<Both. Neither. It depends on which controller. The one in the MV2000 seem
<to do nothing, so the OS does everything.

Correct as the FDC/HDC in the mv2000 is a low level interface at the 
chip level so the vax talks directly to the device registers and must 
set up every detail of the transfer (Drive, cylinder, starting sector,
DMA read or write.) and there is no real intelligence. It's about all it 
can do to put sector(s) of data on the disk or in a buffer and notify of 
the status of the attempt.  For the hardware heads look up the SMC9224
universal disk controller chip.  The same chip is used on the RQDX3 but 
has the T-11 chip between the bus and the controller adds a lot of 
intelligence.

<revectored block instead, without the OS knowing. I'm not sure where the
<RQDX3 stands. In the HSCx0, the controller does it all for you, I think.

The RQDX3 (and the RQDX2/1) does it's own bad block sparing, not sure of 
the method used but it's done for you at the MSCP level.

Allison