Native code can take advantage of CPUs and GPUs in a way that Web applications still cannot to enable computationally demanding applications that involve sophisticated graphics.
Portable Native Client differs from Google's Native Client (NaCl) technology in that it creates architecture-independent output. Where native code compiled with NaCl emerges tuned for a specific instruction set, like x86, ARM or MIPS, programs compiled with PNaCl will run on any hardware platform. In other words, they will work on mobile and desktop devices.
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PNaCl compiles native code into an intermediate form. The resulting LLVM-bytecode then gets wrapped in a portable executable file that can be served from a website.
"When the site is accessed, Chrome fetches and translates the portable executable into an architecture-specific machine code optimized directly for the underlying device," explains Google engineer David Sehr in a blog post.
While PNaCl may allow developers to create hardware-independent code, it's not quite platform-independent: PNaCl applications require Google Chrome to run, because other browser vendors haven't integrated the technology.
In the near term, PNaCl makes ChromeOS more competitive with OS X and Windows as computing environments suited to computationally demanding applications. Whether Google's technology will benefit the Web in the years to come remains to be seen.
"PNaCl and Pepper are not open standards, and there are not even any proposals on the table to standardize them in any forum," O'Callahan wrote in a blog post. "They have documentation, but for the details one must defer to the large bundle of Chrome code that implements them. Other vendors wishing to support PNaCl or Pepper would have to adopt or reverse-engineer this code."
As for Apple and Microsoft, its hard to see either company going out of its way to help hasten adoption of Google's technology. If Google can encourage enough developers to create compelling PNaCl applications that consumers flock to, perhaps Apple and Microsoft will be forced to adapt. But don't hold your breath; it could take a while.