The Art of
ASSEMBLY LANGUAGE PROGRAMMING

Chapter Twenty Three

Table of Content

Chapter Twenty Four (Part 2)

CHAPTER TWENTY FOUR:
THE PC GAME ADAPTER (Part 1)
24.1 - Typical Game Devices
24.2 - The Game Adapter Hardware
24.3 - Using BIOS' Game I/O Functions
24.4 - Writing Your Own Game I/O Routines
24.5 - The Standard Game Device Interface (SGDI)
24.5.1 - Application Programmer's Interface (API)
24.5.2 - Read4Sw
24.5.3 - Read4Pots:
24.5.4 - ReadPot
24.5.5 - Read4:
24.5.6 - CalibratePot
24.5.7 - TestPotCalibration
24.5.8 - ReadRaw
24.5.9 - ReadSwitch
24.5.10 - Read16Sw
24.5.11 - Remove
4.5.12 - TestPresence
24.5.13 - An SGDI Driver for the Standard Game Adapter Card
24.6 - An SGDI Driver for the CH Products' Flight Stick Pro'
24.7 - Patching Existing Games
Copyright 1996 by Randall Hyde All rights reserved.

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One need look no farther than the internals of several popular games on the PC to discover than many programmers do not fully understand one of the least complex devices attached to the PC today - the analog game adapter. This device allows a user to connect up to four resistive potentiometers and four digital switch connections to the PC. The design of the PC's game adapter was obviously influenced by the analog input capabilities of the Apple II computer the most popular computer available at the time the PC was developed. Although IBM provided for twice the analog inputs of the Apple II thinking that would give them an edge their decision to support only four switches and four potentiometers (or "pots") seems confining to game designers today - in much the same way that IBM's decision to support 256K RAM seems so limiting today. Nevertheless game designers have managed to create some really marvelous products even living with the limitations of IBM's 1981 design.

IBM's analog input design like Apple's was designed to be dirt cheap. Accuracy and performance were not a concern at all. In fact you can purchase the electronic parts to build your own version of the game adapter at retail for under three dollars. Indeed today you can purchase a game adapter card from various discount merchants for under eight dollars. Unfortunately IBM's low-cost design in 1981 produces some major performance problems for high-speed machines and high-performance game software in the 1990's. However there is no use crying over spilled milk - we're stuck with the original game adapter design we need to make the most of it. The following sections will describe how to do exactly that.

24.1 Typical Game Devices

The game adapter is nothing more than a computer interface to various game input devices. The game adapter card typically contains a DB15 connector into which you plug an external device. Typical devices you can obtain for the game adapter include paddles joysticks flight yokes digital joysticks rudder pedals RC simulators and steering wheels. Undoubtedly this is but a short list of the types of devices you can connect to the game adapter. Most of these devices are far more expensive that the game adapter card itself. Indeed certain high performance flight simulator consoles for the game adapter cost several hundred dollars.

The digital joystick is probably the least complex device you can connect to the PC's game port. This device consists of four switches and a stick. Pushing the stick forward left right or pulling it backward closes one of the switches. The game adapter card provides four switch inputs so you can sense which direction (including the rest position) the user is pressing the digital joystick. Most digital joysticks also allow you to sense the in-between positions by closing two contacts at once. for example pushing the control stick at a 45 degree angle between forward and right closes both the forward and right switches. The application software can sense this and take appropriate action. The original allure of these devices is that they were very cheap to manufacture (these were the original joysticks found on most home game machines). However as manufacturers increased production of analog joysticks the price fell to the point that digital joysticks failed to offer a substantial price difference. So today you will rarely encounter such devices in the hands of a typical user.

The game paddle is another device whose use has declined over the years. A game paddle is a single pot in a case with a single knob (and typically a single push button). Apple used to ship a pair of game paddles with every Apple II they sold. As a result games that used game paddles were still quite popular when IBM released the PC in 1981. Indeed a couple manufacturers produced game paddles for the PC when it was first introduced. However once again the cost of manufacturing analog joysticks fell to the point that paddles couldn't compete. Although paddles are the appropriate input device for many games joysticks could do just about everything a game paddle could and more. So the use of game paddles quickly died out. There is one thing you can do with game paddles that you cannot do with joysticks - you can place four of them on a system and produce a four player game. However this (obviously) isn't important to most game designers who generally design their games for only one player.

