Evaluating Hardware/ Software Tradeoffs - Hardware extensions and the compilers that make them real

Visual/Math ISAs

Someone once noted that many applications manage to turn 32-bit microprocessors into 8-bit processors. Unfortunately, that's true for many standard programming tasks. Examples include: looking for the EOF character in a text block; or searching for a specific character in a string. These operations involve a 32-bit processor searching for an 8-bit or character value, one byte at a time. Yes, you can speed it up by doing AND compares and testing on matches, but it still takes processing time.

Another processing sink for inefficient cycles is 8-bit or 16-bit pixel processing on a 32-bit or 64-bit data path. Rarely can such processing use the full datapath bandwidth of the CPU. Instead it throttles down, processing to the graphic field size, and wastes datapath resources.

The idea behind the visual/graphics ISA extensions is to perform multiple field operations across the datapath width. Thus, for a 64-bit system, one instruction can do eight 8-bit adds, compares, or logical ANDs, for example. That is an 8X speed up.

Sun Microsystems was the first of the major RISC design houses or chip vendors to implement a "Visual Instruction Set" (VIS) with v9 of its SPARC architecture specification. V9, and VIS, first appeared in UltraSPARC, Sun's 4th generation superscalar, high performance RISC. Rumor has it that Sun developed VIS as a counter to Silicon Graphics and that, internally, the project went under a code name of "SGI" which stands for "Sun Graphics Instructions" in reality. Cygnus Solutions was the first to produce a commercial compiler for V9 processors. Others have followed. MIPS, for example, has the MDMX extensions, and Intel (along with competitors) has introduced its MMX extensions.