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COMPUTER BITS BY DARREN YATES Experiments with your games card; Pt.4 This month, we look at the games card port & learn how each bit is defined. We also discuss how you can use the BIOS interrupt routines to get fast information about the port. In the January 1994 issue, we looked at how you can tell whether or not a PC has a games card installed (apart from having a look at the back). This was done using a machine code routine inside a QBASIC program. A similar process is used by games programs to find out this information. So far we’ve spent some time on the analog inputs which read the X and Y coordinates of a joystick. (Remember that the card can handle two joysticks at once). However, we haven’t covered the four digital inputs which are accessible via the fire buttons of the joysticks. These digital inputs are quite easy to use – in fact, much easier to use than the printer port inputs. Let’s take a look at the pinout diagram for the joystick DB15 socket – see Fig.1. As you can see, the four fire buttons in the joysticks (S1-S4) simply pull their corresponding inputs to ground. We don’t need to worry about maintaining 5V logic lines or anything else – we can simply pull each line to ground or leave it open and we can do this with a single transistor. Bits 7 to 4 are initially set to ‘1’ and become ‘0’ when the joystick button is pressed. You can test this by soldering two wires to pins 10 and 12 of the joystick adaptor plug and joining them together while running this short program in QBASIC. WHILE A$=”” A=INP(&H201) PRINT A A$=INPUT$(1) WEND You should see the number ‘243’ flash down the lefthand side of your screen and whenever you join the wires together, the number should change to ‘211’. The numbers themselves are not important but you should see the number change every Games card port Just as your printer card and serial card both have their own port addresses (the printer port is usually 0378H and the serial port 03F8H), so too does your games card and its address is 0201H. If we have a look at Table 1, we can see how each of the 8 bits is used. time you join the two wires. Alternatively, you could plug in a joystick if you have one handy, and run the same program while pressing the fire buttons. You should find that a similar thing happens – the ‘243’ number on the screen should change. What it changes to will depend upon which input your joystick is connect­ed to – either A or B. The four least significant bits each determine (in a way) the coordinate from the joysticks. This probably won’t be obvious from the outset but they work like this. Remember how we looked previously at the 558 timer circuit and how the joystick control formed part of the monostable cir­cuit? Well, the output of each monostable appears at one of these bits. Now the whole idea is that while the bit remains high, a counter should be counting elsewhere in the program to keep track of the time. When the bit goes low, the count represents a pro­portional figure to the joystick position. As you’ll probably agree, while using this port for the digital inputs is fairly straightforward, there is quite a bit more work to be done on the data from this port in order to obtain the analog inputs. If you think about your favourite flight simulator, just imagine how much calculation has to go on for the joy­stick alone to figure out where you are! BIOS interrupt Fig.1: the pin connection details for the DB15 sockets on a games card. Thankfully, this is where the BIOS interrupt routines come in - in particular INT 15H, SERVICE 84H. Table 2 shows how this works. Last time, we were able to obtain hardware information about the games card by using INT 11H and we can February 1994  79 Table 1: Game Adapter AB Joystick Data Byte Protect your valuable issues Silicon Chip Binders Bit Number 7 6 5 4 3 2 1 0 ✔ Status of B joystick button 2 ✔ Status of B joystick button 1 ✔ Status of A joystick button 2 ✔ Status of A joystick button 1 ✔ B joystick Y coordinate* ✔ B joystick X coordinate* ✔ A joystick Y coordinate* ✔ These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. ★ High quality ★ Hold up to 14 issues ★ 80mm internal width ★ SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A14.95 (includes postage in Australia). NZ & PNG orders please add $A5 each for postage. Not available elsewhere. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. ➦ Use this handy form Enclosed is my cheque/money order for $________ or please debit my ❏ Bankcard   ❏ Visa   ❏ Mastercard Card No: ______________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ 80  Silicon Chip Function A joystick X coordinate* also obtain the joystick coordinates as well as the button inputs using this interrupt routine. Interrupt routines Before we dive headlong into more machine code, the idea of interrupt routines may well be new to some readers, so we’ll take a more leisurely approach. Looking at Table 2, this routine, like most of the BIOS and DOS interrupts, uses the four general purpose 16-bit registers inside the 8086/8088/80286/386/486 processor; ie, AX, BX, CX and DX. Each of these can be split in half to give two 8-bit reg­isters – high and low. For the accumulator register AX, we can access the high eight bits by referring to AH and the low eight bits by referring to AL. The other three 16bit registers give, respectively, BH, BL, CH, CL, DH and DL. Remember that there are only four registers but they can be split