This is a bit of an oversimplification, because I don't actually know the details, but it works roughly like this: GPUs are devices on the PCI Express bus, with

This is a bit of an oversimplification, because I don't actually know the details, but it works roughly like this: GPUs are devices on the PCI Express bus, without an operating system getting in the way everything on the PCI Express bus is organised into something called ECAM (https://en.wikipedia.org/wiki/PCI_configuration_space https://en.wikipedia.org/wiki/PCI_configuration_space). Each device there specifies memory regions it supports. These memory regions map to a large number of configuration registers and on-device memory the GPU supports. Since GPUs are really complex most of its behaviour is actually not directly controlled but instead predetermined by its firmware (basically an operating system in its own right). Finally GPUs really execute multiple instruction streams with proprietary instruction formats and can independently load data from main memory with their build in DMA engines. These operations are typically kicked off by pointing various registers on the GPU to memory regions in main memory to tell it where to fetch instructions and data from. Everything else happens for the most part as consequences of instructions executing on the GPU.