[Coco] Re: Re: Questions about the 502 controller vs theDisto SCII

[L. Curtis Boyle]

On Thu, 27 Jul 2006 00:38:01 -0600, Warren Hoslet <dermunda at hotmail.com>  
wrote:

>>> How is ho-halt implemented by the SCII? Does it perform disk I/O from  
>>> a  buffer on the controller card without requiring the 6809 to handle  
>>> the  timing-critical task. If the 6809 has to do the work I would  
>>> think that  it would still have to mask interrupts which would prevent  
>>> keyboard  type-ahead. Since I have no technical knowledge of the SCII,  
>>> I'm just  speculating.
>>>
>>> - Warren
>>
>>     The controller does the sector transfer on it's own to an internal   
>> buffer; the Coco then reads the entire sector's worth of data in one   
>> chunk, with interrupts still functioning. Quite a few of us tried to  
>> get a  native mode 6309 driver for the floppy controller to be able to  
>> read the  controller live without having to enable the HALT line, but  
>> we couldn't  quite get it fast enough.
>
> It isn't too hard to do sector I/O without using the HALT line, but you  
> still need to mask interrupts so I'm not sure what the point would be.

    The main point is that when using the HALT line, it will not latch an  
IRQ source. Masking IRQ's off will still latch an IRQ that you can process  
once you turn them back on. With HALT, they are lost forever (unless the  
IRQ device itself holds it for you).

> I  implemented a read routine that works (with limited testing) on a  
> CoCo 2 with an FD-501. You just have to monitor the DRQ bit in the  
> status register ($FF48) to know when a byte is available. The trick is  
> that after a certian number of cycles have elapsed without a DRQ, you  
> must assume the transfer is complete and stop testing the status  
> register so that you don't clear the INTRQ request when it comes. Below  
> is just the read loop portion of my routine. The DP register is set to  
> $FF, and B contains the mask for the DRQ status bit.
     On read, yes, this does work.  It's on write that we can't get it to  
fully keep up. I believe myself, Robert Gault and Alan Dekok all tried,  
and we just couldn't quite make it.
>
> getbyt  LDA     <$4B      Get byte from FDC
>         STA     ,X+      Put into buffer
>        	NOP
>        	NOP
>        	BITB    <$48    Check FDC status for DRQ
>         BNE     getbyt   Get next byte if available
>        	BITB    <$48    Check again
>         BNE     getbyt
>        	BITB    <$48    Check one last time
>         BNE     getbyt
>
> * At this point assume there are no bytes remaining  (timing is  
> important)
>         SYNC            wait for NMI interrupt
>
>
>



-- 
L. Curtis Boyle