In modern PC gaming engines (Unreal Engine 5, REDengine, Frostbite), rendering a single frame requires tight coordination between two asynchronous computing units:
1. The CPU Draw Call Phase
Before the graphics card can shade a single pixel, the central processor must compute user input, physics simulations, particle trajectories, and spatial visibility culling. It then dispatches DirectX 12 / Vulkan draw calls instructing the GPU where objects reside in 3D world space. If the CPU cannot dispatch draw calls within the 16.6ms frame budget (for 60 FPS) or 6.94ms (for 144 FPS), the GPU stalls, causing frame rate stutter.
2. The GPU Pixel Shader Phase
Once draw calls are received, the graphics card rasterizes triangles and executes fragment shaders. At 1080p (2.07M pixels), modern GPUs finish rasterization so quickly that they spend idle cycles waiting for the CPU. At 4K (8.29M pixels), the GPU workload multiplies by 400%, shifting 90%+ of total frame time onto the GPU.