>>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. 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.
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