Rudder pedals are really nothing more than a specially designed game paddle designed so you can activate them with your feet. Many flight simulator games take advantage of this input device to provide a more realistic experience. Generally you would use rudder pedals in addition to a joystick device.

A joystick contains two pots connected with a stick. Moving the joystick along the x-axis actuates one of the pots moving the joystick along the y-axis actuates the other pot. By reading both pots you can roughly determine the absolute position of the pot within its working range.

An RC simulator is really nothing more than a box containing two joysticks. The yoke and steering wheel devices are essentially the same device sold specifically for flight simulators or automotive games. The steering wheel is connected to a pot that corresponds to the x-axis on the joystick. Pulling back (or pushing forward) on the wheel activates a second pot that corresponds to the y-axis on the joystick.

Certain joystick devices generically known as flight sticks contain three pots. Two pots are connected in a standard joystick fashion the third is connected to a knob which many games use for the throttle control. Other joysticks like the Thrustmaster' or CH Products' FlightStick Pro include extra switches including a special "cooley switch" that provide additional inputs to the game. The cooley switch is essentially a digital pot mounted on the top of a joystick. Users can select one of four positions on the cooley switch using their thumb. Most flight simulator programs compatible with such devices use the cooley switch to select different views from the aircraft.

24.2 The Game Adapter Hardware

The game adapter hardware is simplicity itself. There is a single input port and a single output port. The input port bit layout is

The four switches come in on the H.O. four bits of I/O port 201h. If the user is currently pressing a button the corresponding bit position will contain a zero. If the button is up the corresponding bit will contain a one.

The pot inputs might seem strange at first glance. After all how can we represent one of a large number of potential pot positions (say at least 256) with a single bit? Obviously we can't. However the input bit on this port does not return any type of numeric value specifying the pot position. Instead each of the four pot bits is connected to an input of a resistive sensitive 558 quad timer chip. When you trigger the timer chip it produces an output pulse whose duration is proportional to the resistive input to the timer. The output of this timer chip appears as the input bit for a given pot. The schematic for this circuit is

Normally the pot input bits contain zero. When you trigger the timer chip the pot input lines go high for some period of time determined by the current resistance of the potentiometer. By measuring how long this bit stays set you can get a rough estimate of the resistance. To trigger the pots simply write any value to I/O port 201h. The actual value you write is unimportant. The following timing diagram shows how the signal varies on each pot's input bit:

The only remaining question is "how do we determine the length of the pulse?" The following short loop demonstrates one way to determine the width of this timing pulse:

                mov     cx
-1          ;We're going to count backwards
mov     dx
201h        ;Point at joystick port.
out     dx
al          ;Trigger the timer chip.
CntLp:          in      al
dx          ;Read joystick port.
test    al
1           ;Check pot #0 input.
loopne  CntLp           ;Repeat while high.
neg     cx              ;Convert CX to a positive value.

When this loop finish execution the cx register will contain the number of passes made through this loop while the timer output signal was a logic one. The larger the value in cx the longer the pulse and therefore the greater the resistance of pot #0.

There are several minor problems with this code. First of all the code will obviously produce different results on different machines running at different clock rates. For example a 150 MHz Pentium system will execute this code much faster than a 5 MHz 8088 system. The second problem is that different joysticks and different game adapter cards produce radically different timing results. Even on the same system with the same adapter card and joystick you may not always get consistent readings on different days. It turns out that the 558 is somewhat temperature sensitive and will produce slightly different readings as the temperature changes.

Unfortunately there is no way to design a loop like the above so that it returns consistent readings across a wide variety of machines potentiometers and game adapter cards. Therefore you have to write your application software so that it is insensitive to wide variances in the input values from the analog inputs. Fortunately this is very easy to do but more on that later.

