Pentium Modes for Memory Addressing

Pentium processors can run in the following modes:

• Real Mode

  In this mode, registers are thought of as if they were only 16 bits, as on the 8086 processor. Therefore addresses are transmitted as two 16 bit numbers. The first is the base and the second is the offset from that base. Bases tend to be reused to access subsequent locations in the same region, or segment.

• Protected Mode

  This mode can use:

  • Paging

    For more information, see Classroom306's article on the subject.

  • Segmentation

    In this mode the system behaves like Real Mode, except that the full 32 bits of registers/buses are used. With the wider bus, the address space of the system is much greater. Given the intention that the system represent addresses with two numbers like real mode, a further goal is that both numbers remain 16 bits, so they can be stored within a single register at the same time. However, with the greater address space, 16 bits cannot represent all possible bases. Therefore the first number points not to a base address, but to an entry in a base-lookup table. Note that this reference will take up less bits than storing the actual 32-bit base address would, since the table only contains as many entries as there are base addresses at any given time. The table is a descriptor table. If there is one for each process they are local, and if the operating system keeps only one it is global.