16-bit And 32-bit Programming

DOS is a 16-bit operating system. All portions of the operating system are written using 16-bit code, and DOS will only support 16-bit drivers and 16-bit application programs.

Windows 9x contains some 16-bit code and some 32-bit code. Programs written in 32-bit code require more memory and are generally faster than programs written in 16-bit code. The programming that supports VFAT in Windows 9x is written with 32-bit code.

Windows 9x is a hybrid between 16-bit and 32-bit code. Although Windows 9x supports 16-bit device drivers, the preference is to use the 32-bit drivers supplied with Windows 9x, for three main reasons. They are generally much faster, and 32-bit drivers can be stored in extended memory, releasing more of the first megabyte of memory to application programs. Also, 32-bit drivers can be dynamically loaded, meaning that they are loaded into memory when they are needed and then removed when not needed, thus conserving memory.

In contrast, 16-bit drivers must be stored in conventional or upper memory. When Windows 9x is installed over DOS, it searches for these 16-bit drivers and replaces them with the 32-bit drivers if it can.

Comparing Windows 95 And Windows 98

Windows 95 and Windows 98 (collectively referred to as Windows 9x) take us two steps closer to a new operating system but do not completely eliminate DOS. Windows 95 is the marriage of Windows for Workgroups (Windows 3.11) with an updated version of DOS sometimes known as DOS 7.0, together with some completely new additions and improvements to the operating system.

Windows 95 also introduced an improved and more automated method of installing new hardware devices, called Plug-and-Play.

Windows 98 made several improvements on Windows 95 but is fundamentally the same operating system. Windows 98 offers additional support for larger hard drives and more hardware devices, includes more software utilities, and is faster than Windows 95.

The underlying DOS portion of Windows 98 is named DOS 7.1.

Microsoft has announced that Windows 98 will be the last operating system it distributes that has a DOS foundation.

Currently, there have been three editions of Windows 98 released. The first two are Windows 98 and Windows 98 SE (Second Edition). The third edition is Windows Me (Millennium Edition).

The following table summarizes the advantages and disadvantages of Windows 9x.

Windows 9x Core Components

The three core portions of the Windows 9x operating system are:

·                     KERNEL

·                     USER

·                     GDI

In the following figure, you can see that the basic Windows 9x core component, the

KERNEL, uses mostly 32-bit code.

The 16-bit code is only retained as entry points into the KERNEL from 16-bit

application programs.

The USER portion uses mostly 16-bit code, primarily because it uses less memory

than the 32-bit equivalent and does not have a need for significant speed.

The GDI uses a mix of 16-bit and 32-bit code in order to maintain compatibility with

16-bit application programs.

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