24.3 Using BIOS' Game I/O Functions

The BIOS provides two functions for reading game adapter inputs. Both are subfunctions of the int 15h handler.

To read the switches load ah with 84h and dx with zero then execute an int 15h instruction. On return al will contain the switch readings in the H.O. four bits (see the diagram in the previous section). This function is roughly equivalent to reading port 201h directly.

To read the analog inputs load ah with 84h and dx with one then execute an int 15h instruction. On return AX BX CX and DX will contain the values for pots zero one two and three respectively. In practice this call should return values in the range 0-400h though you cannot count on this for reasons described in the previous section.

Very few programs use the BIOS joystick support. It's easier to read the switches directly and reading the pots is not that much more work that calling the BIOS routine. The BIOS code is very slow. Most BIOSes read the four pots sequentially taking up to four times longer than a program that reads all four pots concurrently (see the next section). Because reading the pots can take several hundred microseconds up to several milliseconds most programmers writing high performance games do not use the BIOS calls they write their own high performance routines instead.

This is a real shame. By writing drivers specific to the PC's original game adapter design these developers force the user to purchase and use a standard game adapter card and game input device. Were the game to make the BIOS call third party developers could create different and unique game controllers and then simply supply a driver that replaces the int 15h routine and provides the same programming interface. For example Genovation made a device that lets you plug a joystick into the parallel port of a PC. Colorado Spectrum created a similar device that lets you plug a joystick into the serial port. Both devices would let you use a joystick on machines that do not (and perhaps cannot) have a game adapter installed. However games that access the joystick hardware directly will not be compatible with such devices. However had the game designer made the int 15h call their software would have been compatible since both Colorado Spectrum and Genovation supply int 15h TSRs to reroute joystick calls to use their devices.

To help overcome game designer's aversion to using the int 15h calls this text will present a high performance version of the BIOS' joystick code a little later in this chapter. Developers who adopt this Standard Game Device Interface will create software that will be compatible with any other device that supports the SGDI standard. For more details see "The Standard Game Device Interface (SGDI)".

24.4 Writing Your Own Game I/O Routines

Consider again the code that returns some value for a given pot setting:

                mov     cx
-1          ;We're going to count backwards
mov     dx
201h        ;Point at joystick port.
out     dx
al          ;Trigger the timer chip.
CntLp:          in      al
dx          ;Read joystick port.
test    al
1           ;Check pot #0 input.
loopne  CntLp           ;Repeat while high.
neg     cx              ;Convert CX to a positive value.

As mentioned earlier the big problem with this code is that you are going to get wildly different ranges of values from different game adapter cards input devices and computer systems. Clearly you cannot count on the code above always producing a value in the range 0..180h under these conditions. Your software will need to dynamically adjust the values it uses depending on the system parameters.

You've probably played a game on the PC where the software asks you to calibrate the joystick before use. Calibration generally consists of moving the joystick handle to one corner (e.g. the upper-left corner) pressing a button or key and them moving the handle to the opposite corner (e.g. lower-right) and pressing a button again. Some systems even want you to move the joystick to the center position and press a button as well.

Software that does this is reading the minimum maximum and centered values from the joystick. Given at least the minimum and maximum values you can easily scale any reading to any range you want. By reading the centered value as well you can get slightly better results especially on really inexpensive (cheap) joysticks. This process of scaling a reading to a certain range is known as normalization. By reading the minimum and maximum values from the user and normalizing every reading thereafter you can write your programs assuming that the values always fall within a certain range for example 0..255. To normalize a reading is very easy you simply use the following formula:

The MaximumReading and MinimumReading values are the minimum and maximum values read from the user at the beginning of your application. CurrentReading is the value just read from the game adapter. NormalValue is the upper bounds on the range to which you want to normalize the reading (e.g. 255) the lower bound is always zero.

