Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!unmvax!ncar!ames!lll-tis!oodis01!uplherc!esunix!bpendlet From: bpendlet@esunix.UUCP (Bob Pendleton) Newsgroups: comp.arch Subject: Re: OISC was Re: ZISC computers Message-ID: <1120@esunix.UUCP> Date: 29 Nov 88 15:21:52 GMT Organization: Evans & Sutherland, Salt Lake City, Utah Lines: 45 Several years ago I also came up with the one instruction computer that everyone else has also invented. Since I went to a school that Ivan Sutherland taught at I must assume I picked up the idea as it floated past, but I really thought I invented it, sigh. My variation of the one instruction was the Conditional Move From Here To There And Clear Flags or CMFHTTACF instruction. The idea being that if any flags are set the instruction is skipped, in all cases the flags register is cleared. All instructions are two addresses, a source and a destination. The PC, the flag register, the flags mask register, and all ALU registers are mapped to memory locations. And, of course, some operations have the side effect of setting bits in the flags register. Using this machine as a basis for argument I "showed" that there were good economic reasons, the cost of RAM and such, to try to reduce the length of the addresses in the CMFHTTACF instruction. If your address space is 2^32 then the CMFHTTACF instruction is 64 bits long. The easiest way I could think of to reduce the length of the instruction was to use two address spaces. One very small address space that only had enough addresses to map the ALU registers, the PC, and such. One large address space for memory. And, to add a one bit direction flag. To tell if the move was from the large address space to the small or from the small to the large. This looked suspiciously like the opcode, register index, and address type of instructions we see in typical computers. Hmmm. Well, repeated applications of the cheapness principle rather rapidly resulted in a multilevel machine that looked an awful lot like a a sort of RISC with microcoded coprocessors. This exercise lead me to the conclusion that all modern computers are economically motivated variations of the CMFHTTACF machine. :-) The odd things that go through the minds of grad. students while trying to write a thesis. Bob P. -- Bob Pendleton, speaking only for myself. UUCP Address: decwrl!esunix!bpendlet or utah-cs!esunix!bpendlet Reality is what you make of it.