Tue, 15 Nov 2011, 20:38

I have just gotten through an email exchange with a student that was prompted
by my email to the class yesterday, in which I pointed out that if a player
typed three characters instead of two, it would cause at least one illegal
move.  But no harm would be done since eventually the players would see their
screw-up, perform the obvious correction, and the game would continue just
fine.

In the exchange with this student, I discovered that he has a basic 
misunderstanding of what TRAP x20 does.  Since this is fundamental 
and there may be other students with this misconception, I am writing 
to you all.

First the email exchange, then some comments.

> > > Professor Patt,
> > > 
> > > In response to the last email sent where the validity of a players 
> > > input depends on only the first two characters:
> > > 
> > > In my program I have decided to let the players input ONLY two characters.
> > > So a player will not even have the ability to enter more than two.
> > 
> > How do you propose to accomplish that?  TRAP x20 gets you the first,
> > TRAP x20 gets you the second, after which (A) someone types a key loading
> > KBDR and setting KBSR[15].  How do your propose to prevent A from happening?
> > 
> > Yale Patt
>
> Every time a player is asked to input a value, I set a register to a 
> counter of 2. And every time a trapx20 instruction is executed allowing 
> the player to input a character, the counter is decreased by one. The input 
> instruction then branches out to the rest of my code when the counter has 
> reached 0. Then the player is not allowed to press another key after that. 
>
> <<name withheld to protect the student who is missing an important point>>

Aha!  I think I have figured out where his misconception is.  TRAP x20 does
NOT allow the player to input a character.  Anyone sitting at the keyboard
can type a character if he/she has the strength to push down on the keyboard 
with sufficient force.  The person does not have to get permission.  In fact, 
your little brother could type a character while you and your father are 
playing the game.

What the TRAP x20 service routine does is examine KBSR[15] to see if 
someone has typed a character and if someone has, the ascii code for that
character will be copied from KBDR to R0.  It is true that in a lot of our
discussions we have used TRAP x23 to print a banner on the screen inviting
the user to input a character.  We did this in order to encourage the user
to input a character, rather than have our program sit there patiently waiting
and hoping that someone would eventually type a character.  (Anyone who has
sat staring at the telephone, waiting for it to ring, understands the problem
with just waiting patiently!)  

But TRAP x23 did not make the user type a character any more than some 
well-meaning friend can make that long awaited telephone call happen.

In fact, in the case of TRAP x23, I can think of at least two students in 
our class who probably would stubbornly NOT type a character!  In that case 
the TRAP x23 service routine would never finish and control would never 
return to the user program.

Bottom line: In the same way that the TRAP x20 (or TRAP x23) service routine
does not force a user to type a character, the absence of TRAP x20 (or TRAP x23)
does not prevent a user from typing a character.

In the case of the student who wrote, he thinks he is only allowing the player 
to type two keys.  Wrong!  If the player types three keys, the program will 
accept the first two as the player's move.  The third key will then be put in 
KBDR and sit there waiting for the program to execute another TRAP x20, which 
in this case he expects to be the first character of the other player's turn.  
Unfortunately, it will be the third key of the first player's turn.

The good news is, as I tried to make clear in my email to the class is that
it will not be a problem.  The program will keep rejecting inputs until the
players see what they are doing and correct the situation, and bring the game
back to where it should be.

If some of you had the same misconception, I hope this helps.

Good luck finishing the program before Thanksgiving.

Yale Patt