- Replacement of a single statement with a sequence of statements(promotes regularity)

Transcription

1 ALGOL - Java and C built using ALGOL 60 - Simple and cncise and elegance - Universal - Clse as pssible t mathematical ntatin - Language can describe the algrithms - Mechanically translatable t machine language Prgram Structure - Hierarchical structure - Nested Cntrl Structures allws - Nested Envirnments - 2 statement types Declarative Variable declaratins Prcedure declaratins Switch declaratins Imperative Cmputatinal Cntrl flw f executin - Majr Prblems N input/utput statements(assumed peple wuld design their wn as hardware I/O varies) Sme features t flexible making it hard t understand and implementatin difficult Descriptins nt easily understandable with 2 versins as bth used new ntatins - Majr Cntributins Used anther assignment peratr := (difference between equivalence and assignment) Blck structure that allws fr nested scpes (enclsed with BEGIN-END) Parameter Passing - Replacement f a single statement with a sequence f statements(prmtes regularity) Blck - Each blck defines a nested scpe - Variables nly visible and exists within it, hidden frm utside and gne when ut f blck - Avids variable re-declaratin by defining depths f scpes - Allws larger prgrams t be written w/ errrs and faster - Cmmn data must be redeclared in each subprgram(repetitin f cde, viding abstractin principle) - Allws efficient stack strage management - Dynamic Scping Meaning f statements and expressins evlve during runtime - Static Scping Meaning f statements and expressins always stay the same Syntax - Still have ld cntrl statements(if-then, GOTO) - FOR-LOOP, WHILE-LOOP, SWITCH statements - Machine independent - NO restrictin n variable naming - Cncept f keywrds(readability and maintenance) Data Types and Structures - Int, Real and Blean - Lacked a duble precisin type - REGULARITY principle the majr gal Regular rules w/ exceptins are easier t implement, learn, use and describe - ZERO ONE INFINITY principle Only reasnable numbers in prgramming are 0,1 and infinity - Arrays in ALGOL are generalized(mre than 3 dimensins) and dynamic - Arrays indexed nly by integers - Strng typing and keywrds - Allw recursin - Parameter passing Pass by value Cpy passed, n wrry f actual being verwritten Cst expensive with memry (especially with arrays)

2 Pass by name Used in recursin Cmpiler tries t differentiate the same variables by renaming them Parameter Passing Techniques - Call by value - Call by reference - Call by result Frmal parameter acts as an un initialized variables lcal variable which is given a value during the executin f the prcedure On leaving the prcedure the value f the frmal parameter is assigned t the actual parameter - Call by value-result Frmal parameter affect nly the lcal cpy When prcedure cmpletes, the actual parameter is updated t the final value f the frmal parameter Static Scping - Uses blck structure - Methd f binding name t nn-lcal variables - 2 categries Nested static scpes in subprgrams Scpes in sub prgrams that cannt be nested - The visibility f the identifiers and the prcess f binding f names t declaratins is determined at cmpile time - Allws cmpiler t perfrm type-checking - Easier t read and faster t execute - Nn lcal access that s wrks well in many situatins - Prblems Can mistakenly call a sub prgram that shuld nt have been callable(run time errr detectin) T much data access(causes incrrect data access/ affects prgram readability) Getting arund the prblems can result in the cde bearing little resemblances t the riginal Dynamic Scping - Based n calling a sequence f subprgrams nt n their spatial relatinship, thus the cpe can nly be determined at run time - The binding between the use f an identifier and its declaratin depends n the executin f the prgram at run time - Attributes f nn-lcal variable visible t prgram cannt be determined statically - A statement in a sub rutine that cntains a reference t nn-lcal variable can ptentially refer t a different variable entry every time the statement is executed by the prgram - Majr prblems N way t statically type-check references t nn-lcals Makes it hard t read N prtectin t lcal variable access in sub prgram Take far lnger t access nn lcals Pinters - Nn existent in frtran nr algl - 2 uses Pwer f indirect addressing at higher level than assembly A methd f dynamic strage management - 2 interpretatins Reference t cntents (nrmal pinter) Reference t value in memry cell whse address is in the memry t which the variable is bund(dereferencing pinter) - Dereferencing a pinter is the prcess f fllwing the pinter t the variable r subprgram whse address it hlds - 2 prblems Dangling pinter - 2 pinters pint t same data - Original pinter and data deleted - Other pinter pints t null - Slutin is t use a tmbstne Pinter pints t tmbstne that pints t data Pinter always pints t tmbstne even if data gne Cst in memry Cst in prcessing fr keeping track - Slutin is Lck and Keys apprach Pinters represented as rdered pairs (key, address)key is an integer value Heap dynamic variables have a header cell that stres an integer lck value

