Once upon a time there was a Blit terminal [1], which allowed processes to display content in independent windows. One process managed the windows, others could draw only in own windows; the terminal took care of that.

In an ideal future, GPU could be extended to support concurrent access from several processes, each with own context. The GPU would enforce separation; each process would draw to own window. Without going through syscalls...

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[1] http://en.wikipedia.org/wiki/Blit_(computer_terminal)