1 Chapter 7: Runtime Environments

int * larger (int a, int b) { if (a > b) return &a; //wrong else return &b; //wrong } int * larger (int *a, int *b) { if (*a > *b) return a; //OK else return b; //OK }

3 Memory of a computer Register RAM - Code Area - Data Area

4 General Organization of Runtime Storage code area stack SP global/static data area free space heap

5 Code Area code for procedure1 code for procedure2 code for procedure3. code for procedureN SP entry point entry point entry point entry point

6 Fully Static Runtime Environments code for main procedure code for procedure 1 ………. code for procedure N global data area activation record for main procedure activation record for proc 1 SP Code Area Data Area ……………. activation record for proc N

7 Activation Record - a contiguous block of storage recording the state of an activation of a function, not all the compilers set the same fields. Each execution of a procedure is referred to as an activation of the procedure. If the procedure is recursive, several of its activation may be alive at the same time.

8 What is the contents of the activation record? temporary values local data saved machine status optional access link to refer to non-local data held in other activation records optional control link to refer to the activation actual parameters returned value

General Organization of a Activation Record Local & temp. data Old SP Return value Return address Argument count Parameter t n Parameter t 2 Parameter t n -1.. SP Direction of Growth caller’s responsibility callee’s duty

10 So, local data are accessed by negative offsets from SP (stack pointer, a register holding a particular position of currently activated procedure). A local name X can be referenced by X[SP] ([SP] means the address of SP), where X is the (negative) offset for the name from the location pointed to by SP. Parameter can be referred to by an positive offset from SP. That is, the ith parameter can be referenced by (4+n-i)[SP] (assume each entry takes one unit of space)

Suppose the Memory structure for C program is organized as below, where function P calls Q and Q calls R 0.. Static area for programs and global (static) data.. 2 n -1 Top Extra storage for R Activation record for R Direction Extra storage for Q Activation record for Q SP of growthExtra storage for P Activation record for P (fp) (sp)

TOP is a register holding the top address of the stack. SP is also a permanently allocated register (stack pointer) holding a specific address in the activation record of the currently active procedure. We use SP to indirectly refer to the local variables, parameters, etc. Extra storage is used for storing pointers on the display, variable-length data such as dynamically allocated arrays, etc.

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14 Growth direction 2 n -1 0

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At run-time e.g. p (T1, T2,..., Tn) ==> param T1 => push (T1) param T2 => push (T2)..... param Tn => push (Tn) call p, n => push (n) => push (l 1 ) => push () => push (SP) => goto l 2 l 1 denotes the return address; l 2 denotes the address of the first statement of the called procedure.

17 e.g., p (A+B*C, D) ==> T1 = B * C T2 = A + T1 Param T2 Param D Call p, 2 Temporary variable

At run-time * Assume TOP points to the lowest-numbered used location on the stack and the memory locations are counted by words. 'param T' is translated into 'push(T)' which stands for TOP = TOP - 1; /* now TOP points to an available entry */ *TOP = T; /* save T into the memory 'call p, n' is translated into the following instructions: push (n) /*store the argument count */ push (l 1 ) /* l 1 is the return address */ push () /* leave one space for the return value */ push (SP) /* store the old stack pointer */ goto l 2 /* l 2 is the first statement of the called procedure p */

General Organization of a Activation Record Local & temp. data Old SP Return value Return address Argument count Parameter t n Parameter t 2 Parameter t n -1.. TOP Direction of Growth caller’s responsibility SP

The first statement of the called procedure must be a special three-address code 'procbegin', which (1) sets the stack pointer to the place holding the old SP and (2) sets TOP to the top of the activation record (or the stack), so 'procbegin' stands for: SP = TOP; // now SP points to old SP value TOP = SP + size_of_p; /* size_of_p is the size of p, i.e., the number of words taken by the local data for p */ The first statement of the called procedure

General Organization of a Activation Record Local & temp. data Old SP Return value Return address Argument count Parameter t n Parameter t 2 Parameter t n -1.. SP Direction of Growth caller’s responsibility callee’s duty TOP

The ‘return’ statement The return statement in c can have the form 'return (expression);' This statement can be implemented by three-address code to evaluate the expression into a temporary T followed by: 1[SP] = T /* 1 is the offset for the location of the return value */ TOP = SP + 2 /* TOP now point to the return address */ SP = *SP /* restore SP, SP now points to old location */ L = *TOP /* the value of L is now the return address */ TOP = TOP + 1 /* TOP points to the argument count */ TOP = TOP *TOP /* *TOP is the number of parameter of P. We restore TOP to the top of extra storage for the activation record below */ go to *L

23 Do not expect a procedure to do anything that requires it to know the size of the activation record of another procedure.

