Soft Write Protect bit in a Mode page

Gerry Houlder 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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