Dell Precision 620 - Manual (Page 75)

Memory Allocations
A microprocessor and programs operating under MS-DOS ® (real-mode operation) can address only 1 megabyte (MB) (1024 kilobytes [KB]) of
system memory. This area is divided into conventional memory (sometimes called base memory) and upper memory. All system memory above
this 1 MB is called extended memory and cannot be directly addressed by MS-DOS-based programs without the aid of memory-managing
software.
Table 10 provides a map of the conventional memory area. When the processor or a program addresses a location within the conventional
memory range, it is physically addressing a location in main memory, which is the only area of memory it can address under MS-DOS.
Table 10. Conventional Memory Map
Table 11 provides a map of the upper memory area. Some of these addresses are dedicated to various system devices, such as the system
BIOS. Others are available for use by expansion cards and/or an expanded memory manager (EMM). When the microprocessor or a program
addresses a location within the upper memory area, it is physically addressing a location within one of these devices.
Table 11. Upper Memory Map
IRQ Line Used By/Available
IRQ0 System timer
IRQ1 Keyboard controller
IRQ2 Interrupt controller (enables IRQ8 through IRQ15)
IRQ3
and IRQ4 Serial ports (if enabled in System Setup program)
IRQ5 Available
IRQ6 Diskette drive interface
IRQ7 Parallel port (if enabled in System Setup program)
IRQ8 RTC
IRQ9 ACPI
IRQ10 Available
IRQ11 Available
IRQ12 Mouse controller
IRQ13 Math coprocessor
IRQ14 Primary EIDE interface (if enabled in System Setup program)
IRQ15 Secondary EIDE interface (if enabled in System Setup program)
NOTE: For the full name of an abbreviation or acronym used in the table, see the Glossary .
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 .
Address Range Use