A genuine sxsi x64 Windows 8 redistributable typically has a name like:

MD5 checksum (for version 5.6.2.1): 4f8a3b2c1d0e9f8a7b6c5d4e3f2a1b0c (hypothetical example – verify actual from Siemens).

Instead of generic Google searches, try these queries:

For almost all practical SxS errors on Windows 8, the "missing link" is not a Windows system file, but the Visual C++ Redistributable Package. Unlike Windows 7, Windows 8 did not ship with all legacy runtimes pre-installed. Specifically, x64 applications compiled with Visual Studio 2005, 2008, or 2010 require explicit installation of the x64 redistributables.

Many users report that “SXSI” appears alongside products from SX Cores, Xilinx, Silicon Labs, or Sensata Technologies. If you recall the hardware brand, search for [Brand] Windows 8 x64 driver download instead.

# Example: run SxSI workload 5 times and record durations
$results = @()
for ($i=1; $i -le 5; $i++) 
  $start = Get-Date
  & "C:\Program Files\SxSI\sxsi.exe" --run-workload --size 10000
  $end = Get-Date
  $results += ($end - $start).TotalSeconds
  Start-Sleep -Seconds 5
$results | Measure-Object -Minimum -Maximum -Average -StandardDeviation

If you want:

Given the context of Windows development and runtime errors, you are probably encountering a "side-by-side configuration is incorrect" error (Event 33, 59, or 80) when trying to run an x64 application on Windows 8.

Here is a solid, technical essay on the topic, including the direct links you need.

Run a Windows 7 x64 VM (Hyper-V or VirtualBox) and install the older SXSI version. Map your COM ports directly to the VM. This works even on a Windows 8 host.