x86 Assembly: Part Two-- Flags

1. Say "call read_input" as the FIRST LINE in your assembly program.  This routine may trash (i.e., change) all other registers, so be sure to call it before doing anything else!  read_input, like any subroutine, returns its value in the %eax register.
2. Use %eax and do whatever computation you need to do.
3. In the NetRun GUI, click "Input Data": "Enabled" to display the input data edit box.
4. Enter the program's input data--just numbers--in the input data edit box.
5. Hit "Run!".  The program will read the input you've typed in.

EFLAGS

• ZF-- The "zero flag".  Set whenever the previous arithmetic result was zero.  Can be used by the "jz" (jump if last result was zero) or "jnz" instructions.  "je" (jump if equal) and "jne" (jump if not equal) are just aliases of jz & jnz, because if the difference is zero, then the two values are equal.  For example, this code checks if the input is equal to 4:
      call read_input
      cmp $4,%eax
      je equal
      add $20, %eax # If not equal, add
  equal: # If equal, just return
  
• CF--The "carry flag".  Set to indicate the bit that carries out of an addition or subtraction.  Can be used by the "jc" (jump if carry flag is set) instruction.  Set by all the arithmetic instructions.  Can be added into another arithmetic operation with "adc" (add with carry).  For example, you can preserve the bit overflowing out of a big add like this:
  mov $0x8000ff00,%eax
  add %eax,%eax
  mov $0,%eax
  adc %eax,%eax
  "adc" is used in the compiler's implementation of the 64-bit "long long" datatype, and in general in "multiple precision arithmetic" software, like the GNU Multiple Precision Arithmetic Library.  It'd also be used to implement overflow checking in a careful compiler, like
  The carry and zero flags are also used by the unsigned comparison instructions: "jb" (jump if unsigned below), "jbe" (jump if unsigned below or equal), "ja" (jump if unsigned above), and "jae" (jump if unsigned above or equal) in the usual way.
  
• SF-- The "sign flag", which indicates a negative signed result.  Used together with OF to implement signed comparison.
  
• OF-- The "overflow flag". Set by subtract, add, and compare, and used in the signed comparison instructions "jl" (jump if less than), "jle" (jump if less than or equal to), "jg" (jump if greater than), and "jge" (jump if greater than or equal to) instructions.  If you stare at it hard enough, you can read the definitions, work out exactly what SF and OF do, and convince yourself they do the right thing.  They do.