24 Control Link & Access Link Control (Dynamic) Link - link the caller’s activation record and thus allows us to restore the state (activation record) of the caller upon procedure exit. Access (Static) Link - a link used to access non-local variables

B P... P Q B P... P P Q Control Links Access and Control Links growth direction P Two procedures P and Q are defined in procedure B; P calls itself recursively several times before it calls Q. Procedure B procedure P { P….. Q….. } procedure Q {…… …… } … P …. Access Links Control Link

Nested Block Structures in C e.g. Int test( ) { int x,y,m; ….. { int m=1, n=2; x = m + n; …. } ….. { int w=2, k =4; m = w * k; … } ….. }

Access variables for activation record with access links For local variables e.g., fetch (-1)[SP] // assume (-1) is the offset of a local variable. For non-local variables with static distance 1 e.g., EP = (1)[SP] //EP is a register; assume access link is stored in (1)[SP]. EP = (-1)[EP] // EP now points to what the SP should point to. fetch (-3)[EP] // (-3) is the offset of the non-local variable. For non-local variables with static distance 2 e.g., EP = (1)[SP] EP = *EP // the content of EP is saved in EP EP = (-1)[EP] // EP now points to what the SP should point to. fetch (-5)[EP] // assume (-5) is the offset of the non-local

28 Steps to access a non-local variable at run-time 1. Skip down the access links given by the static distance to get the activation record in which the variable resides. The static distance between the use and the declaration of the variable is a constant computed by the compiler at compile-time. 2. The address of the variable is obtained by adding the fixed offset of the variable to the address obtained in step 1. The offset is also a constant computed by the compiler at compile-time.

29 Variable-Length Data A common strategy for handling variable-length data is shown below, where procedure p has three local arrays. The storage for these arrays is not part of the activation record for p; only a pointer to the beginning of each array appears in the activation record. The relative addresses of these pointers are known at compile-time, so the target code can access array elements through pointers.

control link Pointer to A Pointer to B array A array B activation record for P arrays of P control link TOP Activation record of Q called by P array of q SP Access to dynamically allocated arrays int x[10]; int A[w]; Int B[m]; …… read w; m= w * 10; w*2

31 Passing function as a parameter

Program HP proc p (func h) h proc q func f p(g) p(f) func g q HP q p f Stack configuration after the call to p(f) and (dashed) after the call to h. call to h After the call to p(f) access links

Program HP proc p (func h) h proc q func f p(g) p(f) func g q HP q p f Stack configuration after the call to p(g) and (dashed) after the call to h. call to h p g after call to p(g)

34 Display An array data structure (array of pointers to entries of activation record, e.g., d[ ]) used to store the specific location of (1) currently activated function’s activation record and (2) the activation records of each level of declared nested functions. Therefore, storage for a non-local variable a at nesting depth i is in the activation record pointed to by display element d[i].

e.g. program s (input, output); var a: array[0..10] of integer; x: integer; procedure r; var i: integer; begin... a... end; /* r */ procedure e (i,j: integer); begin x := a[i]; a[i] = a[j]; a[j] := x end; /* e */ procedure q (m,n: integer); var k,v: integer; function p (y,z: integer):integer; var i,j: integer; begin... a v... e (i,j); end; /* p */ begin end; /* q */ begin... end. /* s */

d [1] d [2] q (1,9) saved d [2] S q (1,9) saved d [2] S d [1] d [2] q(1,3) saved d [2] growth direction (a) (b)

q (1,9) saved d [2] S d [1] d [2] q(1,3) saved d [3] growth direction d [3] p(1,3) saved d [2] ( c )

q (1,9) saved d [2] S d [1] d [2] q(1,3) saved d [3] growth direction d [3] p(1,3) saved d [2] ( d ) e(1,3) saved d[2]

39 Assignment #5b Do exercises 6.2, 6.14, 7.2, 7.10 of the textbook.