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From: geoff@desint.UUCP (Geoff Kuenning)
Newsgroups: comp.sys.m68k,comp.sys.intel
Subject: Re: Recent Motorola ad seen in Byte
Message-ID: <658@desint.UUCP>
Date: Tue, 28-Apr-87 02:37:46 EDT
Article-I.D.: desint.658
Posted: Tue Apr 28 02:37:46 1987
Date-Received: Thu, 30-Apr-87 04:39:42 EDT
References: <932@intsc.UUCP> <652@desint.UUCP> <678@edge.UUCP>
Reply-To: geoff@desint.UUCP (Geoff Kuenning)
Distribution: comp
Organization: Interrupt Technology Corp., Manhattan Beach, CA
Lines: 33
Xref: mnetor comp.sys.m68k:429 comp.sys.intel:213

In article <678@edge.UUCP> doug@edge.UUCP (Doug Pardee) writes:

> Picking nits:
> 
> > 	The PDP-11 doesn't have any registers with
> > 	wierd private characteristics.
> 
> I'm no expert on the 11, but aren't registers 6 and 7 the Program Counter and
> Stack Pointer?  Not that this voids the argument -- the '86s are much more
> restrictive with their weird registers.

Well, technically that's true.  The PDP-11 PC has the "special characteristic"
that it autoincrements after an opcode fetch (not, however, after an
operand-address or immediate-operand fetch -- see below).  The SP has two
special characteristics:  the JSR/RTS instructions use it as in implied
register, and the interrupt/RTI operations do the same.

As to the PC, DEC has a patent on "PC as a general register".  By making
the PC just another general register, you lose one register, but gain
a whole bunch in instruction-encoding simplicity.  For example, an immediate
operand is handled by just encoding the operand as (PC)+ -- the operand
is plucked from where the PC points, and the PC increments over it to
the next instruction stream element, nice as you please.  I predict that
when DEC's patent expires, you will see this feature in a lot of other
computers.

However, it is worth noting that *every* computer has "special registers"
of various sorts.  Somebody at Tek mentioned MMU registers, which I consider
a red herring.  There's also the PC and the PSW.  My point was the Intel's
GENERAL registers have special characteristics, and thus aren't really
general.
-- 
	Geoff Kuenning   geoff@ITcorp.com   {hplabs,ihnp4}!trwrb!desint!geoff