Absolute Beginners Guide to A+ Certification. Covers the Hardware and Operating Systems Exam

Because Windows 9x is built on an MS-DOS foundation, it supports the different types of physical memory specifications that result from the design of the original IBM PC and its many descendants:

• Conventional memory ” Memory addresses between 0 and 640KB. MS-DOS programs normally run in this area.

• Upper memory ” Memory addresses between 640KB and 1MB, some of which are already in use for the system BIOS, Plug and Play BIOS, video BIOS, and video RAM. Depending on the system, between 96KB and 160KB of this space is not in use by hardware, but these addresses are not available unless additional memory managers (such as Emm386.exe ) are installed during the startup process.

• Extended memory ” Memory over 1MB. This is the primary memory area used by Windows 9x. Windows 9x/Me loads the XMS (Extended Memory Specification) driver Himem.sys to make this memory available to Windows and compatible MS-DOS programs during Windows startup.

• High memory ” The first 64KB of extended memory after an XMS driver is loaded. The Io.sys file installs Himem.sys and activates the dos=high option to copy the MS-DOS kernel used by Windows 9x into the High Memory Area (HMA).

• Expanded memory ” Memory that can be accessed through a 64KB page frame that can be established in unused upper memory blocks. Expanded Memory Specification (EMS) memory originally required special memory boards , but can now be created from XMS memory through the use of Emm386.exe in Config.sys . This type of memory is useful only for older MS-DOS programs.

Memory management takes place during the startup process of Windows 9x. By default, Io.sys converts memory starting at 1MB into XMS using Himem.sys and then uses the dos=high option to move the MS-DOS kernel into the HMA portion of XMS memory.

If you have MS-DOS TSR utilities that you want to install into upper memory blocks (UMBs), you will need to add the following statement to the Config.sys file:

Device=C:\Windows\Emm386.exe NOEMS

The DOS=UMB statement will be added to the memory configuration by Io.sys to make the UMBs available to MS-DOS TSRs.

If you have MS-DOS applications that need access to EMS memory, you need to add the following statement to the Config.sys file:

Device=C:\Windows\Emm386.exe RAM

This converts XMS memory into a common pool of XMS/EMS memory that can be used by both Windows and DOS applications. The DOS=UMB statement will be added to the memory configuration by Io.sys to make any UMBs not used by the EMS page frame available to MS-DOS TSRs.

To load any TSRs from Autoexec.bat into UMBs, use the Loadhigh ( LH ) statement:

LH C:\Utilities\MyTools.exe