[TUHS] Some notes on running UNIX v6 in 2015, using SimH and a healthy dose of documentation

[random832@fastmail.com (Random832)]

On 2015-12-03, Noel Chiappa wrote:
> I did a very similar exercise using the Ersatz11 simulator; I have a lot
> of stuff about the process here:
>
>   http://www.chiappa.net/~jnc/tech/V6Unix.html
>   
> It contains a number of items that you might find useful, e.g.: "V6 as
> distributed is strictly a 20th Century operating system. Literally. You can't
> set the date to anytime in the 21st century, for two reasons. First, the
> 'date' command only take a 2-digit year number. Second, even if you fix that,
> the ctime() library routine has a bug in it that makes it stop working in the
> closing months of 1999."

I don't think your fix to the date command is accurate. The description says:

>  The 'date' command has been extended to support 2- and 4-digit year
>  numbers (to be upwardly compatible, the 2-digit ones assume 19xx).

But the code says:
>	if ((y < 0) || (yh < 0)) {
>		time(nt);
>		y = localtime(nt)[5];
>		y =+ 1900;
>	}
>	  else
>		y = yh + y;

To match your description it should be
	if ((y < 0) && (yh < 0)) {
		time(nt);
		y = localtime(nt)[5];
	} else if(y < 0) {
		/* yh = two digit year */
		y = 1900 + yh;
	} else
		y = yh + y;

Though I'd be inclined to add "if (y < 1970) y =+ 100;" to bring it in line
with modern systems' 1970-2069 range for two-digit years.

Of course, _any_ ancient unix system (and a fair few modern ones) can't
represent dates past 2038. We've only got 23 years left before attempts to
bring up old systems will need some trickery (maybe set the year to 28 or 56
years before the present... that trick, with increasing windows, will work
until 2100 isn't a leap year)

On the other hand, treating it as an "unsigned" value will last about as long
and is a bit less of a hack. But as you note, doing this sort of thing in V6 C
isn't trivial.