Conditional Control Instructions

Objectives

1. Investigate the flags register

Notes

• Reference
• The FLAGS register in intel contains information about the current state of the CPU
• Many different contiditons set the FLAGS register in intel
• This includes
  • If the last instuction had a carry
  • If the last instruction produced a 0
  • If the last operation produced a negative number.
  • If the last operation had overflow.
  • Wikipedia has many more.
• We don't have direct access to the flags register,
  • But pushf, pushfd, pushfq will push it onto the stack.
  • And then we can pop it off into another register.
• There are logical instructions in assembly
  • and dest, src will and the src with the dest
  • This is a bitwise and.
  • There are also or, not, and xor
• We need to be careful not to mess up the values over function call
  • registers 12, 13, 14, 15 are Preserved
  • If a function changes these, it should restore them.
  • I do this with r12 in my PrintFlags function
• See flags.asm.
• These are checked with
  • jz, (je) - zero
  • jnz, (jne) - not zero
  • jc, jnc - carry
  • jo, jno - overflow
  • js, jns - sign
  • jp, (jpe) - even parity
  • jnp, (jpo) - odd parity
• See loop.asm.