Stefan Schulze Frielinghaus authored Oct 02, 2019 and Ben Gamari committed Nov 03, 2019

Function createIOThread expects its second argument to be of size word.
The natural size of the second parameter is 32bits. Thus for some 64bit
architectures, where a write of the lower half of a register does not
clear the upper half, the value must be zero extended.

(cherry picked from commit d0924b15)