复旦大学软件学院计算机系统课件 Machine-Level Representation of Programs II.ppt

* * * * * * * * * * * * * * * * * Condition codes EFLAGS ZF: Zero Flag The most recent operation yielded zero SF: Sign Flag The most recent operation yielded a negative value * Setting Conditional Codes Implicit Setting By Arithmetic Operations addl Src,Dest C analog: t = a+b CF set if carry out from most significant bit Used to detect unsigned overflow ZF set if t == 0 SF set if t 0 OF set if two’s complement overflow (a0 b0 t0) || (a0 b0 t=0) * Conditional Code lea instruction has no effect on condition codes Xorl instruction The carry and overflow flags are set to 0 Shift instruction carry flag is set to the last bit shifted out Overflow flag is set to 0 * Setting Conditional Codes Explicit Setting by Compare Instruction cmpl Src2,Src1 cmpl b,a like computing a-b without setting destination CF set if carry out from most significant bit Used for unsigned comparisons ZF set if a == b SF set if (a-b) 0 OF set if two’s complement overflow (a0 b0 (a-b)0) || (a0 b0 (a-b)0) * Setting Conditional Codes Explicit Setting by Test instruction testl Src2,Src1 Sets condition codes based on value of Src1 Src2 Useful to have one of the operands be a mask testl b,a like computing ab without setting destination ZF set when ab == 0 SF set when ab 0 * Accessing Conditional Codes The condition codes cannot be read directly One of the most common methods of accessing them is setting an integer register based on some combination of condition codes Set commands * Accessing Conditional Codes After each set command is executed A single byte to 0 or to 1 is obtained The descriptions of the different set commands apply to the case where a comparison instruction has been executed * Accessing Conditional Codes Instruction Synonym Effect Set Condition Sete Setz ZF Equal/zero Setne Setnz ~ZF Not equal/not zero Sets SF Negative Setns ~SF Nonnegative Setl Setnge SF^OF Less Setle Setng (SF^OF)|ZF Less or Equal Setg Setnle ~(SF^OF)~ZF Greater Setge Setnl ~(SF^OF) Greater or Eq