AMD 29000

AMD 29000

   <processor> A {RISC} {microprocessor} descended from the
   {Berkley RISC} design.  Like the {SPARC} design that was
   introduced shortly afterward, the 29000 has a large {register
   set} split into local and global sets.  But though it was
   introduced before the SPARC, it has a more elegant method of
   register management.

   The 29000 has 64 global registers, in comparison to the
   SPARC's eight.  In addition, the 29000 allows variable sized
   windows allocated from the 128 register stack {cache}.  The
   current window or stack frame is indicated by a stack pointer,
   a pointer to the caller's frame is stored in the current
   frame, like in an ordinary stack (directly supporting stack
   languages like {C}, a {CISC}-like philosophy).  Spills and
   fills occur only at the ends of the cache, and registers are
   saved/loaded from the memory stack.  This allows variable
   window sizes, from 1 to 128 registers.  This flexibility, plus
   the large set of global registers, makes {register allocation}
   easier than in SPARC.

   There is no special {condition code register} - any general
   register is used instead, allowing several condition codes to
   be retained, though this sometimes makes code more complex.
   An {instruction prefetch} buffer (using {burst mode}) ensures
   a steady instruction stream.  To reduce delays caused by a
   branch to another stream, the first four new instructions are
   cached and next time a cached branch (up to sixteen) is taken,
   the cache supplies instructions during the initial memory
   access delay.

   Registers aren't saved during interrupts, allowing the
   interrupt routine to determine whether the overhead is
   worthwhile.  In addition, a form of register access control is
   provided.  All registers can be protected, in blocks of 4,
   from access.  These features make the 29000 useful for
   embedded applications, which is where most of these processors
   are used, allowing it the claim to be "the most popular RISC
   processor".  The 29000 also includes an {MMU} and support for
   the {AMD 29027} {FPU}.

   (1995-06-19)