Soft Write Protect bit in a Mode page
Gerry_Houlder at notes.seagate.com
Tue Dec 12 09:18:32 PST 1995
The SSC group has been talking about adding a "Write Protect" bit to a mode
page so that a SCSI device can be write protected via SCSI. They have some good
reasons to do this:
1) Making the WP bit in the MODE SENSE header a "writable" bit can result in
unintentional clearing of write protected condition. Many SCSI drivers will
always clear these header bits when doing a Mode Sense followed by a Mode
Select because all of these bits are supposed to be illegal for MODE SELECT. A
bit within a mode page would not have this problem.
2) Many devices (streaming devices in particular) may have other ways of
setting a write protected condition. A removable media may have a write protect
tab on it or the SCSI device may have a switch. The WP bit in the header is a
good way to indicate when any of these write protect mechanisms are active,
meaning a separate bit to show the status of only the software write protect
bit is needed. A separate bit within a mode page will answer this as well.
I see the need for this on direct access devices as well. Unfortunately, the
SSC group has chosen to put a "Write Protect" bit in a streaming device unique
page. I would like to see this bit put in a page that is common to all device
types so that all could take advantage of it. The most appropriate place seems
to be page Ah (Control Mode page). There are a couple of 1 bit fields available
in this page -- how about using byte 4, bit 3?
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