From the VxWorks Kernel Programmer’s Guide:
Prior to VxWorks 6.0, the operating system provided a single memory space with no segregation of the operating system from user applications. All tasks ran in supervisor mode. Although this model afforded performance and flexibility when developing applications, only skilled programming could ensure that kernel facilities and applications coexisted in the same memory space without interfering with one another.
…whereas now, I guess, any idiot and his dog can write a well-behaved VxWorks application…