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From: phil@sivax.UUCP
Newsgroups: net.micro.mac
Subject: Re: Interrupts on the Mac
Message-ID: <253@sivax.UUCP>
Date: Fri, 23-May-86 14:26:30 EDT
Article-I.D.: sivax.253
Posted: Fri May 23 14:26:30 1986
Date-Received: Fri, 30-May-86 05:05:46 EDT
References: <365@ur-tut.UUCP>
Distribution: net
Organization: System Industries, Milpitas, Ca
Lines: 43

> 
> Hi folks,
> 
> I'm hoping that someone out there will be able to help me with a problem I'm 
> having. I'm trying to bypass the serial driver and work directly with the SCC.
> To this end I've written code which will allow me to read and write directly to
> the SCC. I can send data out or receive it at any baud rate as long as I poll 
> the SCC. Now I want to be able to use interrupts to drive my code.  However, 
> I can't seem to get the interrupts to act the way I think they do, so there 
> must be something I'm missing. First I'll explain how I think this mess works 
> and then I'll describe what happens when I execute my code.
> 
> As I understand it there are seven levels of interrupts on the 68000. The SCC 
> will interrupt using vector 2. When a level 2 interrupt occurs the PC is loaded 
> with the value at location $68. The code which is executed then determines why 
> the SCC interrupted the processor and does a jsr to a value located in the 
> Lvl2DT table. A displacement of 24 bytes into this table holds the address of 
> the code to be called if a character has been received on port a. As I 
> understand it all i should have to do is load that table with the address of 
> my code and set up the SCC to interrupt on receive character available.
> 
> Here's how things work now. I have TMON installed, so that I can stop the 
> execution of my code at any point. In order to catch the interrupts that I'm 
> interested in I replace the level 2 and 3 interrupt vectors at locations $68 
> and $6c to point to my code. All my code does is jmp to the locations that 
> were originally in those two vectors. I do this so that I can set break 
> points at the jumps, so that every time a level 2 or 3 interrupt occurs I 
> fall into TMON. I know this works because every time I move the mouse I drop
> into TMON. However, when I send a character to the modem port my Mac hangs up. 
> I'm at a loss to understand why this happens, any help would be appreciated.
> If anyone has a section of code that does what I need that would be great.
> 
> 
> 				Thanks
> 					Tony Giaccone

I tried to do it too, and got about as far as you did.  My program had to be
content with polling.  Using interrupt processing seems to be a little more
complicated because of the ROM drivers trying to get the interrupt anyway.

If you poll with interrupts off, it works fine.

Phil Hunt