On 05/22/2016 12:03 PM, Lennart Sorensen wrote: > On Sat, May 21, 2016 at 02:48:50PM -0400, James Knott wrote: >> Many years ago, I used to maintain Data General Eclipse systems. The >> CPU used microcode to control AMD bit slice processors and associated >> logic. The microcode instructions were over 100 bits wide. Now >> *THAT'S* RISC. ;-) >> >> BTW, those CPUs had an option called Writable Control Store (WCS) where >> one could create custom instructions. > That sounds more like the opposite of RISC. Much more like VAX or > mainframes used to be as far as I know. Maybe even VLIW, although > probably not.
I also used to work on VAX 11/780 systems back then. With the VAX, the microcode was loaded from an 8" floppy at boot. There were occasional updates for it. I suppose one could also write custom instructions for the VAX (BTW, I'm not an anti-VAXer <g>). I said "RISC" because the core of the CPU was bit slice processors, which were very simple devices, providing basic arithmetic & logic functions and the microcode controlled them, along with some glue logic. > > Now being able to define new instructions using low level RISC features > might make some sense, although how much the savings would be in execution > time or binary size I don't know. I have a hard time imagining much > gain there. > I was thinking it might be used for specific areas. For example, many years ago, computers were built for business and ran COBOL or for science & engineering, with FORTRAN. Back then the technology was so primitive that what we now call a general purpose computer was not practical. So, there may be some instructions that could be added for better performance in certain applications. Incidentally, a few years ago, I read a book about IBM's early computers and the design decisions made for business vs science/engineering computers. Back then business computers worked with some form of decimal digits, but S&E used floating point. --- Talk Mailing List [email protected] https://gtalug.org/mailman/listinfo/talk
