Difference between an API and a device driver

Question

I try to understand how they both relate to each other. As far as I know, they both can be a part of the HAL. In case of a communication between an application and a graphics card - can an API get the job done on its own or do we have to rely on them both? Can an API directly communicate with the hardware or do we always need a driver in-between, which translates the command of the API?

Answer 1

TL;DR

Think of an API as a specification that describes what to do, while a driver is an implementation that describes how to do it.

Details

As a contrived example, imagine we have three different audio cards that we want to play nicely with multiple operating systems. We can define an API for the card manufacturers that says, "Each card must support four methods: mute(), playsound(sound), volumeup() and volumedown()". By defining the API, we get a common interface that allows the operating system designers to support the audio devices without worrying about the hardware details. They know that if they want to mute the sound card, they can call mute(), or if they want to turn the volume up, they call volumeup().

It is then up to the device manufacturers to implement a driver that actually performs those actions. The driver will vary between the three different audio cards because they are different at the hardware level, but the API is consistent so the next higher abstraction level (the OS) doesn't need to know how to deal with the hardware.

For a more concrete example, consider the Advanced Control & Power Interface (ACPI) specification. It defines a common interface for operating systems to manage power consumption and thermal characteristics of hardware devices. There are methods that a device driver or firmware must implement in order to be "ACPI Compliant". This allows Windows operating systems and Linux variants to both perform the same actions on hardware devices without needing to implement their own drivers for the hardware

Note: Windows performs ACPI actions through acpi.sys, which they call an "ACPI Driver". Don't let the terminology confuse you; even though they call it a driver, it is really a window into the ACPI interface. Linux uses the acpi kernel module to do the same thing, and Linux doesn't call it a driver. Perhaps ACPI wasn't the best example, but I don't have anything better at the moment.