[danielmarkbruce]

I could be misunderstanding the context of the question, but I think OP is imagining some sophisticated communication logic involved at the chip level. The CPU doesn't know anything much about the GPU other than it's there and data can be sent back and forth to it. It doesn't know what any of the data means.

I think the logic OP imagines does exist, but it's actually in the compiler (eg the cuda compiler), figuring exactly what bytes to send which will start a program etc.

[coolspot]

Not in the compiler but in GPU driver. A graphic program (or compute) just calls APIs (DirectX/Vulkan/CUDA) of a driver, which then knows how to do that on a low-level writing to particular regions of RAM mapped to GPU registers.

[danielmarkbruce]

Yes! This is correct. My bad, it's been too long. I guess either way the point is that it's done in software, not hardware.

[lxgr]

There's also odd/interesting architectures like one of the earlier Raspberry Pis, where the GPU was actually running its own operating system that would take care of things like shader compilation.

In that case, what's actually being written to shared/mapped memory is very high level instructions that are then compiled or interpreted on the GPU (which is really an entire computer, CPU and all) itself.