> On Nov 1, 2022, at 2:32 PM, ben via cctalk <cctalk@classiccmp.org> wrote:
>
> On 2022-11-01 11:20 a.m., Paul Koning via cctalk wrote:
> tents. The result is that the OS only has to deal with the device every 30
> characters or so (RSTS case) or even less often if the buffer size is larger.
>> Consider disk for another example. With very rare exceptions, disks do DMA:
>> the OS points to the place where the data lives, supplies a transfer length
>> and starting disk position, and says "go do it and tell me when it's
>> finished". Newer devices like the MSCP controllers support a queue of
>> requests, but even simple devices like the RK05 will do a full transfer
>> without CPU involvement.
> How mqny old systems required partial transfer for disk io, for swapping
> overlays in and out?
Arbitrary size reads are common; as you said overlay loads require that.
Partial block writes are not so common. In RSTS, the base OS write operation
does not allow that. But I found out early that RT11 does, and depends on it.
In RT11, if you do a partial block write, the rule is that the rest of the
block is zero-filled. In some disks the controller takes care of that. In the
RF11, the driver does because the device does any word count, so the driver
sets up a separate transfer for the rest of the block using a word containing
zero as the source buffer, and "inhibit bus address increment" set so all the
DMA cycles use that same word. In the RC11, the sector size is 64 bytes, so
the device zero-fills to the end of the sector but the driver has to do the
same sort of stuff as the RF11 driver if there are any additional 64-byte
sectors left before the 512-byte block boundary.
It turns out RT11 Fortran depends on this, though I don't remember the details.
paul
