Re: Odd PSW Address does not trigger a specification exception.

[Harold Grovesteen]

On Tue, 2023-01-31 at 10:24 +0100, Thomas Huth wrote:
> On 30/01/2023 18.31, Harold Grovesteen wrote:
> > While doing some testing, I encountered the situation where an odd
> > PSW
> > address is introduced into the active PSW by means of a BC
> > instruction.
> > The odd address is used by Qemu for successful instruction
> > fetching.
> 
> Do you have, by any chance, a small test program that allows to
> reproduce 
> the wrong behavior?

Yes, of course.  It is an ELF used with the -kernel command line
argument.

> 
> > I am using qemu-7.2.0-rc4. I have noticed in this list some patches
> > on
> > or about 9 Jan 2023 related to displacement addresses and the
> > introduction of "gen_psw_addr_disp". The version of qemu I am using
> > does not contain that function, so I can not validate whether it
> > addresses the odd instruction address I encountered in my testing.
> 
> I assume that's this patch here:
> 
>   https://lists.gnu.org/archive/html/qemu-devel/2023-01/msg01591.html
> 
> ?
Yes.
> 
> ... it hasn't been merged yet, I think, but it also doesn't look like
> there 
> is any code in there to deal with odd addresses, as far as I can see
> at a 
> first quick glance.
> 
>   Thomas
> 
> > z/Architecture Principles of Operation manual -13 on page 5-13
> > states:
> > 
> > "A specification exception due to an odd branch address and access
> > exceptions due to fetching of the instruction at the branch
> > location
> > are not recognized as part of the branch operation but instead are
> > recognized as exceptions associated with the execution of the
> > instruction at the branch location."
> > 
> > The specification exception was not recognized at all. Rather the
> > instruction coded at the odd address, LGFI, was fetched and
> > executed.
> > 
> > It was not until the X'00', also at an odd address, immediately
> > following the LGFI was itself fetched for execution that the
> > program
> > ceased operation with an operation exception program interrupt.
> > 
> > I am hoping that this is sufficient information to determine if
> > this is
> > a known reaction by Qemu to this condition and whether the new
> > patches
> > will address this.  If neither is the case, FYI.
> > 
> > If you require details, please contact me here or off list.
> > 
> > Thanks,
> > Harold Grovesteen
> > 
>