The virtual machine manager (VMM) is the 32-bit protected-mode operating system at the core of Windows 95. Its primary responsibility is to create, run, monitor, and terminate virtual machines. The VMM provides services that manage memory, processes, interrupts, and protection faults. The VMM works with virtual devices, 32-bit protected-mode DLLs, to allow the virtual devices to intercept interrupts and faults to control the access that an application has to hardware devices and installed software.
Both the VMM and virtual devices run in a single, 32-bit, flat model address space at privilege level 0 (also called ring 0). The system creates two global descriptor table (GDT) selectors, one for code and the other for data, and uses these selectors in the CS, DS, SS, and ES segment registers. Both selectors have a base address of zero and a limit of 4 gigabytes (GBs), so all the segment registers point to the same address space. The VMM and virtual devices never change these registers.
The VMM provides multiple-threaded, preemptive multitasking. It runs multiple applications simultaneously by sharing CPU (central processing unit) time between the threads in which the applications and virtual machines run. The VMM is also nonreentrant. This means that virtual devices must synchronize access to the VMM services. The VMM provides services, such as semaphores and events, to help virtual devices prevent reentering the VMM.
The virtual machine manager provides a variety of services, messages, macros, and structures to help virtual devices manage their corresponding hardware devices and services. The following chapters introduce the services and provide details about how to call the individual services, messages, macros, and structures.