[0x04]. Notes on Assembly - The fairytale of an x86 CPU

FLAGS	EIP	ESP	EBP
CS	DS	ES	SS
FS	GS	ESI	EDI
EAX	EBX	ECX	EDX

• A 32bit x86 has 16 registers, divided in 6 groups respectively:
  
  • 1 x EFLAGS register
  • 1 x Instruction Pointer
  • 2 x Stack Pointing Registers
    
  • 6 x Segment Registers
  • 2 x Index Registers
    
  • 4 x General Purpose Registers
• The registers are assigned specific roles:
  
  • EFLAGS register (Extended FLAGS register is a 32bit version of the 16bit FLAGS) contains the state of current processor. Only 18 out of 32 flags have a meaning assigned.
  • EIP - Extended Instruction Pointer points to the next instruction memory address in the Fetch-Execute cycle.
    
  • ESP - Extended Stack Pointer - points to the top of the stack. You can see how it grows down on an x86 architecture in the following example: stack_pointer.c
    
  • EBP - Extended Base Pointer - points to the base of the current Stack Frame. If you assemble func.c as follows:
    

    $ gcc -S func.c -o func.s

    and take a look into func.s file, the f() function will be translated to some thing like that:

    f:
    pushl   %ebp  
    movl    %esp, %ebp
    subl    $16, %esp
    movl    $11, -16(%ebp)
    movl    $22, -12(%ebp)
    movl    $33, -8(%ebp)
    movl    $44, -4(%ebp)
    leave
    ret

    1. Line one saves the old EBP
    2. Old ESP becomes new EBP
    3. Increasing the stack by the size of 1 paragraph
    4-7. Saving local variables in the stack frame locations relative to EBP
    
    
  • ?S - Segment Registers
    
    • CS, Code Segment
      
    • DS, Data Segment
    • SS, Stack Segment
    • ES, Extra Segment
      
    • FS, another Extra Segment
      
    • GS, another Extra Segment
      
  • Extended Index Registers, used for array operations (e.g. strings, which are arrays of bytes)
    
    • Source Index
    • Destination Index
    
    
  • Extended General Purpose Registers
    
    • EAX - accumulator, used for storing intermediate results of I/O access, interrupts or arithmetics.
      
    • EBX - base register, used for addressing
      
    • ECX - counter, used in loops and countdowns.
      
    • EDX - data register