TM78 (TU78) question: Read Opposite operation

[Chris Torek]

>In article <2323 at umd5.umd.edu> I asked:
>>Is the 4.3BSD [TU78] driver wrong once or twice?

It appears I insulted the driver unnecessarily.  I have yet to find
a bug here, but...

In article <10102 at ulysses.homer.nj.att.com> ggs at ulysses.homer.nj.att.com
(Griff Smith) writes:
>The driver, as released in 4.3BSD, does have a few bugs - not in error
>recovery that I know of.

... I spotted a nice bug in mtustart, for cooked devices (not that
anyone uses these, but I was trying to figure out why we got these
duplicates, and this code does seek).  At the bottom it does something
like

	if (x < y)
		count = y - x;
	else
		count = x - y;
	if (count > 0377)
		count = 0377;
	mtaddr->mtncs[MUUNIT(bp->b_dev)] = count | MT_SFORW | MT_GO;

which will only ever space forward.  Hm.

>... The "read opposite" refers to the sense of the original
>command, not to that of the retry.

The TM78 User's Guide is rather ambiguous, but does imply this.
-- 
In-Real-Life: Chris Torek, Univ of MD Computer Science, +1 301 454 7163
(hiding out on trantor.umd.edu until mimsy is reassembled in its new home)
Domain: chris at mimsy.umd.edu		Path: not easily reachable