Xref: utzoo unix-pc.general:2016 comp.sys.att:5142
Path: utzoo!utgpu!watmath!clyde!att!mtunb!jcm
From: jcm@mtunb.ATT.COM (was-John McMillan)
Newsgroups: unix-pc.general,comp.sys.att
Subject: Re: Looks like a bug in the 7300 disk driver
Summary: O woe...
Keywords: fate
Message-ID: <1360@mtunb.ATT.COM>
Date: 9 Jan 89 21:35:07 GMT
References: <813@ttrde.UUCP>
Reply-To: jcm@mtunb.UUCP (was-John McMillan)
Organization: AT&T ISL Middletown NJ USA
Lines: 37

In article <813@ttrde.UUCP> pfales@ttrde.UUCP (Peter Fales) writes:
>
>I have discovered what appears to be a bug in the hard disk device driver
>for the unix-pc.
...
It's a FEATURE, not a bug.  Moreover, this feature will never be changed ;^}

Consider TAPE drives: in many systems, you can only communicate to the
drive in EVEN byte multiples -- unless things have changed in the last
twenty years since I've used a tape drive %v).  Is this an error?

In the UNIX-PC, the RAW -- read that "character-device" -- interface
to the disk directly maps transfers into user RAM.  Therefore, since
transfers to/from the disk are in 512 byte blocks, ya can only read
in 512-byte multiples.

In some other systems, boundary cases -- un-aligned first and last blocks
-- are read into kernel buffers and NOT into user-space.  This permits
Peter's anticipated results.

Finally (fat chance of this ;-):
1)	TSK-TSK -- users are NOT supposed to be accessing the RAW disk
	at all.  And if this is permitted... the special characteristics
	of the RAW driver have to be coped with!

2)	I believe I've seen this feature documented -- and isn't that
	enough to satisfy everyone?

3)	Lord[s], let us not descend into the hell of other groups that
	have wasted untold kilo-bucks thrashing unresolvable differences
	of opinion over system features -- not to mention address-to-index
	computations by name.

PS: I haven't pursued the above issue through the source code so the above
drivel may be the first flawed argument of my life -- oops, I meant HOUR.

jc mcmillan	-- att!mtunb!jcm	-- just frothing for myself, not THEM.