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  Five offset registers are used with the segment registers to contain
  segmented addresses. One register, called the instruction pointer (IP),
  contains the offset of the current instruction in the code segment; two
  registers, called the stack registers, are intimately tied to the stack;
  and the remaining two registers, called the index registers, are used to
  address strings of data.

  The instruction pointer (IP), also called the program counter (PC),
  contains the offset within the code segment where the current program is
  executing. It is used with the CS register to track the location of the
  next instruction to be executed.

  Programs do not have direct access to the IP register, but a number of
  instructions, such as JMP and CALL, change the IP value implicitly.

  The stack registers, called the stack pointer (SP) and the base pointer
  (BP), provide offsets into the stack segment. SP gives the location of the
  current top of the stack. Programs rarely change the value in SP directly.
  Instead, they rely on PUSH and POP instructions to update SP implicitly.
  BP is the register generally used to access the stack segment directly.
  You'll see BP used quite often in the assembly-language examples that
  appear in Chapters 8 through 20.

  The index registers, called the source index (SI) and the destination
  index (DI), can be used for general-purpose addressing of data. Also, all
  string move and comparison instructions use SI and DI to address data
  strings.