Q68121: Checking Game Port with CALL INTERRUPT If STICK & STRIG Fail
Article: Q68121 Product(s): See article Version(s): 4.00 4.00b 4.50 Operating System(s): MS-DOS Keyword(s): ENDUSER | SR# S901115-120 B_BasicCom | mspl13_basic Last Modified: 17-JAN-1991 If you are experiencing problems with BASIC's game port (or joystick) routines, such as STICK or STRIG, this article provides information that may help you to diagnose whether the symptom is the result of a ROM BIOS problem. This information applies only if your computer has an Intel 80286 or 80386 microprocessor chip. This information applies to Microsoft QuickBASIC versions 4.00, 4.00b, and 4.50; to Microsoft BASIC Compiler versions 6.00 and 6.00b for MS-DOS; and to Microsoft BASIC Professional Development System (PDS) versions 7.00 and 7.10 for MS-DOS. The way in which BASIC handles joystick commands (which include the STICK function, STRIG function, and STRIG statement) depends on which kind of Intel microprocessor chip the computer uses. If the computer has an 8086/8088 or 80186/80188 microprocessor, the joystick commands talk directly to the hardware without going through the BIOS. If the computer has either an Intel 80286 or 80386 microprocessor, the joystick commands are handled by going through BIOS interrupt 15 hex. Because of errors in some BIOS joystick functions on some computers, STICK or STRIG may not work at all, or may not work properly. If your computer has an Intel 80286 or 80386 microprocessor chip, you can use the code example below to determine if the problem you are having is ROM BIOS related. The program accesses the game port by directly calling INTERRUPT &H15 with function &H84, which is the BIOS joystick routine. If the problem still occurs when calling the BIOS joystick routine directly, then the problem is with the computer's BIOS joystick function and not with BASIC's STICK or STRIG. To access the BIOS joystick routines, you call interrupt &H15 (21 decimal). The AH register should contain &H84 (132 decimal) to indicate the game port support function. Register values prior to issuing interrupt &H15 should be: AX = &H8400 ' This puts &H84 in the AH register. DX = 0 or 1 If DX = 0, this indicates to "read switch settings." If DX = 1, this indicates "read resistive inputs." Depending on the value of DX when you make the call, different parameters will be returned by the interrupt. If DX = 1, then upon returning from Interrupt &H15, the registers will contain: AX = Port1 x coordinate BX = Port1 y coordinate CX = Port2 x coordinate DX = Port2 y coordinate If DX = 1, then bits 4 - 7 are used to represent switches. The AX register will contain the switch settings in bits 4 through 7. BX, CX, and DX will be unchanged. The following code example shows how to use the CALL INTERRUPT routine to directly call the BIOS joystick routines. To compile the program, use the following compile and link lines: BC joystick.bas ; LINK joystick ,,, QB.LIB ; [For BASIC PDS 7.x use QBX.LIB ;] To run this program in the QB.EXE or QBX.EXE environment, invoke QB or QBX with the /L option (to load the QB.QLB or QBX.QLB Quick library). JOYSTICK.BAS ------------ 'This code prints to the screen the values of AX, BX, CX, DX registers 'when interrupt &H15, function &H84 is called with given values in the 'DX register. This will allow you to observe the values 'corresponding to a specific action taken on a given device attached 'to either of the two game ports supported by an IBM or compatible. '$INCLUDE 'QB.BI' ' For BASIC PDS 7.x, use QBX.BI DIM inregs AS RegType DIM outregs AS RegType inregs.ax = &H8400 'puts &H84 in AH register DO inregs.dx = 1 '1 - read resistive inputs CALL INTERRUPT(&H15, inregs, outregs) CLS LOCATE 16, 10: PRINT "RESISTIVE INPUTS: (STICK)" LOCATE 19, 20: PRINT "AX","BX","CX","DX" LOCATE 20, 20: PRINT outregs.ax, outregs.bx, outregs.cx, outregs.dx inregs.dx = 0 '0 - read switch settings CALL INTERRUPT(&H15, inregs, outregs) LOCATE 6, 10: PRINT "SWITCH SETTINGS: (TRIGGER)" LOCATE 9, 20: PRINT " AX" LOCATE 10,20: PRINT outregs.ax 'only bits 4 - 7 are important LOOP WHILE INKEY$ = "" 'loop till any key is pressed END References: Note: The INTERRUPT routine is considered an external subroutine by the compiler. The routine is located in the files QB.LIB and QB.QLB for QuickBASIC 4.x and in QBX.LIB and QBX.QLB in BASIC PDS 7.x. Programs that execute a CALL INTERRUPT statement when compiled in the QB.EXE editor require the presence of the QB.QLB or QBX.QLB Quick library. This means that QB.EXE and QBX.EXE must be invoked with the /L option, which automatically loads the correct Quick library. Compiled programs that execute CALL INTERRUPT must be linked with the file QB.LIB or QBX.LIB. More information on the use of CALL INTERRUPT can be found under the CALL statement in the language reference manual for each BASIC product. For more information on how to use CALL INTERRUPT, query on the following words: CALL and INTERRUPT and application and note and QuickBASIC For a description of the dependency of STICK and STRIG on computers with certain Intel chips, query on the following: STICK and STRIG and BIOS It may be possible to avoid the BIOS routines altogether by directly accessing the game port using get and put functions. (BASIC's INP and OUT statements cannot do this because of the speed required to read the port.) It may be possible for you to write an assembly subroutine, which could be called from BASIC to talk directly to the game port. Information on programming the game port can be found in the following texts: 1. Page 433 of "The Programmer's PC Sourcebook" by Thom Hogan, published by Microsoft Press, 1988 2. "IBM Technical Reference Options and Adapters" Volume 2, "Game Control Adapter", published by IBM
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