Microprocessor Architecture

This section summarizes design issues related to processors used on systems that meet the requirements in this guide.

Windows NT is designed to run on platforms that use Intel486® (with uniprocessor support only), Intel Pentium, Intel Pentium with MMX technology, Pentium Pro, Pentium II, or compatible processors that use the Intel Architecture instruction set. When Windows NT is running on an Intel Architecture processor, a virtual-86 processor mode allows direct execution of most instructions in MS-DOS® – based applications. In virtual-x86 mode, a few instructions, such as I/O, must be emulated to virtualize the hardware.

Windows NT can also run on computers with RISC processors, such as DEC Alpha. When Windows NT is running on a RISC processor, hardware support is not available for executing such MS-DOS instructions, so Windows NT emulates all these instructions and provides a virtual hardware environment using the Virtual Device Manager (VDM). The Windows NT VDM also supports ROM BIOS interrupt services, MS-DOS Interrupt 21 services, and virtual hardware for devices using virtual device drivers.

Advanced RISC computing (ARC) refers to a RISC computer architecture standard associated with the Advanced Computing Environment (ACE) consortium. For ACE-compliant platforms, the system firmware must support bootstrap loading and execution as an abstracted set of ARC routines and ARC devices.

For both kinds of platforms, a hardware abstraction layer (HAL) interfaces between the hardware and the system. Device drivers for certain types of devices create an alias between the names of their device objects and the corresponding ARC device name by calling the appropriate application programming interface (API).

For some devices in Windows NT, there are no differences in the requirements for supporting any microprocessor platform. For example, a network adapter driver calls DMA-related functions of the NDIS interface library for DMA operations between the host and the network adapter. These functions support maximized portability so the driver can run on both Intel Architecture and DEC Alpha systems.

However, some differences in microprocessor platform requirements must be addressed in the Windows NT device driver. For example:

• Phase 1 of Windows NT system startup is specific to the particular microprocessor platform. On Intel Architecture servers, the hardware boot ROM loads a boot sector, which in turn loads the NTLDR. For RISC platforms, the firmware loads necessary ARC drivers, acquires hardware configuration data, and then loads the OSLOADER.
  
  
• Phase 2 of Windows NT system startup sets up memory, captures hardware configuration data, constructs a description of the hardware in memory, and puts a pointer to this description into the loader block.
  
  
• On Intel Architecture servers running Windows NT, there are two kinds of video miniport drivers: VGA-compatible miniports and miniports that rely on having the system-supplied VGA miniport driver or other VGA-compatible Super VGA (SVGA) miniport driver loaded concurrently.

On RISC platforms, miniport drivers rely on the system-supplied VGA support, if necessary. For all RISC platforms running Windows NT, video miniport drivers need not supply any special support for full-screen MS-DOS–based applications. Instead, video miniport drivers must be set up to configure themselves in the registry.