Game_Music_Emu 0.5.6 Design --------------------------- This might be slightly out-of-date at times, but will be a big help in understanding the library implementation. Architecture ------------ The library is essentially a bunch of independent game music file emulators unified with a common interface. Gme_File and Music_Emu provide a common interface to the emulators. The virtual functions are protected rather than public to allow pre- and post-processing of arguments and data in one place. This allows the emulator classes to assume that everything is set up properly when starting a track and playing samples. All file input is done with the Data_Reader interface. Many derived classes are present, for the usual disk-based file and block of memory, to specialized adaptors for things like reading a subset of data or combining a block of memory with a Data_Reader to the remaining data. This makes the library much more flexible with regard to the source of game music file data. I still added a specialized load_mem() function to have the emulator keep a pointer to data already read in memory, for those formats whose files can be absolutely huge (GYM, some VGMs). This is important if for some reason the caller must load the data ahead of time, but doesn't want the emulator needlessly making a copy. Since silence checking and fading are relatively complex, they are kept separate from basic file loading and track information, which are handled in the base class Gme_File. My original intent was to use Gme_File as the common base class for full emulators and track information-only readers, but implementing the C interface was much simpler if both derived from Music_Emu. User C++ code can still benefit from static checking by using Gme_File where only track information will be accessed. Each emulator generally has three components: main emulator, CPU emulator, and sound chip emulator(s). Each component has minimal coupling, so use in a full emulator or stand alone is fairly easy. This modularity really helps reduce complexity. Blip_Buffer helps a lot with simplifying the APU interfaces and implementation. The "classic" emulators derive from Classic_Emu, which handles Blip_Buffer filling and multiple channels. It uses Multi_Buffer for output, allowing you to derive a custom buffer that could output each voice to a separate sound channel and do different processing on each. At some point I'm going to implement a better Effects_Buffer that allows individual control of every channel. In implementing the C interface, I wanted a way to specify an emulator type that didn't require linking in all the emulators. For each emulator type there is a global object with pointers to functions to create the emulator or a track information reader. The emulator type is thus a pointer to this, which conveniently allows for a NULL value. The user referencing this emulator type object is what ultimately links the emulator in (unless new Foo_Emu is used in C++, of course). This type also serves as a useful substitute for RTTI on older C++ compilers. Addendum: I have since added gme_type_list(), which causes all listed emulators to be linked in. To avoid this, I make the list itself editable in blargg_config.h. Having a built-in list allows gme_load_file() to take a path and give back an emulator with the file loaded, which is extremely useful for new users. Interface conventions ---------------------- If a function retains a pointer to or replaces the value of an object passed, it takes a pointer so that it will be clear in the caller's source code that care is required. Multi-word names have an underscore '_' separator between individual words. Functions are named with lowercase words. Functions which perform an action with side-effects are named with a verb phrase (i.e. load, move, run). Functions which return the value of a piece of state are named using a noun phrase (i.e. loaded, moved, running). Classes are named with capitalized words. Only the first letter of an acronym is capitalized. Class names are nouns, sometimes suggestive of what they do (i.e. File_Scanner). Structure, enumeration, and typedefs to these and built-in types are named using lowercase words with a _t suffix. Macros are named with all-uppercase words. Internal names which can't be hidden due to technical reasons have an underscore '_' suffix. Managing Complexity ------------------- Complexity has been a factor in most library decisions. Many features have been passed by due to the complexity they would add. Once complexity goes past a certain level, it mentally grasping the library in its entirety, at which point more defects will occur and be hard to find. I chose 16-bit signed samples because it seems to be the most common format. Supporting multiple formats would add too much complexity to be worth it. Other formats can be obtained via conversion. I've kept interfaces fairly lean, leaving many possible features untapped but easy to add if necessary. For example the classic emulators could have volume and frequency equalization adjusted separately for each channel, since they each have an associated Blip_Synth. Source files of 400 lines or less seem to be the best size to limit complexity. In a few cases there is no reasonable way to split longer files, or there is benefit from having the source together in one file. Preventing Bugs --------------- I've done many things to reduce the opportunity for defects. A general principle is to write code so that defects will be as visible as possible. I've used several techniques to achieve this. I put assertions at key points where defects seem likely or where corruption due to a defect is likely to be visible. I've also put assertions where violations of the interface are likely. In emulators where I am unsure of exact hardware operation in a particular case, I output a debug-only message noting that this has occurred; many times I haven't implemented a hardware feature because nothing uses it. I've made code brittle where there is no clear reason flexibility; code written to handle every possibility sacrifices quality and reliability to handle vaguely defined situations. Flexibility through indirection ------------------------------- I've tried to allow the most flexibility of modules by using indirection to allow extension by the user. This keeps each module simpler and more focused on its unique task. The classic emulators use Multi_Buffer, which potentially allows a separate Blip_Buffer for each channel. This keeps emulators free of typical code to allow output in mono, stereo, panning, etc. All emulators use a reader object to access file data, allowing it to be stored in a regular file, compressed archive, memory, or generated on-the-fly. Again, the library can be kept free of the particulars of file access and changes required to support new formats. Emulators in general -------------------- When I wrote the first NES sound emulator, I stored most of the state in an emulator-specific format, with significant redundancy. In the register write function I decoded everything into named variables. I became tired of the verbosity and wanted to more closely model the hardware, so I moved to a style of storing the last written value to each register, along with as little other state as possible, mostly the internal hardware registers. While this involves slightly more recalculation, in most cases the emulation code is of comparable size. It also makes state save/restore (for use in a full emulator) much simpler. Finally, it makes debugging easier since the hardware registers used in emulation are obvious. CPU Cores --------- I've spent lots of time coming up with techniques to optimize the CPU cores. Some of the most important: execute multiple instructions during an emulation call, keep state in local variables to allow register assignment, optimize state representation for most common instructions, defer status flag calculation until actually needed, read program code directly without a call to the memory read function, always pre-fetch the operand byte before decoding instruction, and emulate instructions using common blocks of code. I've successfully used Nes_Cpu in a fairly complete NES emulator, and I'd like to make all the CPU emulators suitable for use in emulators. It seems a waste for them to be used only for the small amount of emulation necessary for game music files. I debugged the CPU cores by writing a test shell that ran them in parallel with other CPU cores and compared all memory accesses and processor states at each step. This provided good value at little cost. The CPU mapping page size is adjustable to allow the best tradeoff between memory/cache usage and handler granularity. The interface allows code to be somewhat independent of the page size. I optimize program memory accesses to direct reads rather than calls to the memory read function. My assumption is that it would be difficult to get useful code out of hardware I/O addresses, so no software will intentionally execute out of I/O space. Since the page size can be changed easily, most program memory mapping schemes can be accommodated. This greatly reduces memory access function calls.