Game ScriptingGame Scripting

by: Nicholas Haines

What is Scripting?What is Scripting?

Interpreted Interpreted LanguageLanguage– As the game runsAs the game runs

AdvantagesAdvantages

Ease of useEase of use Makes the game more data drivenMakes the game more data driven

– Instead of hard coding into game engineInstead of hard coding into game engine Allows for in-game “tweaking”Allows for in-game “tweaking”

– Quick resultsQuick results– Does not require a recompileDoes not require a recompile

Allows user modability Allows user modability Game can be patched at a later dateGame can be patched at a later date Many publicly available scripting languagesMany publicly available scripting languages

DisadvantagesDisadvantages

PerformancePerformance– Not noticeable most of the time inNot noticeable most of the time in

real world performancereal world performance Automatic memory managementAutomatic memory management

– Can cause problems if it interrupts a Can cause problems if it interrupts a command or takes awhile to completecommand or takes awhile to complete

Poor debugging toolsPoor debugging tools– Some languages don’t give warningsSome languages don’t give warnings– Tends to be very hard to find the errorsTends to be very hard to find the errors

Optimizing Script ExecutionOptimizing Script Execution

Using PatternsUsing Patterns Object: Reduce the number of instructionsObject: Reduce the number of instructions Look for patterns in the opcode outputLook for patterns in the opcode output

– Replace with optimized versionReplace with optimized version Check for addresses calculated multiple Check for addresses calculated multiple

times and save to a spare registertimes and save to a spare register– Replace matching with reference to registerReplace matching with reference to register

May shorten code by up to 50%May shorten code by up to 50%– (assuming decoding script is the bottleneck of (assuming decoding script is the bottleneck of

the virtual machine [VM])the virtual machine [VM])

Non-Branched, Constant Non-Branched, Constant ScriptsScripts

Native function calls can be Native function calls can be intercepted inside the VMintercepted inside the VM

Store as a function pointer along its Store as a function pointer along its argumentsarguments

Big list of stored functions is called an Big list of stored functions is called an Execute ListExecute List

Will now traverse execute list instead Will now traverse execute list instead of having to interpret the opcode of having to interpret the opcode againagain

Branched ScriptsBranched Scripts Conditional jumps cannot be Conditional jumps cannot be

predictedpredicted A A Jump TableJump Table can be used to can be used to

decrease decode time during decrease decode time during executionexecution

Contains the offset of native Contains the offset of native opcode executing specific opcode executing specific script opcodescript opcode

Use pure jump table opcodes Use pure jump table opcodes for most common instructionsfor most common instructions

Combine jump table/parse for Combine jump table/parse for less common instructions to less common instructions to reduce the table sizereduce the table size

Note: jump table values change Note: jump table values change wheneverwhenever

the VM’s code is changedthe VM’s code is changed

Example to be decoded mov [vm_reg0], vm_reg1

(using x86 asm)

;get contents of vm register 0 into eax mov eax, [ecx+0];get contents of vm register 1 into ebx mov ebx, [ecx+4];solve indirection mov [eax], ebx;increment vm instruction pointer add edx, 4;transfer program control to next

opcode jump [edx]

Branched into Non-Branched into Non-BranchedBranched

Monster::PlayerSpotted(bool hear_player, bool see_player, Monster::PlayerSpotted(bool hear_player, bool see_player, float* pos)float* pos)

{{

if(!see_player)if(!see_player)

{{

if(hear_player)if(hear_player)

{{

//we have heard something..check out//we have heard something..check out

//situation..keep an eye on the place//situation..keep an eye on the place

FocusAttention(pos); //native functionFocusAttention(pos); //native function

}}

elseelse

{{

//someone told us about the//someone told us about the

//player (monster friend or so)//player (monster friend or so)

//let's help our friends//let's help our friends

WalkTo(pos); //native functionWalkTo(pos); //native function

}}

}}

elseelse

{{

//we can see the enemy..let's kill him!//we can see the enemy..let's kill him!

