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From: Stephen Fuld <sfuld@alumni.cmu.edu.invalid>
Newsgroups: comp.arch
Subject: Re: Why VAX Was the Ultimate CISC and Not RISC
Date: Tue, 11 Mar 2025 13:46:24 -0700
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On 3/11/2025 11:56 AM, MitchAlsup1 wrote:
> On Tue, 11 Mar 2025 18:15:06 +0000, Stephen Fuld wrote:
> 
>> On 3/11/2025 10:44 AM, MitchAlsup1 wrote:
>>>>> When you do change names, can you spell LD and ST instead of MOV ??
>>>>
>>>> Yes, please LD / ST it is so much clearer what is going on. Less 
>>>> trouble
>>>> getting confused by the placement of operands.
>>>>
>>>> I always put the memory operand second, which breaks the pattern of
>>>> having the destination operand first. Otherwise the destination is
>>>> first.
>>>>
>>>> I go cross-eyed reading code that is a whole lot of moves.
>>>
>>> I agree.
>>
>> I wonder if the different preferences is at least partially due to
>> whether the person has a hardware or a software background?
> 
> Even when both LD and ST are written MOV there is a different OpCode
> for the inbound MOV versus the outbound MOV, so, in effect, they are
> really different instructions requiring different pipeline semantics.
> 
> Only (O N L Y) when one has a memory to memory move instruction can
> the LDs and STs be MOVs. VAX had this, BJX* does not.
> 
> One should argue that different pipeline semantics requires a different
> OpCode--and you already have said OpCode having different bit patterns
> and different signedness semantics different translation access rights
> , ... At the HW level about the only thing LD has in common with ST is
> the way the address is generated--although MIPS did something different.

You are making my point.  No software guy talks about "pipeline 
semantics" :-)  Note that I am not saying you are wrong, just noting the 
difference.



> 
>>                                                         The idea is
>> that when hardware guys see the instruction, they think in terms of
>> register ports (read versus write), what is required of the memory
>> system (somewhat different for loads versus stores), etc.  However
>> software guys think of a language construct, e.g. X = Y, which is
> 
> MARY and MARY2 used X = Y to mean the value in X is deposited into Y.
> Both were left to right only languages. This should surprise most !!
> {{Although to be fair, Mary used the =: operator to perform assign.}}

And see my point about COBOL in the post above.


-- 
  - Stephen Fuld
(e-mail address disguised to prevent spam)