To get better results especially when using a joystick you should obtain three readings during the calibration phase for each pot - a minimum value a maximum value and a centered value. To normalize a reading when you've got these three values you would use one of the following formulae:

If the current reading is in the range minimum..center use this formula:

If the current reading is in the range center..maximum use this formula:

A large number of games on the market today jump through all kinds of hoops trying to coerce joystick readings into a reasonable range. It is surprising how few of them use that simple formula above. Some game designers might argue that the formulae above are overly complex and they are writing high performance games. This is nonsense. It takes two orders of magnitude more time to wait for the joystick to time out than it does to compute the above equations. So use them and make your programs easier to write.

Although normalizing your pot readings takes so little time it is always worthwhile reading the analog inputs is a very expensive operation in terms of CPU cycles. Since the timer circuit produces relatively fixed time delays for a given resistance you will waste even more CPU cycles on a fast machine than you do on a slow machine (although reading the pot takes about the same amount of real time on any machine). One sure fire way to waste a lot of time is to read several pots one at a time; for example when reading pots zero and one to get a joystick reading read pot zero first and then read pot one afterwards. It turns out that you can easily read both pots in parallel. By doing so you can speed up reading the joystick by a factor of two. Consider the following code:

                mov     cx
1000h       ;Max times through loop
mov     si
0           ;We'll put readings in SI and
mov     di
si          ; di.
mov     ax
si          ;Set AH to zero.
mov     dx
201h        ;Point at joystick port.
out     dx
al          ;Trigger the timer chip.
CntLp:          in      al
dx          ;Read joystick port.
and     al
11b         ;Strip unwanted bits.
jz      Done
shr     ax
1           ;Put pot 0 value into carry.
adc     si
0           ;Bump pot 0 value if still active.
add     di
ax          ;Bump pot 1 value if pot 1 active.
loop    CntLp           ;Repeat while high.
and     si
0FFFh       ;If time-out
force the register(s)
and     di
0FFFh       ; containing 1000h to zero.
Done:

This code reads both pot zero and pot one at the same time. It works by looping while either pot is active. Each time through the loop this code adds the pots' bit values to separate register that accumulator the result. When this loop terminates si and di contain the readings for both pots zero and one.

Although this particular loop contains more instructions than the previous loop it still takes the same amount of time to execute. Remember the output pulses on the 558 timer determine how long this code takes to execute the number of instructions in the loop contribute very little to the execution time. However the time this loop takes to execute one iteration of the loop does effect the resolution of this joystick read routine. The faster the loop executes the more iterations the loop will run during the same timing period and the finer will be the measurement. Generally though the resolution of the above code is much greater than the accuracy of the electronics and game input device so this isn't much of a concern.

The code above demonstrates how to read two pots. It is very easy to extend this code to read three or four pots. An example of such a routine appears in the section on the SGDI device driver for the standard game adapter card.

The other game device input the switches would seem to be simple in comparison to the potentiometer inputs. As usual things are not as easy as they would seem at first glance. The switch inputs have some problems of their own.

The first issue is keybounce. The switches on a typical joystick are probably an order of magnitude worse than the keys on the cheapest keyboard. Keybounce and lots of it is a fact you're going to have to deal with when reading joystick switches. In general you shouldn't read the joystick switches more often than once every 10 msec. Many games read the switches on the 55 msec timer interrupt. For example suppose your timer interrupt reads the switches and stores the result in a memory variable. The main application when wanting to fire a weapon checks the variable. If it's set the main program clears the variable and fires the weapon. Fifty-five milliseconds later the timer sets the button variable again and the main program will fire again the next time it checks the variable. Such a scheme will totally eliminate the problems with keybounce.

The technique above solves another problem with the switches: keeping track of when the button first goes down. Remember when you read the switches the bits that come back tell you that the switch is currently down. It does not tell you that the button was just pressed. You have to keep track of this yourself. One easy way to detect when a user first presses a button is to save the previous switch reading and compare it against the current reading. If they are different and the current reading indicates a switch depression then this is a new switch down.

24.5 The Standard Game Device Interface (SGDI)

The Standard Game Device Interface (SGDI) is a specification for an int 15h service that lets you read an arbitrary number of pots and joysticks. Writing SGDI compliant applications is easy and helps make your software compatible with any game device which provides SGDI compliance. By writing your applications to use the SGDI API you can ensure that your applications will work with future devices that provide extended SGDI capability. To understand the power and extensibility of the SGDI you need to take a look at the application programmer's interface (API) for the SGDI.

