From: already5chosen@yahoo.com   
      
   On Fri, 19 Dec 2025 03:30:26 -0000 (UTC)   
   John Levine  wrote:   
      
   > According to John Savard  :   
   > >Therefore, I came up with the idea of using some opcode space for   
   > >block headers which could contain information about the lengths of   
   > >instructions, so as to make decoding variable-length instructions   
   > >fully non-serialized, thus giving me the best of both worlds.   
   >   
   > Sounds like the first two bits of the opcode in S/360 which tells you   
   > the instruction format which also tells you how long the instruction   
   > is.   
   >   
   > They've added lots of new instructions since then with somewhat   
   > different formats, but those bits still tell you how long the   
   > instruction is. The first byte tells you what the fornat is so you   
   > know what address calculations to do.   
   >   
   >   
      
   With very long pipelines that IBM is using starting from z10 (17 years   
   ago) it probably makes no difference.   
   The fact that there are only 3 options for instruction length is   
   important and simplifying things relatively to more than dozen of   
   options in x86, but how many bits one has to access in order to   
   determine the length of instruction is irrelevant or close to   
   irrelevant as long as they all reside near beginning rather than   
   anywhere, like in VAX.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)