Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!sdd.hp.com!spool.mu.edu!snorkelwacker.mit.edu!bloom-beacon!eru!hagbard!sunic!dkuug!imada!micro
From: micro@imada.dk (Klaus Pedersen)
Newsgroups: comp.sys.atari.st
Subject: Re: The cause of the phantom typist
Message-ID: <1991Apr3.184636.7591@imada.dk>
Date: 3 Apr 91 18:46:36 GMT
References: <3865050@janhh.hanse.de>
Sender: news@imada.dk (USENET News System)
Organization: Dept. of Math. & Computer Science, Odense University, Denmark
Lines: 57

jan@janhh.hanse.de (Jan Willamowius) writes:

> struct { /* GEM AES storage segment */
>   ..
>   struct tRec /* application recording as described in Appl_tRecord() */
>   { void(* t_who)(); /* routine */
>   long   t_how;     /* arguments */
>   } recs[32];
>   ..
> }

>>   void /* enqueue input for forker */
>> forkq( void(* who)(), long how)
>> { reg struct tRec* rec;
>>   extern fpcnt, fpt; /* fork-pipe-tail index */
>>   if( fpcnt < 32)
>>   { rec= (long )((fpt++ << 3) + &D.recs);
>>     if( fpt == 32) fpt= 0;
>>     rec->t_who= who;
>>     rec->t_how= how;
>>     fpcnt++;
>> } }
> (copyright by Digital Research Inc, 1985, April 15 (!))
> What happens if forkq() is called non-interrupt, the fpt index is at 31 and
> the code gets reentered (interrupted) between fpt++ and if( fpt == 32) ..?

You might be right there...

> To exorcise one may
> 1) Replace expressions like ++i; if(i == ANYPOWEROFTWO) i= 0;
>    by ++i &= (ANYPOWEROFTWO - 1)

????? in what language do you want to write that ?????

What you are writing is :
    ++i = ++i & (APO2-1);
 - no matter how you try to hide it...

But ok, I understand what you mean - what you want is a interlocked add cyclic.
  IAC 1,APO2,i  - but no compiler can generate that to a 68K. Some
signal processors can (and they are not restricted to powers of 2 (Analog 
Devices).


> 2) make sure the interrupts are disabled whenever forkq() is called
>    (from chkkbd(), mchange() and ap_tplay())

This is a lot easier than making a new CPU with new instructions and a new
C-like compiler that can generate them...

> 3) recompile, assemble and link the system.

might be hard to do with '1)'...

---

Klaus  (micro@imada.dk)