Shareware Grab Bag/Shareware Grab Bag.iso/010/ibmchess.arc/CHESS.DOC

Text File | 1985-08-13 | 14KB | 237 lines

                              CHESS
        (COMPUTE! Magazine September 1985 by John Krause)

     A computer chess game is great for those who can't always find a
human opponent.  But "CHESS" is more than just a substitute for a live
player.  You might call it a "chess processor."  It processes chess
positions as easily as a word processor manipulates text.  It contains
all the features a chess player could ever want.  Its thinking routines
are written entirely in machine language for greater speed, and they
use basic principles of artificial intelligence to simulate an actual
human chess player.
     CHESS consists of two programs.  A BASIC program is used to create
CHESS.BLD which contains the machine language.  To play CHESS, make
sure the CHESS.BLD file is on the disk with CHESS.BAS, then simply load
and run CHESS.BAS.
     After running CHESS, you'll see a title screen for a few seconds
while the computer prepares itself.  Then the board is displayed with
the pieces in their starting positions.  You're in command of the white
pieces versus the computer's black pieces on skill level 1, the easiest
level.  You should see a frame around the square in the lower-left
corner of the board.  This is the cursor which takes the place of your
hand for moving and capturing pieces.
     Use the cursor keys to move the frame cursor atop the piece you
wish to move.  Press and release the Enter key.  Now move the cursor to
the square on which you want to place the piece and hit Enter again.
Your piece moves to the new square, and the computer responds instantly
with a countermove.
     One of the most valuable features of CHESS is that it checks for
illegal moves.  If you try to make an illegal move, the computer buzzes
and keeps your piece on its square.  This feature is not perfect,
however.  It won't catch illegal moves involving castling or en passant
captures.  But it will catch 99 percent of all illegal moves, including
those that put your king in check, as well as the more obvious ones
such as moving a pawn backwards.  If the computer accepts your move,
it's probably legal, but not necessarily so.  If the computer rejects
your move, however, you can be sure that it is illegal.
     If you're a beginner at chess, you'll find the move-checking
feature especially valuable.  Just by trying various moves and noting
which ones the computer accepts, you can get a good idea of the way
each piece can move.
     Information about the current game is displayed at the top of the
screen.  MOVE# indicates the number of the move currently being made,
counting from the start of the game.  In chess, a move by both sides is
considered one move.  So, the move number is changed only after both
sides have moved.
     TO MORE indicates which side has the move.  W means it is white's
turn, and B means it is black's.
     Normally after you move, the computer automatically makes the
next move.  This can be turned off by pressing the T key to switch to
two-player mode.  Now you can play against another person with the
computer acting as referee to check for illegal moves.  To switch back
to one-player mode, press T again.
     You can also let the computer make moves for you by pressing the
M key.  The side that the computer plays depends on whose turn it is.
By repeatedly pressing M, you can watch the computer play itself.
     One of the advantages of a computer opponent over a human is that
you can tell the computer exactly how hard you want it to try to beat
you, and it obediently plays at that level of difficulty.  This is
important because it's no fun if you always love or always win
effortlessly.
     LEVEL shows the current skill level from 1 to 5.  You can change
the level at any time by pressing keys 1-5.  The difference between
levels is the number of moves ahead that the computer looks.  On level
1, for example, it looks ahead one full move or two half-moves (its
move and your reply).  Each succeeding level looks ahead one more
half-move than the previous level.
     Alas, the smarter play on the higher levels doesn't come without
a price.  The further ahead the computer looks, the move moves it must
examine and, hence, the longer it thinks.  Here's a rundown of the
five levels:
          Level 1:  Beginner.  Thinking time: one second.  Look-ahead:
two half-moves.  Fast but dumb.
          Level 2:  Intermediate.  Thinking time: five seconds.  Look-
ahead: three half-moves.  Provides a reasonable challenge for impatient
players.
          Level 3:  Tournament.  Thinking time: two minutes.  Look-
ahead: four half-moves.  Since the usual time limit for tournament play
is 40 moves in two hours, an average of three minutes per move, this
level is best suited for serious players.
          Level 4:  Mate in two.  Thinking time: 20 minutes.  Look-
ahead: five half-moves.  Capable of solving most mate-in-two problems.
          Level 5:  Postal chess.  Thinking time:  two hours.  Look-
ahead: six half-moves.  Simulates chess by mail where there is no time
limit.  Can avoid checkmate in two moves.
     These thinking times are averages.  The actual thinking time varies
greatly depending on the position.  For example, level 5 takes only five
seconds with just two kings on the board.  Also, these times are for the
PC only.  Since the PCjr runs at about two-thirds the speed of the PC,
the thinking times for the PCjr are greater than the values shown above.
     It happens to everyone.  It's inevitable.  You've played for an
hour, somehow managing to maneuver into a superior positions in what
you consider to be the best game of your life, only to throw it all
away in a single, spectacular blunder.
     Don't panic.  You can take back the last half-move by pressing the
B key.  If you're in one-player mode, you need to press B again to take
back your move and the computer's reply.  In fact, you can press B
repeatedly to take back several moves until you reach the starting
position.  This is possible because the computer records every move
made in the game.
     Another use for this feature is to allow the computer to suggest
a move for you.  If you don't have a good idea of where to move next,
press M and the computer will move for you.  If you like that move,
press M again to continue with the computer's next move.  But if you
think you've found a better move, press B to take back the suggested
move and make your own move.
     Pressing the F key does the opposite of B.  It moves forward
through the move list up to the most advanced position.  Note that
every time a new move is made, the resulting position becomes the most
advanced.  So if you use B to backtrack to a previous position, and
then make a new move, all subsequent stored moves are erased because
they are no longer relevant.
     If you have a printer, you can print the move list by pressing the
