Essentially all of the processing of the video data, barring the container format which the CPU uses to know what part of the data to send to the GPU or the Audio chip or codec.

And HW acceleration is generally a preset baked in version of the encoder or decoder. These are mostly codec specific.

So, no using hardware from previous versions.

Now, you can see some software that tries to use the GPU itself, instead of the dedicated hardware acceleration, to decode, but that isn't the HW accelerated, and may not operate in real time.

At the same time, that will consume much more power, eliminating some of the advantages or the pure HW rendition, especially important for mobile.

I could see an argument being made for encoding, if it is 2x or faster than the CPU, but I haven't looked at any in a while, so don't know the speeds.

One of the biggest gains of having dedicated hardware is that the computation doesn’t happen on the general hardware.

This is what makes it viable on mobile devices where system responsiveness and power efficiency are high priority.

Generally these hardware decoders haven’t been retoolalble.

Typically things like quantization and motion estimation.