[Info-vax] Native compilers

[Johnny Billquist]

On 2022-03-08 20:50, chris wrote:
> On 03/08/22 15:10, John Reagan wrote:
>> On Monday, March 7, 2022 at 7:46:02 PM UTC-5, chris wrote:
>>> On 03/05/22 18:05, John Reagan wrote:
>>>> MOVAL (R3)+,(R3)+
>>> Again depends on when the autoincremnt happens in the sequence...
>> VAX architecture defines the behavior exactly.  The human brain?  Not 
>> so much.  :)
>>
> 
> As one would expect and pdp11 and 68k worked the same way, as
> it's written in fact...

Well... No. This was one of the major problems of the PDP-11, and one 
reason the VAX have all these specifications.

The PDP-11 handbooks instead have large tables with differences between 
different implementations. Lots of undefined behavior. Not nice.

Of course, it is not that you have someone write code that actually use 
these undefined sequences very often, but it do happen.

This specific example don't exist on the PDP-11 however, as there is no 
MOVA instruction (but I sometimes wish I did have one...). But the one 
John had his problem example on, which was MOVL (R7)+,R7 is, I believe 
an example of where on the PDP-11 you might be in trouble.

   Johnny