Hal.dll

The Microsoft Windows operating system supports a form of shared libraries known as "dynamic-link libraries", which are code libraries that can be used by multiple processes while only one copy is loaded into memory. This article provides an overview of the core libraries that are included with every modern Windows installation, on top of which most Windows applications are built.

HAL.DLL is a kernel-mode library file and it cannot be used by any user-mode program. NTDLL.DLL is only used by some programs, but it is a dependency of most Win32 libraries used by programs.

The Windows Hardware Abstraction Layer (HAL) is implemented in Hal.dll. The HAL implements a number of functions that are implemented in different ways by different hardware platforms, which in this context, refers mostly to the chipset. Other components in the operating system can then call these functions in the same way on all platforms, without regard for the actual implementation.

For example, responding to an interrupt is quite different on a machine with an Advanced Programmable Interrupt Controller (APIC) than on one without. The HAL provides a single function for this purpose that works with all kinds of interrupts by various chipsets, so that other components need not be concerned with the differences.

The HAL is loaded into kernel address space and runs in kernel mode, so routines in the HAL cannot be called directly by applications, and no user mode APIs correspond directly to HAL routines. Instead, the HAL provides services primarily to the Windows executive and kernel and to kernel mode device drivers. Although drivers for most hardware are contained in other files, commonly of file type .sys, a few core drivers are compiled into Hal.dll.

Kernel mode device drivers for devices on buses such as PCI and PCI Express directly call routines in the HAL to access I/O ports and registers of their devices. The drivers use HAL routines because different platforms may require different implementations of these operations. The HAL implements the operations appropriately for each platform, so the same driver executable file can be used on all platforms using the same CPU architecture, and the driver source file can be portable across all architectures.

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