On 05/22/2016 12:03 PM, Lennart Sorensen 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
On Sat, May 21, 2016 at 02:48:50PM -0400, James Knott wrote: 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.