Real Mode Limitations
In Real Mode, a segment has the following characteristics:
Its start address must be in the first megabyte of memory space.
The segment length is fixed at 64KB.
The segment can be read or written by any program.
In order to have the maximum flexibility, the OS must be able to define a program's segments as residing anywhere within the 4GB memory address range.
In Real Mode, segments cannot reside in extended memory (i.e., memory above the first megabyte).
Programs and the data they manipulate frequently occupy more than 64KB of memory space, but each segment has a fixed length of 64KB in Real Mode, neither shorter nor longer. If the OS only requires a very small segment for a program's code, data or stack area, the smallest ...
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