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Memory Areas

In the 1980s the maximum amount of memory that could be addressed by the processor was 640KB, as the processor was 8-bit. Now the amount of memory that can be addressed is measured in megabytes. For backward compatibility the original 640KB has remained and the remaining memory allocated according to the settings within the BIOS and operating system.

Memory Description
Conventional memory The first 640KB of system memory. Used by operating system kernels and standard DOS programs, device drivers and TSR (Terminate and Stay Resident) programs.
Upper memory area The upper 384KB of the first megabyte of memory, located right above conventional memory. Reserved for the system BIOS and device drivers and special uses such as ROM shadowing. Also called expanded memory or reserved memory.
High memory area The first 64KB (less 16 bytes) of the second megabyte of memory. It is the first 64KB of extended memory but can be accessed in real mode.
Extended memory All memory above 1MB and any memory that is above the high memory area. Used by Windows for programs and data that run in protected mode.


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