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NT and VMS
As in UNIX and most commercial OSs, NT has two modes of execution, as Figure 2 shows. In user mode, applications execute, and OS/2, DOS, and POSIX execute and export APIs for applications to use. These components are unprivileged because NT controls them and the hardware they run on. Without NT's permission, these components cannot directly access hardware. In addition, the components and hardware cannot access each other's memory space, nor can they access the memory associated with NT's kernel. The components in user mode must call on the kernel if they want to access hardware or allocate physical or logical resources.
The kernel executes in a privileged mode: It can directly access memory and hardware. The kernel consists of several Executive subsystems, which are responsible for managing resources, including the Process Manager, the I/O Manager, the Virtual Memory Manager, the Security Reference Monitor, and a microkernel that handles scheduling and interrupts. The system dynamically loads device drivers, which are kernel components that interface NT to different peripheral devices. The hardware abstraction layer (HAL) hides the specific intricacies of an underlying CPU and motherboard from NT. NT's native API is the API that user-mode applications use to speak to the kernel. This native API is mostly undocumented, because applications are supposed to speak Win32, DOS, OS/2, POSIX, or Win16, and these respective OS environments interact with the kernel on the application's behalf.
VMS doesn't have different OS personalities, as NT does, but its kernel and Executive subsystems are clear predecessors to NT's. Digital developers wrote the VMS kernel almost entirely in VAX assembly language. To be portable across different CPU architectures, Microsoft developers wrote NT's kernel almost entirely in C. In developing NT, these designers rewrote VMS in C, cleaning up, tuning, tweaking, and adding some new functionality and capabilities as they went. This statement is in danger of trivializing their efforts; after all, the designers built a new API (i.e., Win32), a new file system (i.e., NTFS), and a new graphical interface subsystem and administrative environment while maintaining backward compatibility with DOS, OS/2, POSIX, and Win16. Nevertheless, the migration of VMS internals to NT was so thorough that within a few weeks of NT's release, Digital engineers noticed the striking similarities.
Those similarities could fill a book. In fact, you can read sections of VAX/VMS Internals and Data Structures (Digital Press) as an accurate description of NT internals simply by translating VMS terms to NT terms. Table 1 lists a few VMS terms and their NT translations. Although I won't go into detail, I will discuss some of the major similarities and differences between Windows NT 3.1 and VMS 5.0, the last version of VMS Dave Cutler and his team might have influenced. This discussion assumes you have some familiarity with OS concepts (for background information about NT's architecture, see "Windows NT Architecture, Part 1" March 1998 and "Windows NT Architecture, Part 2" April 1998).
NT's processes are virtually the same as VMS's processes (Table 2, page 118, shows a comparison of VMS and NT processes). In NT, as in VMS, the process scheduler implements 32 priority levels. The process with the highest priority is always running, and processes with equal priority are scheduled in a round-robin pattern. The system considers the 16 high-priority levels realtime or fixed priorities, because the process scheduler doesn't manipulate priority in processes the system assigns to that range. The 16 low-priority levels (except 0, which the system reserves for the idle thread that executes when nothing else can) are dynamic because the scheduler, often with the input of device drivers, bumps priorities up in reaction to various conditions, such as when the process receives input from a device. This bumping procedure is called boosting. A defining aspect of the NT and VMS schedulers is that they never lower a process' priority below the priority level the application programmed. To handle CPU hogging, in which a process burns CPU cycles without regard to other processes in the system, the scheduler boosts the priority of starved processes that haven't executed for a defined period. Both VMS 5.0 and NT 3.1 schedulers support symmetric multiprocessing (SMP), which let them execute processes simultaneously on different CPUs in order to increase applications' performance.
A major difference between NT process management and VMS process management is that NT processes contain one or more threads of execution, and NT's scheduler gives CPU time to threads, not processes. Digital didn't introduce kernel threads into VMS until version 7.0 in 1995. This addition is one of several enhancements Digital has made to VMS since NT's release that appear to be in response to NT capabilities. In turn, Microsoft added lightweight user-mode threads support to NT 4.0 in 1996, which it copied from the VMS implementation of threads.
