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From: crickman@umn-cs.UUCP (Robin Crickman)
Newsgroups: net.arch
Subject: Re: Re: Addressing modes
Message-ID: <890@umn-cs.UUCP>
Date: Sat, 22-Feb-86 21:57:16 EST
Article-I.D.: umn-cs.890
Posted: Sat Feb 22 21:57:16 1986
Date-Received: Wed, 26-Feb-86 07:38:27 EST
References: <187@anwar.UUCP> <1441@gitpyr.UUCP>
Organization: Computer Science Dept., U of Minn, Mpls, MN
Lines: 21

> function, and we thought about conditionality.  First of all, why can't a jump
> be treated as a special case of LOAD/STORE/MOVE, because most machine implement
> it that way anyway (MOV 1200, PC).  Of course, that makes conditional jumps
> impossible.  So, just set one (or two) bits of every instruction aside to make
> it possible to conditionally execute any instruction.  Is it worth the trouble
> to decode it?  It could be treated so that as the instruction is decoded, the
> condition field is checked, and if the condition is not true, the instruction
> is aborted (it should only take one machine cycle to check the flag, right?).
> For one-cycle instructions, they would have to be executed only if the condition
> was "always execute", otherwise they would have to be delayed a cycle for the
> condition field to be decoded.  Is it worth it?  Does anyone care?
> 
> Scott Dorsey
> ICS Programming Lab, Georgia Insitute of Technology, Atlanta Georgia, 30332

How about a conditional SKIP instruction (if TRUE, skip the next opcode, else 
execute it), which was the only kind of conditional instruction on the pdp-8?
I believe the pdp-8 used the sort of jump described above.  Was it a RISC 
machine?  It certainly had a reduced instruction set!

John Hasler (guest of ...ihnp4!umn-cs!crickman)