[Simh] CMP R3,(R3)+

[Johnny Billquist]

On 2018-07-30 15:51, Timothe Litt wrote:
> On 30-Jul-18 09:30, Paul Koning wrote:
>> Yes, that is the standard way to do this.  I have never seen the code you quoted before and I can't imagine any reason for doing that.
> A memory address test's verification pass.  Check that  memory contains 
> address of self. Of course, you need a
> 
>      bne fail
> following the compare :-)

Not to mention that it will succeed or fail depending on which PDP-11 
model you run the code on? :-)

Just as Paul, I have never seen anyone actually do CMP R3,(R3)+. Most 
assemblers will give a warning just because the actual values used will 
vary depending on model.

>> Either option of course only works if R3 contains a valid memory address, and it must be even.
> I should have noted that "valid memory address" includes "even" for 
> words.  But if the code provided works on any 11 (obviously, not the 
> 11/20), that constraint is met.

I thought about that one for a moment as well, but since the results of 
the compare itself was obviously irrelevant, using a TST instead of a 
CMP does not make the code any more error prone than before.
The CMP would have barfed on an illegal or odd address already. Using a 
TST will cause the same effects, except that condition codes will be 
different at the end. In all other aspects they produce the same result.

   Johnny

-- 
Johnny Billquist                  || "I'm on a bus
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