The memory managers in NT and VMS are also similar. Both OSs implement virtual memory address maps that the system splits between the currently executing application and the kernel. Both NT and VMS rely heavily on memory-mapped files, especially for mapping the code for executing applications and implementing copy-on-write functionality (because of VAX hardware limitations, VMS provides less efficient copy on demand funtionality). Physical memory management in NT and VMS relies on demand-paged virtual memory. VMS's memory manager assigns each process upper and lower limits (called working sets) for the amount of physical memory the system can assign them. This feature compartmentalizes applications so that an application with heavy memory demands minimally affects other processes. NT's memory manager incorporates working sets, along with many subtleties of the VMS working-set tuning algorithms.
As with the process manager, notable differences exist between NT's and VMS's memory manager. VMS's Balance Set Manager moves entire processes' memory footprints out of memory to paging files and back to memory in response to the overall memory demands of the system. Microsoft did not carry this mechanism, known as swapping, into NT's Balance Set Manager, although some of NT's Balance Set Manager's secondary responsibilities are the same as the secondary responsibilities of VMS's Balance Set Manager.
NT's I/O Manager is closely based on VMS's I/O Manager. Both OS's I/O Manager support a layered-driver model throughout the device driver stacks for different device types and implements asynchronous packet-based I/O commands, and its device drivers dynamically load and unload. Stackable and loadable drivers make NT and VMS very extensible. Either OS can divide functionality among several device drivers, with each driver implementing a different abstraction level. For example, the system can insert a fault-tolerant disk driver between a file system driver and a disk driver. This configuration lets the fault-tolerant disk driver receive a request the system sends to one logical drive (e.g., the C drive), then send the request to multiple physical drives to implement mirroring or striping. Asynchronous I/O enables applications and the kernel subsystems to initiate device requests and work while the requests are in progress, rather than wait idly for the requests to complete. NT's device driver architecture and interrupt-request priority scheme are based on VMS. Descriptions of these aspects of the I/O Manager are applicable to both OSs with little variation.
As you can see by comparing Figure 2 and Figure 3, page 117, the Executive subsystems exhibit the most significant resemblance between VMS and NT. But many minor similarities exist in which it is clear that Microsoft derived NT's capabilities from VMS. For example, both NT and VMS represent resources as objects that the system manages through an Object Manager, which implements uniform reference counting and accounting. The Object Manager regulates resource allocation and calls the Executive subsystem functions that request notification of certain object operations. VMS object management is not formalized, like it is in NT, and the VMS Object Manager is just a loose connection of functions. Microsoft extended NT's Object Manager so that it provides a uniform naming model for all kernel resources.
NT's security subsystem is based on objects with discretionary access control lists. DACLs determine which users can perform various operations on those objects. Digital added a DACL enhancement to VMS's security model in version 4.0 in 1984. Therefore, VMS's security implementation is the predecessor to NT's. Microsoft even included systems tools similar to VMS's in NT, including the Performance Monitor, which is based on MONITOR, the extensible VMS performance monitor. VMS included a utility called BACKUP long before Microsoft developed NT's backup utility.
"Why the Fastest Chip Didn't Win" (Business Week, April 28, 1997) states that when Digital engineers noticed the similarities between VMS and NT, they brought their observations to senior management. Rather than suing, Digital cut a deal with Microsoft. In the summer of 1995, Digital announced Affinity for OpenVMS, a program that required Microsoft to help train Digital NT technicians, help promote NT and Open-VMS as two pieces of a three-tiered client/server networking solution, and promise to maintain NT support for the Alpha processor. Microsoft also paid Digital between 65 million and 100 million dollars.
The Evolution of NT and VMS
The saga goes on. Now that Compaq has acquired Digital, will VMS continue to evolve, or will NT seal the fate of its predecessor? One thing is certain: NT will continue to grow, leaving its origins further and further behind.