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Message-ID: <8701290526.AA02333@ucbvax.Berkeley.EDU>
Date: Thu, 29-Jan-87 00:39:08 EST
Article-I.D.: ucbvax.8701290526.AA02333
Posted: Thu Jan 29 00:39:08 1987
Date-Received: Fri, 30-Jan-87 01:44:45 EST
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Subject: Re: Argument lists for system services

The reason for a fixed/required number of arguments for the system services
is efficiency. As they are called hundreds or thousands of times per
second, the necessary access checking of the argument list and the
arguments themselves must be fast. For that reason, the CMKRNL/CMEXEC
handlers have a fixed array indexed by CMKRNL/CMEXEC number, which
contains the required number of arguments. The actual number of arguments
(the byte at (AP)) is checked against this value. Then, the argument
list is PROBERed using the value from the array. A system service is
therefore guaranteed to be able to look at its argument list as it
expects without risking a crash.

(For the same efficiency reason, only 'simple' string descriptors are
supported.)

By the way, a 0 in the argument list does not always indicate an optional
argument: For parameters passed by value (most notorious: QIO's EFN),
you get that value (which you could call a default, if you want).
If you do a number of asynchronous I/O's using that default EFN
and then wait on it, a n y  I/O terminating will set that EFN, and you
have to check the IOSBs to find out which one it was (that's what
they're there for).