Registers in Intel

Objectives

1. Begin to Explore the x86 registers

Notes

• This is Jorgensen chapter 2.
• Intel supports the following data sizes
  • byte - 8 bits
  • word - 16 bits
  • double word 32 bits
  • quad word 64 bits.
• Intel has the following registers
  • *ax - frequently used for return values
  • *bx
  • *cx - used to pass argument 4 to functions
  • *dx - used to pass argument 3 to functions
  • *si - used to pass argument 2 to functions
  • *di - used to pass argument 1 to functions.
  • rN - where n is 8 through 15
  • *bp - the base of the stack
  • *sp - the top of the stack.
• For the *
  • It can be replaced with r, e, nothing, or followed by h or l
  • rax is a 64 bit register
  • eax is a 32 bit register
  • ax is a 16 bit register
  • al is a 8 bit register
  • ah is a second 8 bit reister
  • 
• For r8 - r15
  • r# is the 64 bit register
  • r#d is the 32 bit register
  • r#w is the 16 bit register
  • r#b is the 8 bit register.
• the mov instruction
  • There are many forms.
  • Each will generate a different op code.
  • They depend on the arguments
  • For now we will do a move immediate
    • mov register, immediate value
    • the immediate value, if followed by an h is hex.
    • mov rax, 1111111h
  • A move to rax changes the entire register
  • A move to eax changes the entire register
  • A move to ax only changes the lower 16 bits
  • A move to al only changes the lowest 8 bit
  • A move to ah only changes the second lowest 8 bits.
• Negative numbers
  • Note a move of a negative number is not what you think.
  • Only a move to rax will fully set the negative number.
• Look at regDemo.cpp.
• Some GDB notes
  • list or l n will list the code
  • break # or b # for break
    • # can be a line number
    • Or it can be a label
  • run will restart the program.
  • print or p will print
    • registers start with a $
    • You can change the output type with /?
      • o - octal
      • x - hexadecimal
      • u - unsigned decimal
      • t - binary
      • f - floating point
      • a - address
      • c - char
      • s - string
    • p /x $rax
  • step n or s n
    • Without an argument steps a single line
    • This will step into function call
  • next n will step at the code level.