Introduction

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.

Flag 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)
 

For a plaintext copy: a_tut2.txt