After the operating system and boot drivers are loaded from the disk into memory by NTLDR or OSLOADER, control is passed to the Windows NT Executive.
The HAL is an Executive component that exports the platform-specific functionality. It supports limited output functionality to allow the Kernel to display informational and error messages as other components of the kernel-mode Executive are initialized.
The NTLDR’s initial BIOS call (see Section 1.2.1) is made in x86 real mode.
HAL support for video output also is necessary if the machine crashes. The HAL can be called by the Kernel to reinitialize the video adapter to an alphanumeric mode and to display information, such as a register dump and stack backtrace.