Introduction to Registers
By Bill T.
May 10, 1998


Introduction
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Registers are located directly on the CPU.  They are used to store data for
processing.  The general registers can be used to store anything needed, but
some registers are specialized for certain functions.


Registers
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General Purpose Registers:
  EAX  Accumulator:   Used for general purpose and slightly specialized for
                      mathematical results
  EBX  Base:          Used for general purpose and specialized for indexing
  ECX  Counter:       Used for general purpose and specialized for counting
  EDX  Data:          General purpose register

  Each of these registers are 32-bit. They each can be divided into a 16-bit
  register: AX, BX, CX, DX.  They can also be furthur divided into two 8-bit
  registers, AH (high) and AL (low).
  EAX is 32-bit, therefore it is 4 bytes, called a DWORD.
  AX is 16-bit, 2 bytes, called a WORD.
  AL and AH are each 8-bit or 1 BYTE.

Offset Registers:
  EBP  Base Pointer:        General purpose, holding offsets, and indexing
  ESP  Stack Pointer:       Points to the top of the stack
                            The next item pushed on the stack goes here
  ESI  Source Index:        Used for general purpose as well as the source in
                            data movement operations
  EDI  Destination Index:   Used for general purpose as well as the destination
                            in data movement operations
  EIP  Instruction Pointer: Points to the instruction to be executed next

  Each of these registers asre 32-bit, but can also be accessed as a 16-bit
  register (BP, SP, SI, DI, IP).

Segment Registers:
  CS  Code Segment:  Segment containing the code of the current program
  DS  Data Segment:  Segment containing the data of current program
  ES  Extra Segment: Function depends upon program, may be unused
  FS  Extra Segment:
  GS  Extra Segment:
  SS  Stack Segment: Segment which contains the stack memory

  These registers are each 16-bit.  Each locate the start of a 64K segment in
  memory.  You cannot perform math on segment registers or use them to hold the
  results of other operations.  Segments may be stored anywhere in memory.

Flags Register:
  o  Overflow Flag
  d  Direction Flag
  i  Interrupt Enable Flag
  t  Trap Flag
  s  Sign Flag
  z  Zero Flag
  a  Auxillary Flag
  p  Parity Flag
  c  Carry Flag

  The flags usually represent the result of various instructions. In addition
  to those listed there are several other flags which are not frequently used.

Other Registers:
  CR0-CR3  Control Registers
           These 32-bit registers handle things such as processor mode,
           paging and so on.
  DR0-DR7  Debug Registers
           These are 32-bit registers which are used for debugging
           purposes. DR0-DR3 are used to hold breakpoint addresses and
           DR4-DR7 determine the results of a breakpoint.
  TR6-TR7  Test Registers
           These 32-bit registers are used to test memory paging.
  IDTR     Interrupt Descriptor Table Register (52-bit)
  GDTR     Global Descriptor Table Register (52-bit)
  LDTR     Local Descriptor Table Register (68-bit)
  TR       Task State Segment Register (68-bit)


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Copyright (C) 1998
Bill T.  (billasm@usa.net)