From: james.harris.1@gmail.com   
      
   On 12/08/2021 19:31, Joe Monk wrote:   
   >   
   >>> The S/370 has "base registers". These are under   
   >>> application control - not managed by the OS. I   
   >>> had previously thought you meant that segment   
   >>> registers and base registers are classified as   
   >>> "relocation registers" but now I'm not sure.   
   >   
   >> x86 and S/370 have no relocation registers. In case   
   >> of 360 it is not clear why. One guess is that   
   >> IBM thought that "more is better" and assumed that   
   >> base registers will be more flexible than relocation   
   >> register. Or they underestimated need to relocate   
   >> code. 8086 probably was intended to run single   
   >> program, so no need for relocation. In case   
   >> of 8086 they added segmentation, and having   
   >> _both_ segmention and relocation would be   
   >> a bit odd.   
   >   
   > S/360 and later have no relocation registers because they dont need them.   
   Remember the only 360 to have DAT was the model 67.   
   >   
   > A base address merely refers to an address within the address space of the   
   problem program (CSECT), or to a data area (DSECT).   
      
      
   A nitpick, perhaps, but I think a DSECT is a dummy section, not a data   
   section. IIRC a DSECT only becomes relevant when the assembler is told   
   to use it to define offsets from a base register.   
      
   I guess - and someone will probably correct me if this is wrong - that a   
   DSECT is like a structure or record template: it doesn't reserve any   
   storage but tells an assembler where fields are relative to a base   
   register.   
      
   On the discussion about relocation registers, AISI they are different   
   from base registers in that relocation via (limit + offset) protects   
   other parts of memory from a program while addressing off a base   
   register provides no such protection. But that's down to interpretation   
   of terms; there may be other ways of defining relocation.   
      
      
   --   
   James Harris   
      
   --- SoupGate-Win32 v1.05   
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