22. x86 Protected Mode Flat Memory Model

• On 32-bit processors, Windows and Linux use the so-called protected mode flat memory model.

• Under flat memory model,

  • entire address space is described by a 32-bit segment, which provides 2³² = 4 gigabytes of address space.

  • program can (in theory) access up to 4 gigabytes of virtual or physical memory.

• In protected mode,

  • segment registers contain selector values rather than actual physical segment addresses.

  • Selector values cannot be calculated by the program; they must be obtained by calling the operating system.

  • Programs that update segment values or attempt to address memory directly do not work in protected mode.