From: utzoo!decvax!cca!fred.umcp-cs@Udel-Relay@sri-unix
Newsgroups: net.unix-wizards
Title: compat (v7run)
Article-I.D.: sri-unix.3253
Posted: Mon Sep 13 14:36:07 1982
Received: Tue Sep 14 05:23:00 1982

From:     Fred Blonder 
Date:     9 Sep 82 18:25:32-EDT (Thu)
From: Charles F. Von Rospach 

	Does anyone have any documenation on 'compat'? All I can find
	out about it is that it emulates PDP11 V7 Unix so you can run
	Zork. Is this correct? Is it robust enough to handle something
	like a CBASIC interpreter for PDP11, or are we in for trouble?
	Where can I get detailed info on compat?

In our experience ``compat'' is pretty reliable, but there's a minor bug
in it: It places TWO null pointers on the stack after the arguments
instead of one. This means that when the PDP-11 program looks for the
pointer to the first environment variable, it sees the second null pointer,
and can't find the environment at all, even though it's really there on the
stack, offset by one. The fix is to delete line 76 from unixstart.c

	76d76
	< 	*(--ssp) = 0;
	---

as distributed with 4.1BSD.