🚀 go-pugleaf

Several days ago I posted about a problem I was having in getting
Xinu up on a 3/60.  It came up OK, but I couldn't get any input
into the shell, and Xinu in-general was behaving "weird".

After about 1 day of working on this problem, I have discovered the
solution.


1st of all, the board in question was NOT a 3/60, but was a 3/50.
(I had ordered a couple of 3/50's from a reseller, and they packaged
them in a 3/60 case.  They really were 3/50's).

Now this fact really made me curious, becuase Xinu runs pretty
well on a 3/50.

Just for kicks I brought up SunOS 4.1.1 on each new 3/50 CPU board
and they ran fine.  But Xinu would still not run correctly.
Hmmmmmmmm?

I examined the CPU boards really close against the known-working
3/50 board.  The only real difference I could spot was (substantially)
different labeling on the Zilog serial chips.  Checking the Zilog
data manuals didn't provide any help as I could find no information
that distinguished the differing chips.

So I debugged awhile, and pretty much "accidentally" found the bug.
The bug is in ttyiin(), the tty input routine.  There is a place
in the code where the character is read from zscc_data, then
IMMEDIATELY afterwords, zscc_control is set to 1.  The solution
was to insert a DELAY() between these 2 statements:

INTPROC ttyiin(devm)
    long    devm;
{
    STATWORD ps;
 
    register struct tty *iptr;
    register struct zscc_device *zptr;
    register int    ch;
    int err;
    int cerr;
    int ct;
 
    iptr = &tty[devm];
    zptr = iptr->ioaddr;
    ch   = zptr->zscc_data;
 
    DELAY(2);          /* Add DELAY here to fix bug on some 3/50's)
 
    /* get the contents of RR1 */
    zptr->zscc_control = 1;
    DELAY(2);
    err = zptr->zscc_control;
 
---------end code segment---------


Evidently, there is some timing sensitivity with some versions of
the Zilog chip.


JR


-- 


   /------------------------------------------------------------------\
  |  John Reed                           {ucsd,uunet}!pacdata!johnr    |
  |  Pacific Data Products               johnr%pacdata.uucp@ucsd.edu   |
  |                    ---------------------                           |
  |  George Herbert Walker Bush, with the letters re-arranged spells:  |
  |                                                                    |
  |                 Huge Berserk Rebel Warthog                         |
  |                                     -- From Cosmic Trigger II      |
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