[Coco] Largest supported HDD size?

RETRO Innovations go4retro at go4retro.com
Sun Dec 11 21:59:38 EST 2016

On 12/1/2016 8:10 PM, Darren A wrote:
> On Thu, Dec 1, 2016 at 6:47 PM, RETRO Innovations wrote:
>> Is there a page that describes "no-halt"?  I think I understand the
>> concept, but clarity is great.
> The floppy disk controllers for the CoCo make use of the CPU's HALT line as
> a handshake control during floppy I/O.  Here is how the "Disk Basic
> Unraveled" book describes it:
> "Data transfer between the computer and the disk drives through the FDC is
> accomplished through a series of hardware and software 'tricks'. The slow
> clock speed of the Color Computer will not allow data to be transferred in
> the 'normal' method of getting a byte from the computer, giving it to the
> FDC and then performing status checks until the FDC is not busy. There is
> just not enough time for this when operating at double density. Part of the
> Color Computer's solution is a hardware trick whereby the disk controller
> board will allow the FDC to halt the 6809 while the FDC is storing or
> retrieving data. The halt flag will allow the DRQ signal from the FDC to
> halt the 6809 so that the 6809 will wait while the FDC is processing a data
> request. This trick will allow the 6809 to pass data to the FDC as fast as
> the FDC can take it by executing a fast loop of: grab a byte from RAM, give
> it to the FDC and loop back to get another byte. The analogous loop for
> getting data will also work."
> In reality, the 6809 at 0.89 Mhz is quite capable of running a polling loop
> to handle double-density data transfers with the FDC.  If the designer(s)
> of the CoCo floppy controller had provided a way for software to read the
> DRQ signal (other than from the FDC Status register), there would have been
> no need for the HALT trick.
> - Darren
That makes sense (as far as it explains things to me), but does the IDE 
interfaces have such a construct?  I don't see any such logic in the 
Glenside IDE, so they would all be no-halt, correct?


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