- High-level emulation
High-level emulation (HLE) is an approach for construction of
emulators, specifically of video game consolesystems. In HLE, instead of trying to accurately recreate the hardware, to create a platform on which the native code can be run, the effort focuses on recreating the appropriate "functionality" provided by the system emulated. Thus, the emphasis is shifted from emulating the "method" of reaching certain goals to getting the same (or comparable) "results" as if the native platform was used. By contrast, the traditional way of emulating is termed "Low-level emulation", or "LLE".
The term HLE originates from
UltraHLE, the first emulator for the Nintendo 64console that ran commercial games. Initial discussion about HLE occurred to give context for the reasons behind some video games not functioning properly with the emulator.
Criteria for High-Level Emulation
In order for HLE approach to work, the platform in question must meet certain criteria. Namely, there must exist a higher
abstraction levelthan the raw machine codeto be executed, realised directly in the hardware, or by the operating system. In any case, it must exist outside of the softwareintended to run on the emulated platform, and have certain amount of standardisation and semanticsfor HLE to succeed.
Comparison to traditional models
Compared to LLE, HLE has a very different set of design decisions and trade-offs. First, as the complexity of modern (fifth generation and above) video consoles rapidly increases, so does their computational power, and also the distance to the commodity PCs, which are the most common host systems for the emulators. Thus, the requirements on the quality of the emulated services increases, together with the difficulty of doing so. Hardware chips in consoles are usually extremely specialised towards specific functionality needed by games written for them, often in directions which are completely different than those taken by the hardware in an average PC machine. For example,
3D graphicsmight be realised by an extremely fast integerprocessor, coupled with the assumption of main system memory being the same as graphics memory, taking away the separate step of loading textures.
Emulating such an architecture programmatically on a PC, characterised by the emphasis put on floating-point operations, and specialised graphics hardware with memory separate from the system memory would be extremely difficult, especially taking into account the scarcity of documentation typical for specialised, proprietary hardware. Even if such emulator could be created, it would be much too slow for use. Instead, with HLE approach, the emulator would be rather written to take the data to be processed, along with the operations list, and implement it using the means available on the host systems, ie. floating-point maths and
GPU. The result is not only a much better match with the host platform, but often also significantly better results, as floating-point computation yields higher quality graphics suitable for high resolution displays available for PCs.
Advantages and disadvantages of HLE
Among the advantages of HLE technique, chiefly are the ability to utilise the existing host facilities much better and easier, the ability to optimise the results as the code and hardware improves, and much less or no work at all needed to achieve the desired end result, if an appropriate function is already provided by the host, as would be common in 3D graphics functionality. The progress of implementations is also much more independent of the detailed hardware documentation, instead relying only on the listing of possible functions available to the programmer, which is already provided by a
software development kitavailable for each platform.
The disadvantages include much higher reliance on standardisation among target applications, and the presence of sufficiently high-level mechanisms in the emulated platforms. If there is no such mechanism, or applications fail to utilise it in one of the already supported ways, they will not work correctly, even if other, superficially similar applications function with no problems. Thus a significant amount of
tweaks might be required to get all of the desired titles to run satisfactorily.
As a side-effect, HLE removes the common source of legality issues, by not requiring the users to provide it with the
bootstrapsoftware used by the original platform to create environment for applications to run in. Because the emulator itself provides such environment, it no longer needs system ROM images, bootstrap cartridge images or other software obtained from a physical copy of the emulated system, a process which usually resulted in an unclear status in the light of copyright law.
As the console systems progress into more and more complexity, the importance of HLE approach increases. Modern (6th and 7th generation) video consoles are already far too complex and powerful to facilitate their emulation using the traditional approach. Additionally, some systems (notably
Xbox) are themselves little more than a standardised PC machine, making it wasteful to try and recreate the hardware using PC as the host machine. Thus HLE increasingly becomes the only sensible approach.
* [http://www.zophar.net/n64.html Nintendo 64 Emulators]
* [http://www.emulator-zone.com/doc.php/n64/ultrahle.html History of UltraHLE]
* [http://www.emuunlim.com/UltraHLE/old/techinfo.htm UltraHLE - Technical Information] : an overview of low-level CPU emulation and high-level co-processor emulation in UltraHLE
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