PDP-11 info needed.

From: Megan <mbg_at_world.std.com>
Date: Tue Jul 7 19:51:20 1998

>Eh? While the command set of the UDA50 is MSCP, I wasn't aware that MSCP
>command packets were sent to RA drives. I don't see why it would be
>impossible to make an SDI controller that used some other command set.

Sorry, I was talking at a different abstract level.. just like the RQZX1
talks MSCP on the host side, and SCSI on the disk side, the UDA50 talks
MSCP on the host side... I've never worried about what it actually talks
on the disk side...

>IMHO the RL11 is dumber than the RK11. For one thing the RK11 supports
>spiral reads/writes. And to move heads you load the disk address register
>with the cylinder you want to go to, not the number of cylinders you want
>to move. With the RL11 you've got to remember where you are.

Not just that, but the driver is responsible for trying offsets from
the center track in case a read fails... Back when we first added support
for the RL01/02 to RT-11, it was explained to us that with the RL11, some
of the smarts are moved out to each disk unit. This way the controller
doesn't have to have all the smarts... supposedly more cost-effective (?!)

Anyway, if you lose track of where you are, you have to reset to track
zero and re-seek. There are several offsets (positive and negative)
that the driver is supposed to try when it fails to read (and maybe
write) a block. As already mentioned, the driver has to take care
of track to track changes... which means that the driver has to take
a user request and process it as multiple single-track operations
until complete...

Where the RK driver was simple, the RL driver was a PITA... and the
convolutions in the MSCP handler are even more impressive...

                                        Megan Gentry
                                        Former RT-11 Developer

| Megan Gentry, EMT/B, PP-ASEL | Internet (work): gentry_at_zk3.dec.com |
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Received on Tue Jul 07 1998 - 19:51:20 BST

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