1 Feb
2022
1 Feb
'22
2:17 p.m.
On Feb 1, 2022, at 5:08 PM, Chuck Guzis via cctalk
<cctalk at classiccmp.org> wrote:
On 2/1/22 13:40, Paul Koning via cctalk wrote:
I didn't know that one. The only drive I really know is the 844, an RP04 lookalike,
which does have friendly size sectors, laid out by the controller ("BSC").
LBA addressing, in CDC? Where is that? On the 6000 series, I remember classic c/h/s
addressing. The OS would convert those to "logical track and sector" addresses,
sure. But that was a file system structure thing really. PLATO ignored all that overhead
and laid its own file system directly on top of the disks, with the file system block
offset to c/h/s mapping done in the PP. So yes, it necessarily knew the drive layout.
For that matter, with "logical tracks" the OS still had to know the layout; it
just got buried into the logical to physical mapping system request code, in the CP
monitor for extra inefficiency.
paul
With MSCP, DEC switched to addressing disks by
sector offset, as SCSI did later, rather than by geometry (cylinder, track, sector) on
devices like the RK05 and RP06. If the OS sees only an LBA, it doesn't matter whether
the drive uses zone recording; such complexity can be hidden inside the controller
firmware. But I don't know if that was actually done, either at that time or in later
generations.
Good grief, it took DEC all that time? CDC was doing it in the 1960s.
Had to, because of the wide variety of RMS available. I think that
one of the early 2311 clone drives (854?) used 256-byte (8 bit byte)
hard-sectored media, which isn't very friendly to systems with 60 bit
words. I recall that several sectors were used to create a logical
60-bit word addressable sector, with a substantial part of the last
sector of a logical PRU left unused.