DOS ain't dead

x86 flags after bit shifts with count = 0 (Developers)

posted by mht , Wroclaw, Poland, 05.12.2008, 08:55

Original Intel manual does not say explicitly that flags are left unchanged. It explains that SHR ARG, COUNT (other shift instructions are described similarly) is performed as follows:

argbefore := ARG;
temp := COUNT;
while (temp <> 0) do
begin
  CF := LowestBit(ARG);
  ARG := ARG div 2;
  temp := temp - 1;
end;
if (COUNT = 1) then
  if (HighestBit(argbefore) <> HighestBit(ARG)) then
    OF := 1
  else
    OF := 0
else
  OF := UNDEFINED;

and also says elsewhere that for all shift instructions, AF is undefined and SF, ZF and PF are set as usual.

Complete thread:

• x86 flags after bit shifts with count = 0 - mht, 22.11.2008, 15:15
  • x86 flags after bit shifts with count = 0 - Rugxulo, 25.11.2008, 06:47
    • x86 flags after bit shifts with count = 0 - ecm, 25.11.2008, 13:31
      • x86 flags after bit shifts with count = 0 - mht, 25.11.2008, 22:11
    • x86 flags after bit shifts with count = 0 - rr, 25.11.2008, 21:21
      • x86 flags after bit shifts with count = 0 - mht, 25.11.2008, 21:59
        • x86 flags after bit shifts with count = 0 - rr, 25.11.2008, 22:10
          • x86 flags after bit shifts with count = 0 - mht, 28.11.2008, 07:17
            • x86 flags after bit shifts with count = 0 - mht, 05.12.2008, 08:55
      • x86 flags after bit shifts with count = 0 - RayeR, 25.11.2008, 22:22
        • x86 flags after bit shifts with count = 0 - rr, 26.11.2008, 20:57
          • x86 flags after bit shifts with count = 0 - Rugxulo, 28.11.2008, 00:09
            • x86 flags after bit shifts with count = 0 - marcov, 28.11.2008, 15:41
            • x86 flags after bit shifts with count = 0 - rr, 30.11.2008, 18:33
              • x86 flags after bit shifts with count = 0 - Rugxulo, 30.11.2008, 20:53
                • portable handhelds' battery life - Rugxulo, 30.11.2008, 21:06
                • x86 flags after bit shifts with count = 0 - rr, 30.11.2008, 22:38
              • x86 flags after bit shifts with count = 0 - RayeR, 30.11.2008, 22:41