From: ptkwt@... (Phil Tomson)
Date: 2004-07-16T15:52:16+09:00
Subject: Re: Do you want ":=" operator?

In article <cd7qpj$7cc@odbk17.prod.google.com>,
Xiangyu Yang <xiangyu.yang@gmail.com> wrote:
>It seems that I need more words to explain my CPU model. I'll show why
>it is *impossible* to impletement *register assignment* by "=".

Quite true, you would lose all your 'future' (as in events scheduled 
to happen in the future).  I mistakenly used '=' in my example whereas I 
should have used the hypothetical ':='

>A HW register updates its output to its input value only when an event
>occurs (typically a clock rising/falling edge). My CPU can process
>serveral instrucions at the same time. For example, "R0=R0&R1; R2=R0"
>means that, at next clock period, R0 will get the AND result and R2 the
>current R0 value (not same as AND result). To support this feature,
>registers must simulate the true HW properties like this:
>
>class Reg
>attr_accessor :value, :new_value, :data_enb
>
>def initialize()
>@value=@new_value=0
>@data_enb=false
>end
>
>def assign(data)
>@new_value = data
>@data_enb=true
>end
>
>def update
>@value, @data_enb=@new_value, false if @data_enb
>end
>
>end
>
>"R0=R0&R1; R2=R0" will actually does "R0.assign(R0&R1);
>R2.assign(R0.value)" (suppose Reg.& defined), and at later time all the
>regs' update() will be called to simulate the clock event.
>In short, Rx does not just represent a value, but a real register
>*object*, so the variable Rx should always reference to the same
>object. That's why "=" cannot be used despite of any forms.
>What I want is an *object modification operator* ":=" provided by
>syntax to save the translation effort, which is not so easy to do in
>complex cases. What a nice thing if my CPU program "R0:=R0&R1; R2:=R0"
>is just ruby program!

Indeed.  I understand exactly, but I suspect there are about 3 of us on 
this list who have done hardware design (or used HDLs).  I think perhaps 
we're having a hard time conveying what we're trying to do and how a ':=' 
operator could really help.  Maybe we need to come up with a more general use 
for a ':=' operator that everyone can relate to.  I'm sure some examples exist.

BTW: It's been mentioned, but you might find that RHDL would save you a 
lot of work since it already takes care of event scheduling, process 
launching based on sensitivity lists, concurrency (using continuations), 
etc.  The lack of a ':=' operator aside, Ruby's mix of features is really 
great for hardware modelling.  It would have been very much more difficult 
to implement something like RHDL in Perl or Python, for example (no 
continuations or code blocks).

Phil