Differentiate between real mode and protected mode.
- Following a system reset the 386 is initialized in Real Mode. In this mode the chip looks virtually identical to an 8086. That is, it has the following features:
- The address space is limited to 1 MB using address lines AO-A19 (the high address lines A20-A31 are inactive).
- The segment memory addressing mechanism of the 8086 is retained with each segment limited to 64 KB.
- Two new features are available to the programmer in Real Mode—access to the 32-bit register set of the 386 and the addition of two new segments called F and G. This will be explained in more detail when the programming model is introduced later in this chapter.
- The primary difference between Real Mode and Protected Mode is the latter's new addressing mechanism and protection levels.
- Although memory segments are still retained, each segment may range from a single byte to 4 GB (the full physical address space of the 386).
- The addresses stored in the segment registers are now interpreted as pointers into a descriptor table.
- Each segment's entry in this table is eight bytes long and identifies the 32-bit base address of the segment, the segment size, and the access rights. Memory addresses are computed by adding the offset specified by the instruction to the segment base address.
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