Path: utzoo!mnetor!uunet!husc6!mit-eddie!uw-beaver!cornell!batcomputer!pyramid!voder!apple!bcase
From: bcase@apple.UUCP (Brian Case)
Newsgroups: comp.arch
Subject: Re: Condition Codes in General Registers
Message-ID: <7446@apple.UUCP>
Date: 19 Feb 88 18:42:42 GMT
References: <191@telesoft.UUCP> <1556@gumby.mips.COM> <208@telesoft.UUCP> <868@ut-emx.UUCP> <682@l.cc.purdue.edu>
Reply-To: bcase@apple.UUCP (Brian Case)
Organization: Apple Computer Inc., Cupertino, USA
Lines: 24

In article <682@l.cc.purdue.edu> cik@l.cc.purdue.edu (Herman Rubin) writes:
>Let me make some additional suggestions apropos of Crowl's posting.
>It should be possible to address an arbitrary byte of a register
>(this is useful for other purposes).  One way of doing this is to
>make a small address a reference to the register file; on virtual
>machines this would be costless, and the amount of memory lost on
>non-virtual machines would be negligeable.  Some machines have done
>this, but I do not believe that they have done it well; either this
>can only be used in certain situations, or the user's registers are
>in blocks and/or separated from main memory.  If we do this, we can
>keep many condition codes active for transfers.

Mapping the register set of a machine into the memory address space of a
simple, pipelined machine is difficult and costly (in space and time)
to implement and, I staunchly claim, vitually useless (except in the case
of the CRISP where the concept is central to the architecture, but in that
case there "is no register file.").  Believe me, even if your machine did
allow memory-mapped, byte access to the register file of a fast, modern
processor, you wouldn't use it because it would be so much slower than
any other kind of register access (you can't know it is really a register
access until the address is computed (and maybe translated), but the
register file is way back there in pipeline stage two, so the machine has
to sit and wait while you insert at least one bubble to perform the
unexpected (to the lock-step pipeline) register access).