[Coco] megaread HOW TO ?

[Dave Philipsen]

So I think there are two issues at play here. The keyboard is scanned by an interrupt driven routine under OS9. 

1) If you halt the CPU, you can deny it the ability to service interrupts because it could be that an interrupt comes in while the CPU is halted. 

2) If you mask interrupts you also obviously prevent the CPU from doing the keyboard scan on a regular basis which could lead to lost characters. 

From all the conversations I’ve looked back at on Discord, the CoCoSDC does function as a ‘no-halt’ controller under OS9. However, as David Ladd pointes out, it probably masks interrupts to prevent anything from screwing around with MPI slot switching while a sector is being read. If that is the case, we could experiment with a modified driver that could be used for non-MPI systems which doesn’t mask the interrupts. 



> On Apr 6, 2020, at 7:23 AM, Allen Huffman <alsplace at pobox.com> wrote:
> 
> On Apr 6, 2020, at 12:22 AM, Dave Philipsen <dave at davebiz.com> wrote:
>> 
>> But ‘no-halt’ has to do with whether the HALT line gets asserted during disk access. I have heard, though not verified, that the CoCoSDC does have a no-halt mode that is used under OS9. The only reason it uses HALT is to retain compatibility with DECB ROMs.
> 
> If true, it could just be the driver. I can do typeahead under the Cloud-9 SuperIDE but I lose characters much more often under the CoCoSDC.
> 
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