From: g4ugm@dave.invalid   
      
   On 21/09/2025 00:40, Lawrence D’Oliveiro wrote:   
   > On Sat, 20 Sep 2025 21:13 +0100 (BST), John Dallman wrote:   
   >   
   >> ... the IBM Z instruction set only has three instruction lengths -   
   >> 2, 4 and 6 bytes, which has not changed since System/360 - and you   
   >> can always discover the length of each instruction from its first   
   >> two bytes. That makes having multiple instructions being decoded   
   >> simultaneously easier, which is a bottleneck in x86 and x86-64, the   
   >> other long-lasting CISC instruction set.   
   >   
   > Mainframes were never designed for high CPU performance.   
   >   
   > Look at the current Top500 list of the world’s fastest machines; what   
   > architectures do you see? IBM POWER offers a few contenders; also ARM,   
   > I think MIPS, and of course the most common is x86-64. At some point   
   > no doubt a RISC-V machine is likely to make an appearance.   
   >   
   > No IBM Z. Not before, not now, not ever.   
   >   
      
   No, but these machines are all special purpose. Look at some of the Z   
   technical documents, the way the system is build is fascinating.   
      
   The real advantage of the 360/370 etc. architecture was the way it did   
   IO. The original channel with its own dedicated processor and 8-bit bus   
   running at 1Mhz yielding 8 Mbits/sec was rapid for its era.   
      
   Then the use of block mode terminals so the management of input fields   
   was all done in the terminal controller. The Mainframe never saw an   
   interrupt until a complete form was filled in.   
      
   I think DEC or was it HP forgot this with the Alpha. I remember looking   
   at Alpha for Microsoft Exchange on Windows/NT. It was really hard to   
   justify using an Alpha because Exchange is very IO intensive. You   
   couldn't get enough RAID to use the CPU.   
      
   But we digress, I don't believe the techniques IBM use to perpetuate the   
   use of Z would have worked with VMS. Remember IBM too has had its   
   failures. No one runs AIX on Z or X86 these days but at one time it was   
   flavour of the month in IBM. I think OS/2 is in a similar position to   
   VMS. Now on its third owner/manager after IBM...   
      
   Dave   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)