Scalable Processor ARChitecture

    (SPARC) An instruction set architecture designed
   by Sun Microsystems for their own use in 1985.  Sun was a
   maker of 680x0-based Unix workstations.  Research
   versions of RISC processors had promised a major step
   forward in speed but existing manufacturers were slow to
   introduce a RISC type processor, so Sun went ahead and
   developed its own, based on the University of California at
   Berkley's RISC I and RISC II 1980-2.  In keeping with
   their open philosophy, they licenced it to other companies,
   rather than manufacture it themselves.  The evolution and
   standardisation of SPARC is now directed by the non-profit
   consortium SPARC International, Inc.

   SPARC was not the first RISC processor.  The AMD 29000
   came before it, as did the MIPS R2000 (based on Stanford's
   design) and Hewlett-Packard Precision Architecture CPU,
   among others.  The SPARC design was radical at the time, even
   omitting multiple cycle multiply and divide instructions (like
   a few others), while most RISC CPUs are more conventional.

   SPARC implementations usually contain 128 or 144 registers,
   (CISC designs typically had 16 or less).  At each time 32
   registers are available - 8 are global, the rest are allocated
   in a "window" from a stack of registers.  The window is moved
   16 registers down the stack during a function call, so that
   the upper and lower 8 registers are shared between functions,
   to pass and return values, and 8 are local.  The window is
   moved up on return, so registers are loaded or saved only at
   the top or bottom of the register stack.  This allows
   functions to be called in as little as 1 cycle.  Like some
   other RISC processors, reading global register zero always
   returns zero and writing it has no effect.  SPARC is
   pipelined for performance, and like previous processors, a
   dedicated condition code register holds comparison results.

   SPARC is "scalable" mainly because the register stack can be
   expanded (up to 512, or 32 windows), to reduce loads and saves
   between functions, or scaled down to reduce interrupt or
   context switch time, when the entire register set has to be
   saved.  Function calls are usually much more frequent, so the
   large register set is usually a plus.

   SPARC is not a chip, but a specification, and so there are
   various implementations of it.  It has undergone revisions,
   and now has multiply and divide instructions.  Most versions
   are 32 bits, but there are designs for 64-bit and
   superscalar versions.  SPARC was submitted to the IEEE
   society to be considered for the P1754 microprocessor
   standard.

   SPARC(R) is a registered trademark of SPARC International,
   Inc. in the United States and other countries.

   [The SPARC Architecture Manual, v8, ISBN 0-13-825001-4].

   (1994-11-01)