[Simh] CMP R3,(R3)+

[Paul Koning]

> On Jul 30, 2018, at 9:51 AM, Timothe Litt <litt at ieee.org> 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 :-)

Oh, ok.  Yes, for a memory test that makes sense, but now the incompatibility matters and you have to split the instruction into two.

> ...
> 
> Those constructs bring back memories... particularly of debugging such clever code that didn't have the corresponding comment.  I often worked on several machines with slightly different ideas of condition codes; switching took some effort.  Clever coding is fine - as long as you document it.
> 
> BLISS got pretty good at being clever - but never at commenting its assembler code.  Some of its contortions caused CPU architects to pause before agreeing that the code should work.  On a few occasions, SHOULD and DID diverged...

One example I remember that puzzled me the first time I saw it is CMP (SP)+, (PC)+  which is "pop a word from the stack and skip the next (one-word) instruction".

Then there is the classic "one word to write all of memory" -- 014747 dropped into the last word of memory and executed.  For extra credit, there is a one-word program that *clears* all of memory.

	paul