Re: "Hold" operation for A-datapath

Ed Casas (edc@ece.ubc.ca) Tue, 7 Nov 2000 08:56:37 -0800 

Date: Tue, 7 Nov 2000 08:56:37 -0800
From: Ed Casas <edc@ece.ubc.ca>
Subject: Re: "Hold" operation for A-datapath



> Do we really need a "hold" operation for the A-datapath, which
> select the previous value of "a" for the current "a" value?

Yes.

> Isn't it only needed only when the decoder is acting as a state
> machine, not as in this case where it is a combinational logic
> circuit?

Some instructions (e.g. branches) don't alter the value of the A
register.  So a "hold" function is essential for the A datapath.

This question was asked previously about the PC datapath.  The
difference between A and PC is that every single instruction
alters PC so, in principle, there's no need for a "hold"
operation.

> If we need it, how are we suppose to set the condition (in the
> decoder) for this operation?

If an instruction doesn't alter A, then the decoder should give
the A datapath the "hold" function.

-- 
Ed Casas   edc@ece.ubc.ca   http://casas.ece.ubc.ca   +1 604 822-2592