24.5.1 Application Programmer's Interface (API)

The SGDI interface extends the PC's joystick BIOS int 15h API. You make SGDI calls by loading the 80x86 ah register with 84h and dx with an appropriate SGDI function code and then executing an int 15h instruction. The SGDI interface simply extends the functionality of the built-in BIOS routines. Note that and program that calls the standard BIOS joystick routines will work with an SGDI driver. The following table lists each of the SGDI functions:

SGDI Functions and API (int 15h ah=84h)
DH Inputs Outputs Description
00 dl = 0 al- Switch readings Read4Sw. This is the standard BIOS subfunction zero call. This reads the status of the first four switches and returns their values in the upper four bits of the al register.
00 dl = 1 ax- pot 0

bx- pot 1

cx- pot 2

dx- pot 3
Read4Pots. Standard BIOS subfunction one call. Reads all four pots (concurrently) and returns their raw values in ax bx cx and dx as per BIOS specifications.
01 dl = pot # al= pot reading ReadPot. This function reads a pot and returns a normalized reading in the range 0..255.
02 dl = 0

al = pot mask
al = pot 0

ah = pot 1

dl = pot 2

dh = pot 3
Read4. This routine reads the four pots on the standard game adapter card just like the Read4Pots function above. However this routine normalizes the four values to the range 0..255 and returns those values in al ah dl and dh. On entry the al register contains a "pot mask" that you can use to select which of the four pots this routine actually reads.
03 dl = pot #

al = minimum

bx= maximum

cx= centered
- Calibrate. This function calibrates the pots for those calls that return normalized values. You must calibrate the pots before calling any such pot functions (ReadPot and Read4 above). The input values must be raw pot readings obtained by Read4Pots or other function that returns raw values.
04 dl = pot # al = 0 if not calibrated 1 if calibrated. TestPotCalibrate. Checks to see if the specified pot has already been calibrated. Returns an appropriate value in al denoting the calibration status for the specified pot. See the note above about the need for calibration.
05 dl = pot # ax = raw value ReadRaw. Reads a raw value from the specified pot. You can use this call to get the raw values required by the calibrate routine above.
08 dl= switch # ax = switch value ReadSw. Read the specified switch and returns zero (switch up) or one (switch down) in the ax register.
09 - ax = switch values Read16Sw. This call lets an application read up to 16 switches on a game device at a time. Bit zero of ax corresponds to switch zero bit 15 of ax corresponds to switch fifteen.
80h - - Remove. This function removes the driver from memory. Application programs generally won't make this call.
81h - - TestPresence. This routine returns zero in the ax register if an SGDI driver is present in memory. It returns ax's value unchanged otherwise (in particular ah will still contain 84h).

24.5.2 Read4Sw

Inputs: ah= 84h dx = 0

This is the standard BIOS read switches call. It returns the status switches zero through three on the joystick in the upper four bits of the al register. Bit four corresponds to switch zero bit five to switch one bit six to switch two and bit seven to switch three. One zero in each bit position denotes a depressed switch a one bit corresponds to a switch in the up position. This call is provided for compatibility with the existing BIOS joystick routines. To read the joystick switches you should use the Read16Sw call described later in this document.

24.5.3 Read4Pots:

Inputs: ah= 84h dx = 1

This is the standard BIOS read pots call. It reads the four pots on the standard game adapter card and returns their readings in the ax (x axis/pot 0) bx (y axis/pot 1) cx (pot 2) and dx (pot 3) registers. These are raw uncalibrated pot readings whose values will differ from machine to machine and vary depending upon the game I/O card in use. This call is provided for compatibility with the existing BIOS joystick routines. To read the pots you should use the ReadPot Read4 or ReadRaw routines described in the next several sections.

24.5.4 ReadPot

Inputs: ah=84h dh=1 dl=Pot number.

