Information Technology Reference
In-Depth Information
H.7 Core system components
The core of Windows 95/98 has three components: user, kernel, and GDI (graphical device
interface), each of which has a pair of DLLs (one for 32-bit accesses; the other for 16-bit ac-
cesses). The 16-bit DLLs (dynamic link libraries) allow for Win16 and MS-DOS
computability.
Figure H.8 shows that the lowest-level services provided by the Windows 95/98 kernel
are implemented as 32-bit code. In Windows 95/98 the names of the files are GDI32.DLL,
KERNEL32.DLL and USER32.DLL; these are contained in the \WINDOWS\SYSTEM di-
rectory.
Windows 95/98 core
User
GDI
Kernel
Figure H.8
Core components
H.7.1 User
The user component provides input and output to and from the user interface. Input is from
the keyboard, mouse, and any other input device and the output is to the user interface. It
also manages interaction with the sound driver, timer, and communications ports.
Win32 applications and Windows 95/98 use an asynchronous input model for system in-
put. With this, devices have an associated interrupt handler (for example, the keyboard inter-
rupts with IRQ1) which converts the interrupt into a message. This message is then sent to a
raw input thread area, which then passes the message to the appropriate message queue. Each
Win32 application can have its own message queue, whereas all Win16 applications share a
common message queue.
H.7.2 Kernel
The kernel provides for core operating system components including file I/O services, virtual
memory management, task scheduling and exception handling, such as:
•
File I/O services.
•
Exceptions - these are events that occur as a program runs and calls additional software
which is outside of the normal flow of control. For example, if an application generates
an exception, the Kernel is able to communicate that exception to the application to per-
form the necessary functions to resolve the problem. A typical exception is caused by a
divide-by-zero error in a mathematical calculation, an exception routine can be designed
so that it handles the error and does not crash the program.
•
Virtual memory management - this resolves import references and supports demand pag-
ing for the application.
