The concepts of real versus protected mode
memory addressing come up frequently in discussions of memory space
located above conventional memory. Conventional memory is the first
640 KB of memory.
Real Mode
Real mode memory addressing means that software, such as
DOS or its applications, can address only 1,024 KB (1 MB) of RAM. In
other words, real mode means that the microprocessor chip addresses
the first 1,024 bytes of "conventional and upper memory" by actually
assigning real addresses to real locations in memory. A 286 system
running in real mode, for example, acts essentially the same as the
8088/86 chips and can run older software with no modifications.
Protected Mode
The counterpart to real mode is protected mode memory
addressing. Unlike real mode, protected mode allows one program to
fail without bringing down the whole system. The theory behind
protected mode is that what happens in one area of memory has no
effect on other programs. In other words, when running in protected
mode, a program is limited to its own memory space allocation, but it
can access memory above 1MB. The 80286 could address up to 16 MB of
memory, but software could use the chip to access even more memory.
Note that before programs that run concurrently are truly safe from
one another's actions, the microprocessor (along with other system
chips) requires an operating system that can work to provide that
protection. Just about every operating system other than DOS runs in
protected mode.
