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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>
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Organization: CCI Office Systems Group, Reston, VA
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> 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