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What exactly is a HAL and how would I write my own?

When I program STM32 microcontrollers I use ST's toolchain which provides all the tools necessary in order to build (and flash) the application binary. The toolchain also comes with ST's [own library](https://github.com/STMicroelectronics/stm32f4xx-hal-driver/blob/master/Src/stm32f4xx_hal_gpio.c) (source and header files) that allows me to control the micro's peripherals by using their functions. They group these files under a common name, "HAL", which stands for Hardware Abstraction Layer.

Looking into, for example, the gpio hal I see code like this.

```C
/* stm32f4xx_hal_gpio.c */
void HAL_GPIO_TogglePin(GPIO_TypeDef* GPIOx, uint16_t GPIO_Pin)
{
  uint32_t odr;
  assert_param(IS_GPIO_PIN(GPIO_Pin));

  /* get current Output Data Register value */
  odr = GPIOx->ODR;

  /* Set selected pins that were at low level, and reset ones that were high */
  GPIOx->BSRR = ((odr & GPIO_Pin) << GPIO_NUMBER) | (~odr & GPIO_Pin);
}
```

So in this function they set/reset some register bits (alter the hardware) and the result of setting these bits is that a pin is toggled (on a higher abstraction layer).

So is a HAL really just a collection of functions that when called set/reset specific register bits for my specific microcontroller? If so, is it really necessary to call it a HAL?