The Flags Register

Notes

• This is Jorgensen 2.3.1.5 and other places
• Note: Jorgensen calls this the rFlag(s) register
  • Other places I have seen FLAGS
  • gdb calls it eflags
• The flags register is used to communicate information from the CPU
  • It is a 64 bit special purpose register
  • It is usually set by math operations
    • But some come from interrupts
  • Jorgensen gives us a few of the bits
    • carry is bit 0
    • parity is bit 2
    • zero is bit 6
    • sign is bit 8
  • But most of the bits are reserved for future use.
  • Look at the table on wikipedia.
• When the CPU does anything, these flags will be set
  • And set when other instruction is executed the flags are changed.
  • The conditional jump instructions examine these flags.
• In gdb we can examine the flags register
  • p $eflags
  • Run simple.asm
  • Before the first add, PF, ZF and IF
    • Note that IF is the Interrupt enable Flag.
  • After the first add, PF, SF, IF
  • After the first inc: SF, If
  • After the second inc : PF AF ZF IF
• j*
  • Look at this cheat sheet for the jump instructions.
  • Each of these will examine the flags register and jump accordingly
  • We will only consider signed numbers for now.
  • We will come back and look at unsigned later.
  • Note: this must immediately follow the instruction that sets the flags register.
• A short side trip on labels
  • The nasm page.
  • letters, numbers, _, $, #, @, ~, . and ?
  • must start with a letter, _ or ?
  • Colon at the end is optional
  • NASM supports local labels
    • start with a .
    • refer back to the previous non-local label

    • loop1: 
      .top:
             cmp rax, 10
             jge .out
      
             ...
      
             inc rax
             j .top
      .out:

    • It will yell at you if you don't end the label with :
    • You could also do jmp loop1.top
• cmp
  • Compares two values
  • First can be register, memory
  • Second can be register, memory, immeiate
  • But both can not be memory
  • This will set the flags register but not change the values of either operand.
  • It does this by subtraction.