On Mon 11 Mar 2002 at 08:11:34 -0800, Eric Smith wrote:
> Might as well just document the sequence of instructions NetBSD uses for
> the busy wait.  Assuming it's short and simple, SIMH can easily be
> modified to detect it and DTRT.

That's one thing that IBM's VM/370 did (VM/370 is the operating system
that virtualises IBM 370-family computers on 370's. In very great
detail). It detected for instance a common busy-wait loop on a TIO (Test
IO) instruction. Another thing it did was implement the SVC (SuperVisor
Call) instruction, which was always documented as reserved for things
like field-service diagnostics. The SVC would be like a system call in a
"normal" operating system.

The hardware that VM/370 was emulating was also more flexible that SIMH.
For instance you could break to CP (control-program) level and issue a
command like (I have forgotten the exact syntax) DEFINE 191 DASD 3380 20
to define a DASD (direct access storage device) of hardware model 3380,
with a size of 20 cylinders, at IO channel 191. There would be no
pre-set limit on the number of disks or printers or card punches you
could define. Interesting bit: most communication with the outside world
went via punched cards. Incoming mail was put in your card reader,
outgoing mail was punched and then transfered to the reader of RSCS, the
Remote Spooling and Communications Subsystem, another virtual machine,
which took care of passing it on.

One thing SIMH maybe could do is change the meaning of an uncommon sort
of instruction that's effectively a No OPeration, which could be
inserted in the busy-wait. SIMH could treat it like a WAIT instruction.

-Olaf.
-- 
___ Olaf 'Rhialto' Seibert - rhialto@polder --They that can give up essential 
\X/ land.nl --liberty to purchase a little temporary safety
--------------deserve neither liberty or safety. - Benjamin Franklin, 1759
---People will accept your ideas much more readily if you tell them that
---Benjamin Franklin said it first. - Unknown