Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site rlgvax.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!godot!harvard!seismo!rlgvax!guy From: guy@rlgvax.UUCP (Guy Harris) Newsgroups: net.micro.68k Subject: Re: Why no INC or DEC Message-ID: <374@rlgvax.UUCP> Date: Sun, 20-Jan-85 01:11:09 EST Article-I.D.: rlgvax.374 Posted: Sun Jan 20 01:11:09 1985 Date-Received: Mon, 21-Jan-85 02:52:44 EST References: <242@talcott.UUCP> Distribution: net Organization: CCI Office Systems Group, Reston, VA Lines: 19 > Why doesn't the 68000 have increment or decrement instructions? An INC or DEC instruction would take one word to specify the opcode, size, and address mode/register, and an extra word for any offset, if any. This is exactly as many words as ADDQ #1,or SUBQ #1, would take - so it has a more general form of INC and DEC (you can increment or decrement by any value from 1 to 8 (ADDQ #0 is useless, so the instruction adds 1+<3 bits from instruction> - easy to do by just jamming the input carry bit high in the adder). > I know that the 68000 was designed (at least to some degree) with the > spirit of RISC, Say what? Yes, its instruction set is smaller than the VAXes, but then so is the instruction set of a PDP-11 or IBM 370 (excluding microcode assists, etc.). Guy Harris {seismo,ihnp4,allegra}!rlgvax!guy