protected mode

   An operating mode of Intel 80x86 processors.  The opposite
   of real mode.  The Intel 8088, Intel 8086, Intel 80188
   and Intel 80186 had only real mode, processors beginning
   with the Intel 80286 feature a second mode called protected
   mode.

   In real mode, addresses are generated by adding an address
   offset to the value of a segment register shifted left four
   bits.  As the segment register and address offset are 16 bits
   long this results in a 20-bit address.  This is the origin of
   the one megabyte (2^20) limit in real mode.

   There are 4 segment registers on processors before the Intel
   80386.  The 80386 introduced two more segment registers.
   Which segment register is used depends on the instruction, on
   the addressing mode and of an optional instruction prefix
   which selects the segment register explicitly.

   In protected mode, the segment registers contain an index into
   a table of segment descriptors.  Each segment descriptor
   contains the start address of the segment, to which the offset
   is added to generate the address.  In addition, the segment
   descriptor contains memory protection information.  This
   includes an offset limit and bits for write and read
   permission.  This allows the processor to prevent memory
   accesses to certain data.  The operating system can use this
   to protect different processes' memory from each other, hence
   the name "protected mode".

   While the standard register set belongs to the CPU, the
   segment registers lie "at the boundary" between the CPU and
   MMU.  Each time a new value is loaded into a segment register
   while in protected mode, the corresponding descriptor is
   loaded into a descriptor cache in the (Segment-)MMU.  On
   processors before the Pentium this takes longer than just
   loading the segment register in real mode.  Addresses
   generated by the CPU (which are segment offsets) are passed to
   the MMU to be checked against the limit in the segment
   descriptor and are there added to the segment base address in
   the descriptor to form a linear address.

   On a 80386 or later, the linear address is further processed
   by the paged MMU before the result (the physical address)
   appears on the chip's address pins.  The 80286 doesn't have a
   paged MMU so the linear address is output directly as the
   physical address.

   The paged MMU allows for arbitrary remapping of four klilobyte
   memory blocks (pages) through a translation table stored in
   memory.  A few entries of this table are cached in the MMU's
   Translation Lookaside Buffer to avoid excessive memory
   accesses.

   After processor reset, all processors start in real mode.
   Protected mode has to be enabled by software.  On the 80286
   there exists no documented way back to real mode apart from
   resetting the processor.  Later processors allow switching
   back to real mode by software.

   Software which has been written or compiled to run in
   protected mode must only use segment register values given to
   it by the operating system.  Unfortunately, most application
   code for MS-DOS, written before the 286, will fail in
   protected mode because it assumes real mode addressing and
   writes arbitrary values to segment registers, e.g. in order to
   perform address calculations.

   Such use of segment registers is only really necessary with
   data structures that are larger than 64 kilobytes and thus
   don't fit into a single segment.  This is usually dealt with
   by the huge memory model in compilers.  In this model,
   compilers generate address arithmetic involving segment
   registers.  A solution which is portable to protected mode
   with almost the same efficiency would involve using a table of
   segments instead of calculating new segment register values ad
   hoc.

   To ease the transition to protected mode, Intel 80386 and
   later processors provide "virtual 86 mode".

   (1995-03-29)