Logical Memory

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Chances are that the current A+ exams will not test you on the use of logical memory, as did previous tests. The current exams are likely to test your knowledge of loading device drivers and configuration files that support more current operating systems. (See Section 2, A+ Operating Systems Technologies Study (Exam 220-302) of this book for information on operating systems.) However, newer operating systems must remain backward compatible to support older applications, so it is a good idea for you to understand the concepts of logical memory.

Operating systems and software divide areas of memory into logical sections in which applications and programs can run. Today's popular operating systems, such as Windows NT, Windows 9.x, Windows 2000, Windows Me, and Windows XP, automatically divide and maintain logical memory areas. Older operating systems, such as DOS, PC DOS, and Windows 3.x, required manual configuration of logical memory areas by skilled technicians who were very familiar with the configurations of DOS and logical memory management software tools.

Logical memory is divided into four basic divisions, as shown in Table 4.4.

Table 4.4: Logical Memory Divisions

Memory Area	Memory Description
Conventional memory	The first 640K of system memory addresses are used to load and run device drivers, programs, and applications. This is also referred to as lower memory area.
Upper memory area (UMA)	The first 384K of memory above conventional memory are used for device drivers, video RAM, and ROM BIOS. This is also referred to as expanded or reserved memory.
High memory area (HMA)	The first 64K of extended memory, minus 16 bytes, provide ‘real-mode' support to operating system.
Extended memory (XMS)	All memory addresses above 1MB and up to 4GB. This area is used primarily for programs and applications.

Conventional Memory

In the early days of personal computing, the original PC and software developers created 640K of addressable memory space and named it conventional memory. At the time, it was assumed that 640K would be more than enough memory to store the entire operating system, software device drivers, and applications. This amount of memory was acceptable in the early 1980s, when operating systems were small and applications ran one at a time. As time progressed, the need for addressable memory space increased greatly. Today's computing systems require very large amounts of memory for operating systems, Graphical User Interfaces (GUIs), and multitasking applications.

Upper Memory Area (UMA)

The upper memory area, also referred to as reserved memory, is the first 384K of memory addresses directly above conventional memory. The first section of upper memory addresses is reserved for video RAM and ROM. The top section of memory addresses in upper memory is reserved for the system BIOS. (You may have heard the term shadowing before. In computer terms, shadowing refers to moving ROM BIOS information into the reserved area of memory.) BIOS programs for expansion boards other than video are located or ‘mapped' to the memory addresses between video RAM and the system BIOS. Table 4.5 lists the areas in reserved memory with associated computer hexadecimal memory addresses. Unused memory addresses in upper memory are referred to as Upper Memory Blocks (UMBs).

Table 4.5: Reserved (Upper) Memory Map

Reserved Memory Area	Assigned Memory Address Range (Hexadecimal)
System BIOS	F000-FFFFF
Optional BIOS area	C8000-EFFFF
Video BIOS	C0000-C7FFF
Color text	B8000-BFFFF
Mono text	B0000-B7FFF
VGA/EGA	A0000-AFFFF

As applications grew more sophisticated, the need for more conventional memory space increased. To meet this demand, developers redesigned the UMA into expanded memory. Special DOS memory management programs and device drivers, such as EMM386.exe, Memmaker, and Himem.sys, were developed to move device drivers out of conventional memory and into expanded memory, freeing up space for the operating system and applications.

• EMM386.EXE uses Limulation (conversion of extended memory to expanded memory) to open access to the UMBs. This makes it possible to load programs and device drivers into memory using the AUTOEXEC.BAT and CONFIG.SYS files of DOS.

• Memmaker was introduced with DOS 6.0. It is a utility that allows you to free up conventional memory by loading device drivers and terminate-and-stay-resident programs (TSRs) into UMBs.

• Himem.sys is a memory device driver that also opens the HMA and directs programs to memory addresses in extended memory. Himem.sys must be loaded in the config.sys file for access to extended memory.

High Memory Area (HMA)

The high memory area is the first 64K of extended memory minus 16 bytes. This area of memory is controlled by Himem.sys and is the only area of extended memory available to a processor running in real mode.

Extended Memory (XMS)

Extended memory includes all addressable memory above reserved memory (above 1MB) and up to 4GB. You must have at least a 286 processor to take advantage of extended memory.

If you are interested in learning more about logical memory and its divisions, you may find the following PCGuide.com Web site informative: http://www.pcguide.com/ref/ram/logic-c.html

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