3 When allcated a lck value is created and placed bth in lck cell and the heap variable and in the key cell f the pinter Every access t the dereferenced pinter cmpares the key value f the pinter t the lck value in the heap dynamic variable. If match then legal, else its an errr When deallcated, the lck value is cleared t an illegal lck value S if there is anther pinter pinting t invalid data, the key value will n lnger match and access is denied Memry Leak - 2 pinters t 2 data - Frce the 2 pinters t pint t 1 data - The ther data is nw inaccessible as n pinter pints t it - Slutin is reference cunter r garbage cllectin - Garbage cllectr When need mst, wrks wrst Uses an algrithm cllects heap cell with an indicatr bit Mre efficient by use f the pinter rtatin and slide peratins 3 phases Cells set themselves t indicate as rubbish Every pinter in the prgram is traced int the heap and all reachable cells are marked as nt being garbage All cells in the heap that have nt been specifically marked as still being used are returned t the lst f available space Garbage cllectin desn t wrk all the time Prblems (single size cells) Takes t lng (cst in executin and memry due t indicatr bit) Prcess yields nly a small number f cells as available space Prblems(variable size cells) Different sizes, scanning is a prblem If pinter desn t pint t anything, cannt fllw chain Maintaining list f available space when it becmes a list f variable sized lengths, segments r blcks Allcatin slwed because f requests t large blck Reference Cunter - Fr each memry lcatin, allcate a cunter t keep track f the number f pinters assciated - Prblems (single size cells) Cst in memry fr cunters Cst in updating the cunters Des nt wrk with circular linked list - Prblems (variable size cells) Available space list maintenance prblems - 3 cnditins Can change address f pinter - Makes pinter pint at smething else Can change value f the data Cannt change memry lcatin f the data Weak and Strng Typing - Strng Eliminate errrs befre runtime Each name in a prgram has nly a single type that is knwn at runtime Type f variables knwn at cmpile time, allws cmpiler t catch errrs - Weak Type f variable nt knwn until runtime, Errrs undetected by cmpiler Activatin Recrd - Need fr dynamically created ARs fr string recursive instances - T knw the state f the prcedure activatin it is necessary t knw The cde that makes up the bdy f the prcedure The place in the cde where the activatin f the prcedure is nw executing The value f the variables visible t this activatin - Cde des nt vary between different instances therefre nt included in AR - Cntains variable parts which define a particular executin Instructin Part - Designates the current instructin being executed Envirnment Part

4 - Defines bth the lcal and nn-lcal cntext t be used fr this activatin f the sub prgram it determines hw the instructins are interpreted - Lcal Cntext Lcal variabkles, actual parameters and registers Easy t search - Nn-Lcal cntext Difficult t search as it needs t be extracted frm the saved states Uses static links - Dynamic Links A subprgram needs a reference t its caller Prvide a pinter t the caller s AR recrd Pinter is stred in the AR f the subprgram - Dynamic Chain Sequence f dynamic links Tells exactly the rder f the subprgrams Search within scpe fr variable, if nt fund g t parent up t the ancestr until fund else return errr. - Static Link Pints t the bttm f the activatin recrd instance f the static parent 2 ways t access nn-lcal variables(have t find the definitin and the right ne) - Static Chain Nesting level knwn at cmpile time, cmpiler knws if a variable is nn-lcal and the length f the chain needed t reach it Static depth indicates hw deeply it is nested in the utermst scpe Nesting depth/chain ffset is the difference f static depth and t the variable(ancestr) Access t a nn-lcal beynd the static parent scpe is cstly Steps - Find the instance in the AR in which the variable was allcated - Use lcal ffset f the variable within the AR - Search the static chain until a static ancestr AR has that variable - Displays Static links are cllected in a single array instead f AR Cntents is a list f addresses f the accessible AR, ne fr each scpe and in rder f nesting Access f nn-lcal nly invlves 2 steps regardless f scpe and reference lcatin f variable Access t a nn-lcal beynd the static parent scpe is cstly All subrutine invked/ended requires the display t be mdified(cst at prcessing) A subrutine terminatin als requires the saved pinter in the AR t be placed back in display A pinter at N pints t an AR recrd with static depth f N Disadvantage at memry fr implementing an array Steps - Link t crrect AR in display is fund using a statically cmputed value called the display ffset which is clsely related t the chain ffset - Lcal ffset within the AR is cmputed and used like in static chain - Nn-lcal reference is represented by an rdered pair f integers [display ffset lcal ffset Dynamic Link Chain Static Link chain

5 Display Cmpund Assignment Operatr - A = A + B - Same as A += B Nesting Selectrs - Cmpiler desn t bther abut indentatin can result in imprper semantics but cmpiler desn t cmplain - Use curly braces t indicate prper scpe f nesting - Prvides disambiguatin