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The pages on this site contain documentation for very old MS-DOS software, purely for historical purposes. If you're looking for up-to-date documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

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  Accumulator           [E]AX (AH/AL)  Multiply, divide, I/O, fast
                                       arithmetic
  Base                  [E]BX (BH/BL)  Pointer to base address (data
                                       segment)
  Count                 [E]CX (CH/CL)  Count for loops, repeats, and shifts
  Data                  [E]DX (DH/DL)  Multiply, divide, and I/O
 
  Source Index          [E]SI          Source string and index pointer
  Destination Index     [E]DI          Destination string and index pointer
  Base Pointer          [E]BP          Pointer to stack base address
  Stack Pointer         [E]SP          Pointer to top of stack
 
  Flags                 [E]<Flags>     Processor flags
  Instruction Pointer   [E]IP          Memory location of current
                                       instruction
 
  Code Segment          CS             Segment containing program code
  Data Segment          DS             Segment containing program data
  Stack Segment         SS             Segment for stack operations
  Extra Segment         ES             Extra program data segment
    "                   FS             Extra program data segment (386+)
    "                   GS             Extra program data segment (386+)
 
  Control Registers     CR(0-3)        Paging, caching, and
                                       protection (386+)
  Debug Registers       DR(0-7)        Data and instruction
                                       breakpoints (386+)
  Test Registers        TR(3-7)        Testing the TLB and cache (386+)
 
  Global Descriptor     GDTR           Address and limit of GDT (286+)
  Local Descriptor      LDTR           Address, limit, and selector of
                                       LDT (286+)
  Interrupt Descriptor  IDTR           Address and limit of IDT (286+)
  Task Register         TR             Address, limit, selector, and
                                       attributes of current task
 
  The E register prefix refers to the full 32-bit register (386+).
 
  See also: Flag Summary
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