Plan 9's rio window system expected most graphical clients to use the draw functions, which operate by writing to /dev/draw. /dev/draw wasn't even a framebuffer

Plan 9's rio window system expected most graphical clients to use the draw functions, which operate by writing to /dev/draw. /dev/draw wasn't even a framebuffer—it was a control file that you wrote messages into. It clearly wouldn't have met Carmack's requirements though as the thread you link to points out, the Plan 9 team would have been happy for him to map the framebuffer into the address space of his process.