A working knowledge of the Windows operating system begins with an understanding of the system architecture. This lesson introduces key aspects of the Windows architecture that affect operating system functionality and performance.
After this lesson you will be able to:
Estimated lesson time 105 minutes
- Describe how Windows 95 and Windows NT execute code.
- Describe how Windows 95 and Windows NT perform preemptive multitasking.
- Describe how Windows 95 and Windows NT manage memory.
- Compare and contrast the way Windows 95 and Windows NT run applications.
- Describe how device drivers provide hardware independence.
- Explain how the Win32 API benefits developers.
- Describe the purpose and the structure of the Windows registry.
The Windows operating system uses two modes to maintain operating system efficiency and integrity:
Figure 1.2 Rings 0 and 3 of the Intel Protection model
The 0 (zero) privilege level, also known as kernel mode , is the most privileged level. Privilege level 3, or user mode , is the least privileged. When code is running at a given privilege level, it is said to be running in that ring. For example, code running at privilege level 0 is said to run in ring 0. The Windows operating system only uses rings 0 and 3.
Kernel mode (ring 0) is the privileged mode of operation in which code has direct access to all hardware and all memory addresses. Software that runs in kernel mode has the following attributes:
Windows 95 and Windows NT
Because Windows 95 is designed to be as
User mode (ring 3) is a less privileged processor mode than kernel mode and has no direct access to hardware. Code running in user mode can run only in its own address space. It uses the Windows APIs to request system services.
User mode processes:
To protect against malfunctioning applications or unauthorized user access, user mode processes cannot directly access hardware. Hardware access
Windows helps protect the operating system by limiting the areas of memory that a user mode process can access. This limitation is accomplished by assigning only certain addresses to the user mode process.
Virtual memory, also known as virtual RAM, allows hard disk space to be used as if it were additional memory. The details of virtual memory are covered later in this chapter.
User mode processes are lower in priority; therefore, they have less access to the CPU than processes that run in kernel mode. This ensures that the operating system does not have to wait while an application finishes processing. In addition, any software component that fails in user mode cannot halt the entire system and may not affect any other software component running on the system at the same time.