Re: Multi-Tasking on Apple IIgs?

["Bryan Parkoff" <nospam@nospam.com>]

Michael,

    You got the point that interrupt is required.  Have you thought the idea 
that 65xx processor should have its own timer?  It is like external 
controller card such as clock or timer controller card.  The clock or timer 
controller card can interrupt 65xx processor before it stores all registers 
including PC registers into stack and then go to another application.  It 
should do the same for third application and worth so further.  It goes back 
to restore registers and PC register from stack to run first application and 
so forth.
    Am I right?  Do you think that it is done through NMI request?

Bryan Parkoff

"Michael Black" <et472@FreeNet.Carleton.CA> wrote in message 
news:d0q6ej$div$1@theodyn.ncf.ca...
>
> "Bryan Parkoff" (nospam@nospam.com) writes:
>>     Is there a way to write assembly language that can run two 
>> applications
>> at same time as multi-tasking?  It has to run first application and then
>> switch to run second application at the same time.  It may be the Program
>> Counter register that it can be modified.  I don't know how like Intel 
>> did.
>>
>> Bryan Parkoff
>>
>>
>
> Any microprocessor can multi-task.  Of course, if you have limited memory
> it can be troublesome, since you either can't do much, or you have to
> move stuff in and out of memory.  You generate an interrupt, which causes
> the CPU to deal with it, saving the register values and then doing the
> something else.  Do it fast enough, and it looks like everything is
> working at the same time.
>
> Microware had a multi-tasking monitor rom for the 6800, before going
> on to create OS-9 for the 6809.  That was an 8-bit CPU, albeit better
> than the 6502 or 6800.
>
> Multi-tasking is not really a complicated thing, though much of the time
> when it's added we are seeing a much fancier operating system which makes
> it seem like multi-tasking is complicated.
>
> The real issue in making an existing computer multi-tasking is that you
> can't start with a clean slate.  It may be problematic to impose it
> on an existing operating system or ROM monitor, and making sure that
> it doesn't affect any application running on top.  If you are writing
> from scratch, you write everything so it doens't do weird things (like
> self-modifying code).
>
>  Micahel
>