or, more accurately, ‘selected’ as eight bits wide by using the ‘H’ and ‘L’ suffixes. In the 80386/486 processors, the AX to DX registers are themselves the lower 16-bit ‘splits’ of 32-bit registers EAX through to EDX, the ‘E’ prefix standing for ‘extended’. It’s not possible though to access the higher 16-bit sections of each of these extended registers. If we look back at Table 2, before the interrupt can be called using the instruction INT 15H, we have to load the number 84H into the upper 8-bit section of AX, namely AH. This tells the PC that we want a particular service out of those available from INT 15H. You can think of this service number as a house number and the interrupt number as a street name. We still have a further choice to make and that deals with the amount of information returned. By setting the DX register to ‘0’, the only information returned from the interrupt routine is just the joystick fire button settings and these are returned in Table 2: Joystick Support (Interrupt 15h, Service 84h) Registers on entry: AH:84h DX: 00h = read switches 01h = read joystick position Registers on Return: If reading switches (DX=0): AL = switch settings (bits 4-7) If reading position (DX=1); AX=A(X) value BX=A(Y) value CX=B(X) value DX=b(Y) value Memory affected: None Note: This service is not available on PC machines released prior to 1983. register AL. However, if we set DX to ‘1’, then register AX returns the X-axis coordinate from joystick A, BX the y-axis, CX the x-axis from joystick B and DX the y-axis. Note that the joystick button settings are ignored in this case. One good thing about this routine is that it returns all four coordinates at once, giving you the possibility of having four analog inputs sampled at the same time. Example OK, let’s take our new-found know­ ledge and write a short program. Let’s keep it simple and just return the settings of the fire buttons. Remember that these appear in register AL. The program, called BUTTON.BAS, uses a machine-code program inside QBASIC to get the information we want. If you’ve been following this series, you’ll notice the similarity between this program and the Games Card Finder program presented in the January 1994 issue. Going through it briefly, the machine code program is stored in the integer array ASMPROG. Lines 3 and 4 of the machine code load 84H into register AH and 0H into DX. After that, the INT 15H call is made. Again, the VARPTR and VARSEG provide the specific address information of the first element in the program array. This is so control is transferred to the correct position and so the program starts and runs correctly. The joystick fire button information is returned in the variable BUTTON. Each fire button bit is then separated out and passed to the BUTTON array from 1 to 4. As the comments in the program show, bit 7 from port 201H corresponds to button 1 which gives the value 128 if the button is not pressed, bit 6 corre­sponds to button 2 which gives 64 and so on – down to 32 and finally a value of 16 in BUTTON(4). If any of these buttons have been pressed however, the corresponding BUTTON array will give a value of ‘0’. This information is then printed on the screen for each of the four buttons. The FOR..NEXT loop cuts down on the program lines and simply checks each BUTTON array in turn, calculating what the correct number should be in that array element if that button wasn’t pressed. Button.Bas: Joystick Button Finder Program ‘Joystick button finder ‘Copyright 1993 SILICON CHIP ‘Written by DARREN YATES B.Sc. ‘ This program uses the BIOS interrupt 15, service 84 to obtain ‘ the status of the four fire buttons without using the BASIC ‘ commands. DEFINT A-Z DIM ASMPROG(1 TO 10) DIM button(1 TO 4) ‘The machine-code program is stored in the array ASMPROG and read ‘and read into the array. ASMBYTES: DATA &h55 : ‘PUSH BP save base pointer DATA &h8b,&hec : ‘MOV BP,SP get our own DATA &hb4,&h84 : ‘MOV AH,84H set service number DATA &hba,&h00,&h00 : ‘MOV DX,0000 select button input data only DATA &hcd,&h15 : ‘INT 15H make ROM-BIOS call DATA &h8b,&h5e,&h06 : ‘MOV BX,[BP+6] get argument address DATA &h88,&h07 : ‘MOV [BX],AL save list in argument DATA &h5d : ‘POP BP pop argument off stack DATA &hca,&h02,&h00 : ‘RET 2 and make far return to BASIC ‘get the starting offset of the array start = VARPTR(ASMPROG(1)) ‘poke machine code program into the array ASMPROG DEF SEG = VARSEG(ASMPROG(1)) RESTORE ASMBYTES FOR index = 0 TO 18 READ byte POKE (start + index), byte NEXT index ‘run the machine-code program start = VARPTR(ASMPROG(1)) CALL absolute(button, start) DEF SEG ‘variable BUTTON now contains info on the joystick buttons button(1) = button AND &H80 button(2) = button AND &H40 button(3) = button AND &H20 button(4) = button AND &H10 PRINT STRING$(18, 205) ‘this section selects the correct bit for each button ‘ bit 7 = button 1; bit 6 = button 2; bit 5 = button 3; bit 4 = button 4 ‘ if that bit is 0 then button is pressed FOR number = 1 TO 4 IF button(number) = 2 ^ (8 - number) THEN PRINT “button “; number; “ is open” ELSE PRINT “button “; number; “ is pressed” END IF NEXT number That’s it for this month. If you’re not sure about any of the foregoing, then read the article again – this topic is fairly complex the first time you come across it but it gets easier once you become more familiar with it. Next time, we’ll look at installing a games card into a PC and give some guidelines on using the in-built 5V SC power supply. February 1994  81