! Smaller, simpler, subcomponent of program! Provides abstraction. n hide low-level details, give high-level structure

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1 Function Chpte 1 Functions Oiginl slides fom Gegoy Byd, Noth Colin Stte Univesity Modified slides by Chis Wilcox, Colodo Stte Univesity! Smlle, simple, subcomponent of pogm! Povides bstction n hide lo-level detils, give high-level stuctue n esie to undestnd ovell pogm flo n enbles sepble, independent development! C functions n zeo o multiple guments pssed in n single esult etuned (optionl) n etun vlue is lys pticul type! In othe lnguges, clled pocedues, suboutines, 2 Exmple of High-Level Stuctue min() SetupBod(); /* plce pieces on bod */ DetemineSides(); /* choose blck/hite */ /* Ply gme */ Stuctue of pogm do is evident, even ithout WhitesTun(); knoing implementtion BlcksTun(); hile (NoOutcomeYet()); Functions in C! Decltion (lso clled pototype) Fctoil( n); type of etun vlue nme of function types of ll guments! Function cll -- used in expession = x + Fctoil(f + g); 1 evlute guments 2, execute function 3 use etun vlue in expession 3 1

2 Function Definition! Stte type, nme, types of guments n must mtch function decltion n give nme to ech gument (doesn't hve to mtch decltion) Fctoil( n) i; esult = 1; fo (i = 1; i <= n; i++) esult *= i; etun esult; gives contol bck to clling function nd etuns vlue Why Decltion?! Since function definition lso includes etun nd gument types, hy is decltion needed?! Use might be seen befoe definition Compile needs to kno etun nd g types nd numbe of guments! Definition might be in diffeent file, itten by diffeent pogmme n include "hede" file ith function decltions only n compile septely, link togethe to mke executble 6 Exmple double VlueInDolls(double mount, double te); min() function decltion (pototype) function cll (invoction) dolls = VlueInDolls(fncs, DOLLARS_PER_FRANC); pf("%f fncs euls %f dolls\n", fncs, dolls); function definition (code) double VlueInDolls(double mount, double te) etun mount * te; Implementing Functions: Ovevie! Activtion ecod (stck fme) n infomtion bout ech function, including guments nd locl vibles n stoed on un-time stck Clling function push ne ctivtion ecod copy vlues o guments cll function get esult fom stck Clled function execute code put esult in ctivtion ecod pop ctivtion ecod fom stck etun 7 8 2

3 Run-Time Stck! Recll tht locl vibles e stoed on the un-time stck in n ctivtion ecod! Stck Poe () is poe to the next fee loction in the stck, nd is used to push nd pop vlues on nd off the stck! Fme poe () is poe to the beginning of egion of the ctivtion ecod tht stoes locl vibles fo the cuent function! When ne function is clled, its ctivtion ecod is pushed on the stck; hen it etuns, its ctivtion ecod is popped off of the stck Run-Time Stck Memoy Memoy Memoy Wtt min min min Befoe cll Duing cll Afte cll 9 Activtion Recod! NoNme(, b), x, y; etun y; Nme Type Offset Scope b x y NoNme NoNme NoNme NoNme NoNme bookkeeping y x dynmic link etun ddess etun vlue b locls gs Activtion Recod Bookkeeping! Retun vlue n spce fo vlue etuned by function n llocted even if function does not etun vlue! Retun ddess n sve poe to next instuction in clling function n convenient loction to stoe R7 in cse nothe function (JSR) is clled! Dynmic link n n clle s fme poe used to pop this ctivtion ecod fom stck

4 Exmple Function Cll! Volt(, ) k; m; etun k; Wtt( ) ; = Volt(,); etun ; 13 Clling the Function! = Volt(, );! ; push second g AND R0, R0, #0 ADD R0, R0, # ADD,, #-1 STR R0,, #0 ; push fist gument LDR R0,, #0 ADD,, #-1 STR R0,, #0 ; cll suboutine JSR Volt ne Note: Clle needs to kno numbe nd type of guments, doesn't kno bout locl vibles 1 Stting the Cllee Function Ending the Cllee Function! ; leve spce fo etun vlue ADD,, #-1 ne ne ; push etun ddess ADD,, #-1 STR R7,, #0 ; push clle s fme pt ADD,, #-1 STR,, #0 ; set ne fme poe ADD,, #-1 ; llocte spce fo locls ADD,, #-2 xfcfb x30 m k 1! etun k;! ; copy k o etun vlue LDR R0,, #0 STR R0,, #3 ; pop locl vibles ADD,, #1 ne ; pop dynmic link (o ) LDR,, #0 ADD,, #1 ; pop etun dd (o R7) LDR R7,, #0 ADD,, #1 ; etun contol to clle RET ne -3 xfcfb x30 m k 16

5 Resuming the Clle Function! = Volt(,);! JSR Volt ; lod etun vlue ; fom top of stck LDR R0,, #0 ; pefom ssignment STR R0,, #0 ; pop etun vlue ADD,, #1 ; pop guments ADD,, #2 ne Summy of LC-3 Function Cll Implementtion 1 Clle pushes guments (lst to fist) 2 Clle invokes suboutine (JSR) 3 Cllee lloctes etun vlue, pushes R7 nd Cllee lloctes spce fo locl vibles Cllee executes function code 6 Cllee stoes esult o etun vlue slot 7 Cllee pops locl vs, pops, pops R7 8 Cllee etuns (JMP R7) 9 Clle lods etun vlue nd pops guments Clle esumes computtion 17 18