Platforms is basically the essential framework for programs to run. There's Windows, Apple/MAC, Solaris, and Linux platforms. It's a place to run various softwares, programs, whatever. It's often labeled a collection of APIs or Application Programming Interfaces. In that sense The operating system solves problems with running hardwares as well as softwares. You don't have to manually program the instructions for task performed by a device such as booting a system or burning a dvd.
One example of cpu architexture is seen with the differences between single and dual cored cpus lately. Another is the differences between 32bit and 64bit model cpus. Then there is the differing architextures between Intel and AMD. As mentioned before the role of the OS is to provide the interface for programs to run. The cpu architexture has more to do with how cpus process data bits and code instructions. For instance the older model cpus ran supported single channel memory configurations while all new models support the dual channel or double data rate transfer seen with memory.
