Macs?
What do you mean by closed system? Closed hardware? Closed software?
I'm talking about a closed system that has one set of inflexible hardware and a single OS. Basically, with a PC you can upgrade components, change CPUs, and do all sorts of things. However, with a console the hardware is fixed and the OS is fixed. Developers know exactly what hardware is going to be in the console when they are making their games and they know exactly how much memory the OS is going to use.