How to use GAMES.TPU -- overview --
by Lou DuChez, Pascal-kind-of-guy
GAMES.TPU is a Turbo Pascal unit designed to rework the keyboard
and timer interrupts to better function in game writing. The
keyboard is the biggest problem in a lot of games: the keyboard
buffer reads keys sequentially, but you want simultaneous reads
for action games. Now scan codes for key presses and releases
are transmitted through port 60h, so it seems like it should be
possible just to keep an eye on that memory location.
Unfortunately, interrupt 09h (the keyboard interrupt) ends up
resetting the port to 0 before you can read the contents. So I
wrote a new interrupt handler that does these steps:
1) reads port 60h and records presses / releases;
2) calls the regular keyboard interrupt;
3) clears the keyboard buffer.
All key presses are stored in an array of 128 boolean values.
I've found that you can keep track of about six keys
simultaneously -- more than enough for most games.
I've also found it useful to nullify the "Ctrl-Break" interrupt.
Basically, you don't want to "Ctrl-Break" out of a Pascal program
before resetting the keyboard interrupt, so I made a "Ctrl-Break"
interrupt that does absolutely nothing.
Finally, when you write an action game, you want it to run at the
same speed on all computers, be they original PC's or 486's. The
best way to do that is to monitor the computer's timer; I wrote
an interrupt that will pause until a fixed number of "ticks"
(18.2 per second) go by.
GAMES.TPU -- the Keyboard interrupt
You install the new keyboard interrupt by invoking procedure
INITNEWKEYINT (and reset to the old one by invoking
SETOLDKEYINT). As mentioned before, the status of the keys is
recorded in a boolean array (from 0 to 127) called KEYDOWN. A
"True" indicates the key is down; a "False" indicates it is not.
So your program just has to check this array periodically to see
what keys are down.
Now as for figuring out which array elements correspond to which
keys: I provide two ways. First of all, there is a function
"SCANOF": it takes a character argument, and returns (as a byte)
what scan code corresponds to the character (more accurately, the
key that makes the character). If, for example, you need to know
the scan code of the "1" key, you'd want the value returned by
SCANOF('1') (or SCANOF('!'), since you're trying to see if the
"1"/"!" key is down, and the "Shift" keys aren't an issue). In
particular, you'd know that the "1" key is down if
KEYDOWN[SCANOF('1')] was "True".
The SCANOF function works for all the alphanumeric and
punctuation keys; it doesn't work for the arrows, "NumLock",
function keys, etc. because there's no particular characters to
associate with them. So here are some constants that you can use
instead:
CONSTANT VAL DESCRIPTION CONSTANT VAL DESCRIPTION
escscan $01 "Esc" entscan $1c "Enter"
backscan $0e Backspace rshscan $36 Right Shift
ctrlscan $1d "Ctrl" prtscan $37 "PrntScrn"
lshscan $2a Left Shift altscan $38 "Alt"
capscan $3a "CapLock" homescan $47 "Home"
f1scan $3b F1 upscan $48 Up Arrow
f2scan $3c F2 pgupscan $49 "Pg Up"
f3scan $3d F3 minscan $4a "-" on keypad
f4scan $3e F4 leftscan $4b Left Arrow
f5scan $3f F5 midscan $4c "5" on keypad
f6scan $40 F6 rightscan $4d Right Arrow
f7scan $41 F7 plusscan $4e "+" on keypad
f8scan $42 F8 endscan $4f "End"
f9scan $43 F9 downscan $50 Down Arrow
f10scan $44 F10 pgdnscan $51 "Pg Down"
f11scan $d9 F11 insscan $52 "Ins"
f12scan $da F12 delscan $53 "Del"
scrlscan $46 "ScrollLock" numscan $45 "Num Lock"
tabscan $0f Tab
Is the left arrow down? It is, if KEYDOWN[LEFTSCAN] is "True".
There is a second array of booleans (0 to 127), called WASDOWN:
it records whether or not a key has been depressed in a period of
time. This is more useful for keys that get tapped instead of
continuously held down: for example, a movement key is held, but
a fire button is tapped. Has the Space Bar been pressed? Only
if WASDOWN[SCANOF(' ')] is "True". A procedure to use with the
WASDOWN array is CLEARWASDOWNARRAY: resets all the elements of
WASDOWN to "False". ("FOR COUNTER := 0 TO 127 DO
WASDOWN[COUNTER] := FALSE") So you reset the array with
CLEARWASDOWNARRAY, and WASDOWN will record all the keys that have
been pressed until you call CLEARWASDOWNARRAY again.
GAMES.TPU -- the "Ctrl-Brk" interrupt
This one takes little to no explanation. Call INITNEWBRKINT to
call the new interrupt (essentially, to disable it); call
SETOLDBRKINT to reset it.
GAMES.TPU -- the Timer interrupt
18.2 times per second, the computer generates a "tick", calls
hardware interrupt 08h, updates its clock/calendar, does various
house-cleaning functions, and then calls interrupt 1Bh. 1Bh is
an interrupt for programmers to hook their programs onto for
timing purposes; that's exactly what GAMES.TPU does. Invoke the
new timer handler with INITNEWTIMINT (and disable it with
SETOLDTIMINT); it first calls whatever TSR's might be using that
interrupt already, then it increments a counter. You indirectly
access that counter by the procedure TICKWAIT: the computer waits
until the counter gets to the number of ticks specified before
doing anything else (byte values only). Once that number has
been reached, the counter resets to 0.
I think an example is in order: let's say you have an action game
where each round should take one-half second on any machine. So
at the beginning of the first round, call TICKWAIT(0) (to set the
tick counter to 0). At the end of each round, call TICKWAIT(9)
(to wait out the half second). Now between those steps, while
the computer has been drawing things on the screen, updating
enemy positions, etc., the tick counter has been automatically
incrementing every .055 seconds. On a 4.77Mhz XT, the counter
may have gotten to 6 by the time the program gets to the
TICKWAIT(9) statement, which waits three more ticks before
proceeding to the next step. On a 486, the tick counter might
get only to 1 before encountering the TICKWAIT(9); in which case,
the computer will wait eight ticks before proceeding. So you can
make programs that run at the same speed on any computer via the
timer interrupt and TICKWAIT.
Some sample pseudocode:
program gamething;
uses games;
procedure movefoes;
begin
.
.
end;
procedure firephasers;
begin
.
.
end;
procedure updatescore;
begin
.
.
end;
procedure playgame;
begin
repeat begin
movefoes;
firephasers;
updatescore;
tickwait(2);
end until igotblownup;
end;
begin {main program}
initnewtimint;
tickwait(0);
playgame;
setoldtimint;
end.
Included are GAMES.PAS (compile it yourself -- I wrote it via
Turbo Pascal 6.0, but it ought to work on 4 and 5 too), and
RAIDERS.EXE, a game I wrote using all those interrupts.
Enjoy!
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