The Hardware Abstraction Layer (HAL) isolates processor and platform dependencies from the OS and device driver code. In general, when device driver code is ported to a new platform, only a recompile is necessary. How can this work since device driver code is inherently device-, processor-, and platform-specific? Clearly, the device driver code must rely on code (macros) within the HAL to reference hardware registers and buses. In some cases, the device driver code must rely on abstraction code provided in the I/O Manager (and elsewhere) to manipulate shared hardware resources (e.g., DMA channels). Subsequent chapters in this book will explain the proper use of the HAL and other OS services so that device driver code can be platform-independent.
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