ShootAt(pos); //native functionShootAt(pos); //native function

}}

}}

Generates each Generates each possibility and removes possibility and removes branchingbranching

Code to the left Code to the left becomes:becomes:

Monster::PlayerSpotted(bool hear_player=true,Monster::PlayerSpotted(bool hear_player=true,

bool see_player=false,bool see_player=false,

float* pos)float* pos)

{{

FocusAttention(pos);FocusAttention(pos);

}}

This can be optimized This can be optimized using execution listsusing execution lists

Advanced Script DebuggingAdvanced Script Debugging

Exchanging Debug Exchanging Debug InformationInformation

Debugger creates Debugger creates a new processes a new processes that uses the VM to that uses the VM to execute scriptsexecute scripts

VM sends string VM sends string messages to messages to DebuggerDebugger

VM needs memory VM needs memory buffers for the buffers for the Debugger to write Debugger to write toto

Useful FeaturesUseful Features

Debug informationDebug information Handling Handling

BreakpointsBreakpoints Variable WatchVariable Watch Call StackCall Stack Step-by-Step Step-by-Step

ExecutionExecution

Step-by-Step Step-by-Step Assembly Assembly ExecutionExecution

Register WatchRegister Watch Memory WatchMemory Watch Step-by-Step High-Step-by-Step High-

Level ExecutionLevel Execution Step Over Step Over

ExecutionExecution

Adding Error ReportingAdding Error Reporting

Create A Parser For Your Create A Parser For Your ScriptScript

Use tools like Lex and YaccUse tools like Lex and Yacc Since mostly nonprogrammers use Since mostly nonprogrammers use

the scripting language, make the scripting language, make meaningful error messages.meaningful error messages.

A Simple LanguageA Simple Language

Example code:Example code:

int x;int x;

int y;int y;

y = 0;y = 0;

x = SomeFunction() - x = SomeFunction() - 5;5;

if( x > if( x > OtherFunction() )OtherFunction() )

y = 3;y = 3;

Possible ErrorsPossible Errors– Semicolon missingSemicolon missing– Parentheses missingParentheses missing– Unrecognized Unrecognized

characterscharacters– Unrecognized Unrecognized

keywordkeyword– Function misspelledFunction misspelled– Undeclared variableUndeclared variable– Variable declared Variable declared

more than oncemore than once

Reporting with yyerror()Reporting with yyerror()

Prints “syntax error” to standard outputPrints “syntax error” to standard output Overwrite yyerror() with a useful Overwrite yyerror() with a useful

versionversion Derive a custom scanner functionDerive a custom scanner function Print formatted error messages to a Print formatted error messages to a

specified error filespecified error file– Include line number and copy of the lineInclude line number and copy of the line

This can be stored as a log or displayed This can be stored as a log or displayed in another applicationin another application

Identifying Specific ErrorsIdentifying Specific Errors

Unknown Character ErrorsUnknown Character Errors . yyerror(“Error: Unknown character ‘%c’ “, *yytext);. yyerror(“Error: Unknown character ‘%c’ “, *yytext);

Append grammar to trap missing Append grammar to trap missing semicolonsemicolon

Do the same to catch unrecognized Do the same to catch unrecognized functionsfunctions

Undeclared and Redeclared VariablesUndeclared and Redeclared Variables– Parser must check variable table to see if the Parser must check variable table to see if the

variable has been previously declaredvariable has been previously declared

ReferencesReferences

AI Game Programming WISDOM 2AI Game Programming WISDOM 2Sections 9.1-9.3Sections 9.1-9.3

Game Dev Articles: Game Dev Articles:

Scripting by Falko Poiker Scripting by Falko Poiker http://http://www.relic.com/industry/articles/scripting.phpwww.relic.com/industry/articles/scripting.php

The Secret Life of Game Scripting The Secret Life of Game Scripting

By Brian HookBy Brian Hook

http://www.bookofhook.com/Article/GameDevelophttp://www.bookofhook.com/Article/GameDevelopment/TheSecretLifeofGameScript.htmlment/TheSecretLifeofGameScript.html