Basic Joystick Programming

This article will demonstrate how to program in TI Basic for using a joystick to move a character, and also demonstrate how a program can take input from either a joystick or the keyboard.

Joystick input

The CALL JOYST format is not ideally written for the TI99/4A. The row and column variables are reversed compared to the graphics commands, and the subprogram seems to assume the screen origin is at bottom left (it is actually at top left).

The return variables are placed in the command as follows: CALL JOYST(NUMBER,COLRETURN,ROWRETURN)

whereas the graphics commands are in the form: CALL HCHAR(ROW,COL,CODE)

To move a character to the screen left, we need to decrease the value of the column. If the joystick is moved to the left, the column return variable is indeed negative, while movement to the right gives a positive return.

However, the top of the screen is Row 1, so to move down the screen the row must be increased : move the joystick down and the row return variable is NEGATIVE. This can cause confusion very easily! The sign has to be changed to make this work!

Sample code

If we wish to amend ROW and COL variables using the joystick, it is necessary to use:

100 CALL JOYST(1,COLRET,ROWRET)
110 ROW=ROW-ROWRET/4
120 COL=COL+COLRET/4
               

Notice the different signs used to amend the row and column variables. We have to divide by four because the CALL JOYST will only return 4, 0, or -4.

===Remember: the alphalock MUST be up for the joystick to work===.

Which joystick

To use the above in a program leaves one problem: you need to know which is joystick number one! This can be marked, but you can also scan both joysticks:

100 CALL JOYST(1,CR,RR)
110 CALL JOYST(2,CR2,RR2)
120 ROW=ROW-RR/4-RR2/4
130 COL=COL+CR/4+CR2/4  

This will take a little longer to process and you will have to see the effect in your program before you decide to use it.

Joystick status

When using CALL KEY there is a status return we can check to see if NO key has been pressed. With joysticks there is no status return, only the return variables. A status return can however be created.

Instead of the CALL KEY "IF ST=0 THEN" it is possible to use "IF CR+2*RR=0 THEN"

Why multiply the second return by 2? Check through all the possible returns from the joystick and you will see that a simple addition, subtraction or multiplication will not return a unique answer to equate with "joystick central".

So far we are amending the variables ROW and COL without checking to see if they are valid. To use HCHAR etc they must be from 1 to 24 or 32 respectively. Anything else will produce an error message and halt the program.

It is possible to use lots of lines of coding in TI Basic:

200 IF ROW<1 THEN 210 ELSE 220
210 ROW=1
220 IF ROW>24 THEN 230 ELSE 240
230 ROW=24.....

and so on.

It is easier however to add to the ROW incremental line a value check which will reverse the increment if it places the value outside the limits.

To do this we need to use the relational expressions discussed under "IF THEN" in the previous section.

If a relational expression is TRUE it has a value of -1

If a relational expression is FALSE it has a value of 0

Thus PRINT(2=3) will appear as 0, but PRINT(2=2) will appear as -1.

Dealing with the ROW first, if the variable ROW starts with a value of 1, and the joystick is pushed up, we must reverse the reduction of ROW.

There are two expressions which must be true : if both ROW=1 and RR=4 then after we have added 1 to ROW we must deduct it, to leave it set to 1:

ROW=ROW-RR/4+(ROW=1)*(RR=4)

Now it is impossible for ROW to become less than 1.

This has been developed further in the sample program which you will find at the bottom.

A typical use of the joystick is to move a character around the screen, and this is what the sample program will do. To give greater flexibility, this program checks both joysticks, and also checks the keyboard (keys WERSDZXC).

First the screen is cleared and the row and column variables are set to initial values. Our character is placed on screen and the joysticks and keyboard are scanned.

The next line checks to see if an input has been made: if neither joystick nor the keyboard has been used, the program will go back and look at the joysticks/keyboard again. The plus sign between the relational expressions serves as an 'OR'.

If the status of one keyboard unit is NOT zero, the program continues.

Now the program is divided into two. If the keyboard has been used, the variable ST will have a non-zero value which causes the program to branch to the keyboard section. Otherwise it continues with the joystick section.

The joystick section is a slight development of what has been discussed above. We are checking for both limits to the row variable.

The keyboard section uses similar principles,but the limit checks are a little different: If the ROW variable has a value of 1, it cannot be decreased as (RW<>1) takes a value 0 (false) and no change is made.

In the sample program a character is moved around the screen, but if you wish to leave a line of characters, just delete the line which places a blank (32) in the old position.

That was quite a complex program to develop, so check it over thoroughly. The use of relational expressions can become quite complex, but they can both speed up execution time and save memory usage.

Joystick and Keyboard control program

100 CALL CLEAR 
110 RW=12
120 CL=16
130 CALL HCHAR(RW,CL,42)
140 CALL KEY(1,K,ST)
150 CALL JOYST(1,CR,RR)
160 CALL JOYST(2,CR2,RR2)
170 IF (CR+2*RR<>0)+(CR2+2*RR2<>0)+(ST<>0)THEN 180 ELSE 140
180 IF ST THEN 240
190 CALL HCHAR(RW,CL,32)
200 RW=RW-RR/4-RR2/4+(RW=1)*((RR=4)+(RR2=4))-(RW=24)*((RR<0)+
  (RR2<0))
210 CL=CL+CR/4+CR2/4+(CL=1)*((CR<0)+(CR2<2))-(CL=32)*((CR>0)+
  (CR2>0))
220 CALL HCHAR(RW,CL,42)
230 GOTO 140
240 CALL HCHAR(RW,CL,32)
250 RW=RW-(RW<>1)*((K=5)+(K=4)+(K=6))+(RW<>24)*((K=15)+(K+1=1)+
  (K=14))
260 CL=CL-(CL<>1)*((K=4)+(K=2)+(K=15))+(CL<>32)*((K=6)+(K=3)+
 (K=14))
270 CALL HCHAR(RW,CL,42)
280 GOTO 140