth Emulator.

GBA games are written in C, C++, you name it, as long as you have a proper compiler that can create binary executables for the ARM CPU . I personally prefer C++, though .

I think he means screensaver. I finially figured it out in the other thread when you asked why he would want that code. Then I saw it exits when you do any input, which acts like a screensaver does, hence ss. But it took me almost 2 days to figure that out.

Why don't you say it straight out then?
(my personal association was Schutzstaffel, and that's not an association you want to give people...)

That's teh 2nd time you gave that answer. From now on, I'll be to lazy to answer your posts correctly.