P key.  The list appears in three columns: the move numbers, white's
moves, and black's moves.  Each move is indicated by the square the
piece moved from followed by the square it moved to.  Each square is
specified by its coordinates according to the numbers along the left
side of the board and the letters along the bottom.
     You can also dump the screen image to the printer to get a hardcopy
of a particularly interesting position.  Before loading BASIC from DOS,
type GRAPHICS with the DOS master disk in the drive.  Then run CHESS and
press Shift-PrtSc (Fn-PrtSc on the PCjr) whenever you want to print the
position.
     The computer thinks by analyzing thousands of possible moves and
countermoves and choosing what it considers to be the best move based
on the relative value of pieces.  Most positions don't have just one
best move but several which are equally good, in which case the computer
chooses among them at random.  This random factor insures that every
game will be different, and makes for varied and interesting play.
     The computer announces checkmate when it occurs.  However, there
are a few quirks in the way the computer evaluates a checkmate.  On
levels 3-5, it announces checkmate prematurely.  When this happens, the
computer has determined that it's impossible to avoid checkmate on the
next move or two -- assuming both sides make the best moves.
     Also, the computer doesn't know the subtle difference between
checkmate and stalemate.  Consequently, when a game is stalemated, the
computer announces checkmate even though the game is a draw.  Since the
computer tries as hard as it can to checkmate its opponent, it also
tries to achieve stalemate, possibly by forcing a draw when it could
have won.  Fortunately, this rarely happens, because a stalemate
requires unusual circumstances, such as when one side has only the
king remaining.
     You can start a new game at any time by pressing the N key.  This
sets up the pieces in the starting position with white on the bottom.
If you want to play the black pieces, you can press the I key to invert
the board, so you still play from the bottom.  As with the N command,
the board is reset to the starting position.  However, the N and I
commands retain the move list from the previous game.  This allows you
to replay the game using the F command.  When replaying a game, be sure
to reset the board by pressing I if the game was played in the inverted
mode, or N if normal mode was used.
     You don't have to begin a game from the starting position.  You can
set up any position and begin playing from that point.  If you want, you
can first clear the board by pressing the C key.  To add a piece or
change a piece to a different one, move the cursor to the appropriate
square, hold down either Shift or Ctrl, and press P, N, B, R, Q, or K
for pawn, knight, bishop, rook, queen, or king, respectively.  Holding
down Shift adds one of the lower player's pieces, and Ctrl adds one of
the upper player's pieces.  (Just remember that Ctrl is above Shift on
the keyboard.)  A piece can be removed from the board by pressing the
space bar.  Note that these changes are not stored on the move list.
     These commands allow you to experiment with hypothetical or
downright ridiculous positions.  The position doesn't even have to be
legal.  Live out your fantasy by giving yourself ten queens versus the
computer's lone king.  Or invent your own type of chess by giving each
side two kings, for example (although in this case the computer might
get confused trying to determine a checkmate).
     You can also set up a problem for the computer to solve, such as
the mate-in-two problems published in many newspapers.  To solve a
make-in-two problem, press C to clear the board, set up the position,
press 4 to select level 4, and press M to start the computer thinking.
After several minutes of deep thought, the computer will make a move
(the solution) and announce checkmate.  The only mate-in-two problems
that the computer cannot solve are those which involve castling, en
passant captures, or pawn promotion.
     The computer never castles or captures en passant because, due to
their complexity, these moves are not included in its thinking routine.
But you can make these special moves.  To castle, move the king two
squares to the left or right.  The rook moves automatically.  To capture
en passant, move your pawn diagonally to the proper square.  The
opponent's pawn is removed automatically.  Remember, the computer
doesn't check for illegal moves involving castling or en passant
captures, so if you're a beginner, you should familiarize yourself with
the rules on these special moves.
     When a pawn reaches the opposite side of the board, it's
automatically promoted to a queen.  In the rare event that you would
rather promote to a knight, bishop, or rook, you can easily make the
change by positioning the cursor over the new queen and pressing N, B,
or R with Shift or Ctrl.  Note, however, that underpromotions are not
stored in the move list.
     If you want to stop the present game and continue later, you can
save the game on disk (in drive A) by pressing the S key.  You'll see
the prompt Save:.  Type in a filename for your game and press Enter.
The filename can be up to eight characters long.  Don't type an
extender; .CHS is added automatically.  If a file on the disk already
has the same name, it will be replaced.
     To load a previously saved game, press the L key.  Answer the
Load: prompt with the filename and press Enter.  (Don't type the .CHS
extender.)  The L command restores the game exactly as it was when it
was saved.  Not only the position is restored, but also the move list
and even the position of the cursor.
     If the computer is unable to save or load a game, an error number
is displayed.  See Appendix A of the BASIC Reference Manual for a
description of the error.
     Besides allowing you to continue a game at a later time, the S and
L commands can be used to create a library of your best games.  To do
this, press N or I just before saving.  The game will come up in the
starting position when loaded and can be replayed using the F command.

CHESS Command Summary
---------------------
B             Move backward
C             Clear board
F             Move forward
I             New game (inverted)
L             Load game
M             Computer's move
N             New game
P             Print move list
S             Save game
T             Two players
1-5           Level
Cursor Keys   Move cursor
Enter         Your move
Space Bar     Remove piece
Shift-P       Lower player's pawn
Shift-N       Lower player's knight
Shift-B       Lower player's bishop
Shift-R       Lower player's rook
Shift-Q       Lower player's queen
Shift-K       Lower player's king
Ctrl-P        Upper player's pawn
Ctrl-N        Upper player's knight
Ctrl-B        Upper player's bishop
Ctrl-R        Upper player's rook
Ctrl-Q        Upper player's queen
Ctrl-K        Upper player's king