Dell Latitude XPi CD, Reference Manual

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3-16 Dell Latitude XPi CD Reference and Troubleshooting Guide
Memory Allocations
A microprocessor and programs operating under
MS-DOS (real-mode operation) can address only
1 megabyte (MB) (1024 kilobytes [KB]) of system mem-
ory. This area is divided into conventional memory
(sometimes called base memory) and upper memory. All sys-
tem memory above this 1 MB is called extended memory and
cannot be directly addressed by MS-DOS–based programs
without the aid of some special memory-managing software.
Table 3-5 provides a map of the conventional memory area.
When the microprocessor or a program addresses a location
within the conventional memory range, it is physically
addressing a location in main memory, which is the only main
memory it can address under MS-DOS.
Table 3-6 provides a map of the upper memory area. Some of
these addresses are dedicated to various system devices, such
as the system/video basic input/output system (BIOS). Others
are available for use by expansion cards and/or an expanded
memory manager (EMM). When the microprocessor or a pro-
gram addresses a location within the upper memory area, it is
physically addressing a location within one of these devices.
IRQ12 Generated by the keyboard controller
to indicate that the output buffer of the
integrated trackball or external PS/2
mouse is full
IRQ13 Not available
IRQ14 Generated by the hard-disk drive to
indicate that the drive requires the
attention of the microprocessor
IRQ15 Reserved for the CD-ROM drive
Table 3-4. IRQ Line Assignments
(Continued)
IRQ Line Used/Available
NOTE: For the full name of an abbreviation or acronym used
in this table, see the Glossary in the online System User’s
Guide.
Table 3-5. Conventional Memory Map
Address Range Use
00000h–003FFh Interrupt vector table
00400h–004FFh BIOS data area
00500h–005FFh MS-DOS and BASIC work area
00600h–0FFFFh User memory
10000h–1FFFFh User memory
20000h–2FFFFh User memory
30000h–3FFFFh User memory
40000h–4FFFFh User memory
50000h–5FFFFh User memory
60000h–6FFFFh User memory
70000h–7FFFFh User memory
80000h–8FFFFh User memory
90000h–9FBFFh User memory
NOTE: For the full name of an abbreviation or acronym
used in this table, see the Glossary in the online System
User’s Guide.