What is a HAL and what problem does it solve?

A Hardware Abstraction Layer separates what a peripheral does from how a particular chip implements it: you define an interface (the operations the application needs, set, clear, read for a GPIO) and provide concrete drivers that implement it per MCU, while application logic depends only on the interface. It solves two problems. Portability: swap MCUs by writing a new driver behind the same interface, and the app logic is untouched, no hunting through code for GPIOA->ODR. Testability: because the logic depends on an interface rather than registers, you can inject a fake implementation and unit-test the logic on a PC, in CI, with no hardware. The cost to weigh is a layer of indirection, so good HALs stay minimal, they expose the behavior the app uses, not every register field. It's the pattern that turns firmware from chip-welded register pokes into portable, testable software.