Path: utzoo!mnetor!uunet!husc6!mailrus!ames!amdahl!dlb!ardent!rap
From: rap@ardent.UUCP (Rob Peck)
Newsgroups: comp.sys.amiga
Subject: Re: assembly use of gameport.device
Message-ID: <316@ardent.UUCP>
Date: 18 Feb 88 18:51:04 GMT
References: <8802160312.AA22247@jade.berkeley.edu>
Organization: Dana Computer, Inc., Sunnyvale, CA
Lines: 39

In article <8802160312.AA22247@jade.berkeley.edu>, BBOURBIN@UMDD.BITNET (Brett S Bourbin) writes:
> Last question:  What is the difference between using a:
> 
>                    DoIo( IO_Message) ;
> 
> or using :
> 
>                    SendIO( IO_Message ) ;
>                    WaitPort( MsgPort );
>                    GetMsg( MsgPort );
>

DoIO is a synchronous command.  Your task sleeps while the I/O is
happening.  To paraphrase the RKM, DoIO "internally uses" the
ReplyPort IF THERE IS ONE, meaning that it sends the I/O message to
the device, waits for the reply to be received, and removes the
message from the port without your task ever being aware that this
has happened.  But there does not have to be a ReplyPort specified
for the I/O message (if I remember RKM correctly).

SendIO is asynchronous.  You start the I/O then can go on to do
other things.   The example given works if and only if the port
begin waited on is that which is specified in the mn_ReplyPort.
Only when you feel you cannot do anything more
until the I/O completes do you need to use WaitPort.  If the
I/O is done, no wait occurs - the next instruction gets executed
right away.  In the example provided, it is GetMsg, that removes
the message from the port.  (If you fail to remove the message,
then send it off a second time, it is likely that you will lock
up the system - endless loop of linking a list node onto its own
tail as I recall).   

If you provide no ReplyPort in the message when using SendIO, then
you can tell that the I/O has completed by using CheckIO(IO_Message).
I forget exactly what this function returns, but it effects a polling
function on the message.  Or use WaitIO(IO_Message) that will put
your task to sleep until the I/O finishes.

Rob Peck				...ihnp4!hplabs!ardent!rap