Why care about Big Endian?

Since the demise of PowerPC and Sparc, there are not a lot of Big Endian machines around for most people to access. These days the apart from some large mainframes, the only Big Endian machines left tend to be network routers using MIPS (and even then a lot of MIPS devices are run in Little Endian mode).

Why should you care about Big Endian? Probably for the most part you don’t ever need to. It seems likely that Little Endian has won. However you may not wish to limit yourself. You never know when your project needs to be ported to a different system that you didn’t anticipate.

When I started my programming job 15 years ago the only thing my company cared about was Windows 32 bit. Windows 2000 was the OS of choice. Mac and Linux were irrelevantly small and mobile platforms did not exist. Even 64 bit x64 had not shown up yet. Coding was easy, and fully centred around Windows APIs. Unfortunately this led to some poor assumptions made and bad programming practices. A few years later we needed to support the new x64 (Back when it was called AMD64). That took about 6 months of updating our code base. There were various places where assumptions had been made that a void* (PVOID in Windows.h terms) could be happily exchanged with a DWORD or ULONG (Windows terms for 32 bit unsigned ints).

After we had cleared the 64 bit hurdle we were then set for many years. Cross platform compatibility to us meant Windows with x86 or x64. Then along came the mobile platforms running on ARM chips. The big players were iOS and Android. Both of which have a unix ancestry. Before these were relevant platforms for us to code on we had started new projects and had anticipated that we would need to be cross platform in the future. We wrote a large compatibility library rather than directly using Windows.h and its APIs. We did however get this quite wrong. Our compatibility library was all based around the way things work in Windows and when we finally came to start porting it to other platforms we discovered that again we had assumptions that didn’t hold. Now currently, in 2017, we build for Windows, MacOS, Linux, iOS, and Android with x86, x64, Arm, and Arm64 architectures. Build time has certainly increased! We still have to live with some of the early poor assumptions we made (such that assuming that using Windows wide chars (UTF16) would be future proofing ourselves!) but overall now have a pretty good handle of working over multiple platforms and architectures.

My point after this long ramble, is that you never know where your project will go and what kind of things will be important in the future. Maybe in 10 years time, the new popular computers are all Big Endian. So you might just want to make sure your code works on it to save yourself headaches down the line.

Some people say you should never worry about Endianess and always write in an Endian-netural manner. This maybe good advice, but it is incredibly easy to slip up. It just takes you writing a number to disc straight out of memory to have made  the code no longer Endian-neutral. So without actually being able to test it, how will you know for sure?

There are two ways of dealing with Endianness that I can see

  1. Never copy memory bytes into words. eg never do the following: memcpy( &my32BitValue, myByteArray, sizeof(uint32_t). Instead always deal byte at a time and assemble your 32bit Value with bit shifts and ORs.
  2. Just wrap everything in htons and ntohs style wrappers before using them.

You will still want to test that it actually worked though. This is the rather tricky part as you most likely won’t have access to a Big Endian machine. However it is possible to emulate one using QEMU. In the next article I will give some concise instructions to get you up and going with an emulated MIPS Linux setup.

 

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