Paul said “But the DZ doesn't do DMA.” I apologize. I used DZ in responses when i meant DH.
Sent from my iPhone > On Nov 1, 2022, at 11:56, Paul Koning via cctalk <cctalk@classiccmp.org> > wrote: > > > >> 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 >
