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> What I'm looking for is a description of how a CPU tells a GPU to start executing a program. Through what means do they communicate - a bus? How does such a communication instance look like? For most modern computers, through the PCI Express bus. Take a look at the output of "lspci -v" and you'll see something like: 00:02.0 VGA compatible controller: [...]
[...]
Flags: bus master, fast devsel, latency 0, IRQ 128
Memory at ee000000 (64-bit, non-prefetchable) [size=16M]
Memory at d0000000 (64-bit, prefetchable) [size=256M]
I/O ports at f000 [size=64]
Expansion ROM at 000c0000 [virtual] [disabled] [size=128K]
That is, the GPU on this particular laptop makes available a region of memory sized 16 megabytes at physical address 0xee000000, and another region of memory sized 256 megabytes at physical address 0xd0000000. Whenever the CPU writes to or reads from these memory regions, it is writing to memory on the GPU, not on the normal RAM chips. And not all of that "memory" on the GPU is real memory; some of it are registers, which are used to control the GPU.The same happens on the opposite direction: for code running on the GPU, some regions of memory are actually the RAM normally used by the CPU. In either case, the memory read and/or write transactions go through the PCI Express bus to the other device. The exact details of what is written to (and read from) that memory vary depending on the device. For most GPUs, the driver sets up a list of commands in memory (either "host" memory, which is the RAM on the CPU, or "device" memory, which is the RAM on the GPU accessible through these PCI Express "memory windows"), and writes the address of that command list to a register on the GPU; the GPU then reads the list and executes the commands found in it. These commands can include things like "start N threads of the program found at X with Y as the input" (GPU programs are commonly called "shaders", and they are highly parallel), but also things like "wait for event W to happen before doing Z". |