Sundeep Kamath

Sundeep Kamath

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Dealing with 32-bit, 64-bit and Any CPU compilation options in .Net

Ever wondered why Visual Studio IDE provides 3 options for compilation - x86, x64 and Any CPU ? While most are aware of these options, many still lack clarity on the significance of these options (including me earlier in my career). So, as part of this article I hope to clear some of these doubts.

Windows OS has evolved over the years, right to from the time it started out as a 16-bit OS, then transitioned to 32-bit (x86) and more recently to 64-bit (x64).

Why would you use a 64-bit OS over 32-bit? To understand this, lets understand the capabilities of both 32-bit and 64-bit CPUs.

  • 32-bit CPU
    • Address pointer size is 32 bits -> can access 2^32 (4,294,967,296) discrete addresses.
    • This allows a program to make a data structure in memory up to 4 GB in size.
  • 64 bit CPU
    • Address pointer size is 64 bits -> can access 2^64 (18,446,744,073,709,551,616) discrete addresses.
    • This allows a program to make a data structure in memory up to 16 Exabytes in size.
  • Main advantages (64-bit over 32-bit)
    • Process on a 64-bit CPU can work with a larger set of data compared to 32-bit CPU (only constrained by physical memory).
    • A 64 bit integer makes arithmetic or logical operations using 64 bit types such as C#’s long faster than one implemented as two 32 bit operations.
  • To summarize, for all practical purposes…
    • Applications that use large amounts of memory, like Image and video editing software, 3D rendering utilities, and video games will make better use of a 64-bit architecture and operating system, especially if the machine has 8 or even 16 GB of RAM that can be divided among the applications that need it.

Understanding the behaviour of 32-bit, 64-bit and Any CPU compiled PE files:

  • On a 32-bit machine:
    • Any CPU: runs as a 32-bit process, can load Any CPU and x86 assemblies, will get BadImageFormatException if it tries to load an x64 assembly.
    • Any CPU-32-bit preferred (default): same as Any CPU.
    • x86: same as Any CPU.
    • x64: BadImageFormatException always.
  • On a 64-bit machine:
    • Any CPU: runs as a 64-bit process, can load Any CPU and x64 assemblies, will get BadImageFormatException if it tries to load an x86 assembly.
    • Any CPU-32-bit preferred (default): runs as a 32-bit process, can load Any CPU and x86 assemblies, will get BadImageFormatException if it tries to load an x64 assembly.
    • x86: runs as a 32-bit process, can load Any CPU and x86 assemblies, will get BadImageFormatException if it tries to load an x64 assembly.
    • x64: same as Any CPU.

Any CPU-32-bit preferred This is a new option introduced starting with .NET 4.5 and Visual Studio 11. It is a new subtype of Any CPU and is also the default compilation option in the IDE.

You can check this in the project properties as shown in the screenshot below …

Visual Studio project properties default compilation option

How 32-bit PEs work on 64-bit architectures ?

  • Generally, only 32-bit PEs can be loaded into a 32-bit process, and only 64-bit PEs can be loaded into a 64 bit process.
  • When a 32-bit PE is launched on a 64-bit OS, it runs in the “WOW” (Windows-32 on Windows-64) to present an illusion of a 32-bit operating system to the process.

Why was the Any CPU compilation option introduced?

  • Now before 64-bit Windows OS was introduced there was just the 32-bit PEs (of course, we are not considering 16-bit OS in this discussion) and hence during compilation these PEs were marked as 32-bit and there was no ambiguity.
  • With the introduction of 64-bit Windows OS, a decision had to be made - should the binaries be marked as 32-bit or 64-bit by default?
  • Technically speaking, managed binaries have no hard CPU dependency - they could be either 32 or 64 bit.
  • However, there was a way required to reuse .Net libraries from both 32-bit and 64-bit processes. For this the OS loader support was extended to support architecture-neutral PE files (Any CPU).
  • In case of Any CPU PEs the OS loader decides how to initialize the process. On 64-bit OS’s they are run as 64-bit processes, and on 32-bit OS’s they are run as 32-bit processes.

Hope this post helped clarify a few things!! I’ll also touch upon how to identify the architecture affinity of a PE in a future post.

Abbreviations:

PE : Process Executables (exe’s and dll’s)

References

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