[Simh] CMP R3,(R3)+

[Timothe Litt]

On 30-Jul-18 11:45, Johnny Billquist wrote:
> 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? :-)
Oddly enough, we did have quality control, and it usually worked.

Diagnostics are often CPU-specific.  This was fixed after the 11/20.
I might have done this - it represents a 40% savings in instructions for
the loop:
                  ;11/20 safe                     Many other
11s          Some other 11s
    10$: movr3, r0   10$:  cmp r3,(r3)+     10$: cmp r3,(r3)+
                   cmp r0, (r3)+                 bne    fail 
                           bne fail
                    bne  fail                         sob  r1,
10$                        dec r1
                    dec 
r1                                                                        
bne 10$
                    bne  10$

20-40% reduction in instructions of an inner loop at boot time is worth
a runtime check
for the 11/20 - if I cared (e.g. a BIST is probably in
processor-specific ROM, so no need to check).


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