Real Mode:

1. 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.
2. 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.

Protected Mode:

1. The primary difference between Real Mode and Protected Mode is the latter's new addressing mechanism and protection levels.
2. 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).
3. The addresses stored in the segment registers are now interpreted as pointers into a descriptor table.
4. 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.