[Info-vax] Pointless Nostalgia: System Service Dispatching

[Michael Moroney]

Johnny Billquist <bqt at softjar.se> writes:

>On 2016-06-30 01:05, lawrencedo99 at gmail.com wrote:
>> On Wednesday, June 29, 2016 at 11:36:10 PM UTC+12, Johnny Billquist wrote:
>>> Consider a $QIOW for example. If you get an AST, you do not want the $QIOW
>>> to be reexecuted.
>>
>> That'™s not a problem, because there is no actual $QIOW system call.

>There isn't? Did I place the $ on the wrong side or something now? Or 
>what are you suggesting?

$QIOW (and most $xxxW services which also have a corresponding $xxx service)
are implemented as a $QIO followed immediately by a $SYNCH of the specified
event flag/IOSB pair.  I believe there is a stub routine that makes both 
calls.

I believe only $HIBER can potentially be re-executed.  It does (at least on 
VAX) some funky stack manipulation to place a call to the internal routine
back on the stack rather than the following instruction.  The called
routine determines whether to actually hibernate based on internal states.