This reads the specified pot and returns a normalized pot value in the range 0..255 in the al register. This routine also sets ah to zero. Although the SGDI standard provides for up to 255 different pots most adapters only support pots zero one two and three. If you attempt to read any nonsupported pot this function returns zero in ax. Since the values are normalized this call returns comparable values for a given game control setting regardless of machine clock frequency or game I/O card in use. For example a reading of 128 corresponds (roughly) to the center setting on almost any machine. To properly produce normalized results you must calibrate a given pot before making this call. See the CalibratePot routine for more details.

24.5.5 Read4:

Inputs: ah = 84h al = pot mask dx=0200h

This routine reads the four pots on the game adapter card just like the BIOS call (Read4Pots). However it returns normalized values in al (x axis/pot 0) ah (y axis/pot 1) dl (pot 2) and dh (pot 3). Since this routine returns normalized values between zero and 255 you must calibrate the pots before calling this code. The al register contains a "pot mask" value. The L.O. four bits of al determine if this routine will actually read each pot. If bit zero one two or three is one then this function will read the corresponding pot; if the bits are zero this routine will not read the corresponding pot and will return zero in the corresponding register.

24.5.6 CalibratePot

Inputs: ah=84h dh=3 dl=pot # al=minimum value bx=maximum value cx=centered value.

Before you attempt to read a pot with the ReadPot or Read4 routines you need to calibrate that pot. If you read a pot without first calibrating it the SGDI driver will return only zero for that pot reading. To calibrate a pot you will need to read raw values for the pot in a minimum position maximum position and a centered position. These must be raw pot readings. Use readings obtained by the Read4Pots routine. In theory you need only calibrate a pot once after loading the SGDI driver. However temperature fluctuations and analog circuitry drift may decalibrate a pot after considerable use. Therefore you should recalibrate the pots you intend to read each time the user runs your application. Furthermore you should give the user the option of recalibrating the pots at any time within your program.

24.5.7 TestPotCalibration

Inputs: ah= 84h dh=4 dl = pot #.

This routine returns zero or one in ax denoting not calibrated or calibrated respectively. You can use the call to see if the pots you intend to use have already been calibrated and you can skip the calibration phase. Please however note the comments about drift in the previous paragraph.

24.5.8 ReadRaw

Inputs: ah = 84h dh = 5 dl = pot #

Reads the specified pot and returns a raw (not calibrated) value in ax. You can use this routine to obtain minimum centered and maximum values for use when calling the calibrate routine.

24.5.9 ReadSwitch

Inputs: ah= 84h dh = 8 dl = switch #

This routine reads the specified switch and returns zero in ax if the switch is not depressed. It returns one if the switch is depressed. Note that this value is opposite the bit settings the Read4Sw function returns.

If you attempt to read a switch number for an input that is not available on the current device the SGDI driver will return zero (switch up). Standard game devices only support switches zero through three and most joysticks only provide two switches. Therefore unless you are willing to tie your application to a specific device you shouldn't use any switches other than zero or one.

24.5.10 Read16Sw

Inputs: ah = 84h dh = 9

This SGDI routine reads up to sixteen switches with a single call. It returns a bit vector in the ax register with bit 0 corresponding to switch zero bit one corresponding to switch one etc. Ones denote switch depressed and zeros denote switches not depressed. Since the standard game adapter only supports four switches only bits zero through three of al contain meaningful data (for those devices). All other bits will always contain zero. SGDI drivers for the CH Product's Flightstick Pro and Thrustmaster joysticks will return bits for the entire set of switches available on those devices.

24.5.11 Remove

Inputs: ah= 84h dh= 80h

This call will attempt to remove the SGDI driver from memory. Generally only the SGDI.EXE code itself would invoke this routine. You should use the TestPresence routine (described next) to see if the driver was actually removed from memory by this call.

24.5.12 TestPresence

Inputs: ah=84h dh=81h

If an SGDI driver is present in memory this routine return ax=0 and a pointer to an identification string in es:bx. If an SGDI driver is not present this call will return ax unchanged.

Chapter Twenty Three

Table of Content

Chapter Twenty Four (Part 2)

Chapter Twenty Four: The PC Game Adapter (Part 1)
29 SEP 1996