Post on 30-Dec-2015
description
Implementing Subprograms
Subprogram linkage refers to the subprogram call and return operations of a programming language.
The execution of a called subprogram requires many associated actions.
• parameter passing
• storage allocation of local variables
• saving execution status of calling program
• transfer of control to the subprogram
• return to calling program
• access to non local variables
Subprogram Activation
The subprogram definition is written by the programmer. The definition is a static entity.
The definition serves as a template for the subprogram activation.
During the execution of the program, if the subprogram is called, a subprogram activation is created.
Subprogram Activation
The activation record is dynamically created when the subprogram is called. It is destroyed when execution of the subprogram terminates.
The code segment is statically created and invariant during execution. The code is shared by all activations.
A subprogram activation has a code segment and an activation record.
Implementing Subprograms in ALGOL-like Languages
In programming languages which allow recursion (i.e. Pascal, C++) subprogram linkage is complex
• Different parameter passing methods
• Local variables of subprograms are often dynamically allocated
• Recursion makes multiple activations of a single subprogram possible
• Implementation of static scoping
Activation Record for Subprogram Allowing Recursion
In programming languages which do allow recursion, there can be many activations of a given subprogram at any time.
Local variables
Parameters
Return Address
Dynamic Link
Static Link
Static and Dynamic Links
The static link points to the bottom of the activation record instance of an activation of the static parent. It is used for references to non-local variables.
The dynamic link points to the top of the activation record instance of the caller. It is used for destruction of the current activation record instance.
Program main1; var p: real; procedure A (X: integer); var Y: boolean; procedure C (Q: boolean); begin
end; {of proc C} begin
C(Y); end; {of proc A} procedure B (R: real); var S,T: integer; begin
A(S); end; {of proc B} begin {of MAIN_1 } B(P); end. {of MAIN_1}
Local variable P{ ARIfor MAIN
TOP
Program main1; var p: real; procedure A (X: integer); var Y: boolean; procedure C (Q: boolean); begin
end; {of proc C} begin
C(Y); end; {of proc A} procedure B (R: real); var S,T: integer; begin
A(S); end; {of proc B} begin {of MAIN_1 } B(P); end. {of MAIN_1}
Local variable
Parameter
Return (to main)
Dynamic Link
Static Link
Local variable
T
S
RARIfor B
Local variable P{ ARIfor MAIN
TOP
Program main1; var p: real; procedure A (X: integer); var Y: boolean; procedure C (Q: boolean); begin
end; {of proc C} begin
C(Y); end; {of proc A} procedure B (R: real); var S,T: integer; begin
A(S); end; {of proc B} begin {of MAIN_1 } B(P); end. {of MAIN_1}
Local variable
Parameter
Return (to main)
Dynamic Link
Static Link
Local variable
T
S
RARIfor B
Local variable P{ ARIfor MAIN
TOP
Local variable
Parameter
Return (to main)
Dynamic Link
Static Link
Local variable
T
S
RARIfor B
Local variable P{ ARIfor MAIN
Parameter
Return (to B)
Dynamic Link
Static Link
Local variable
X
Y
ARIfor A
TOP
Program main1; var p: real; procedure A (X: integer); var Y: boolean; procedure C (Q: boolean); begin
end; {of proc C} begin
C(Y); end; {of proc A} procedure B (R: real); var S,T: integer; begin
A(S); end; {of proc B} begin {of MAIN_1 } B(P); end. {of MAIN_1}
Local variable
Parameter
Return (to main)
Dynamic Link
Static Link
Local variable
T
S
RARIfor B
Local variable P{ ARIfor MAIN
Parameter
Return (to B)
Dynamic Link
Static Link
Local variable
X
Y
ARIfor A
TOP
Program main1; var p: real; procedure A (X: integer); var Y: boolean; procedure C (Q: boolean); begin
end; {of proc C} begin
C(Y); end; {of proc A} procedure B (R: real); var S,T: integer; begin
A(S); end; {of proc B} begin {of MAIN_1 } B(P); end. {of MAIN_1}
Local variable
Parameter
Return (to main)
Dynamic Link
Static Link
Local variable
T
S
RARIfor B
Local variable P{ ARIfor MAIN
Parameter
Return (to B)
Dynamic Link
Static Link
Local variable
X
Y
ARIfor A
TOP
Parameter
Return (to A)
Dynamic Link
Static Link
Q
ARIfor C
Program main1; var p: real; procedure A (X: integer); var Y: boolean; procedure C (Q: boolean); begin
end; {of proc C} begin
C(Y); end; {of proc A} procedure B (R: real); var S,T: integer; begin
A(S); end; {of proc B} begin {of MAIN_1 } B(P); end. {of MAIN_1}
Local variable
Parameter
Return (to main)
Dynamic Link
Static Link
Local variable
T
S
RARIfor B
Local variable P{ ARIfor MAIN
Parameter
Return (to B)
Dynamic Link
Static Link
Local variable
X
Y
ARIfor A
TOP
Program main1; var p: real; procedure A (X: integer); var Y: boolean; procedure C (Q: boolean); begin
end; {of proc C} begin
C(Y); end; {of proc A} procedure B (R: real); var S,T: integer; begin
A(S); end; {of proc B} begin {of MAIN_1 } B(P); end. {of MAIN_1}
Local variable
Parameter
Return (to main)
Dynamic Link
Static Link
Local variable
T
S
RARIfor B
Local variable P{ ARIfor MAIN
TOP
Program main1; var p: real; procedure A (X: integer); var Y: boolean; procedure C (Q: boolean); begin
end; {of proc C} begin
C(Y); end; {of proc A} procedure B (R: real); var S,T: integer; begin
A(S); end; {of proc B} begin {of MAIN_1 } B(P); end. {of MAIN_1}
Local variable P{ ARIfor MAIN
TOP
Implementing Nonlocal References
Nonlocal variable references occur in two steps:
1) find the activation record where the nonlocal variable was allocated
2) use the offset of the variable within that activation record to access it
Implementing Nonlocal References
Static Chain
Display
Implementing Nonlocal References
Static Chain - a static chain is a chain of static links that connects certain activation record instances in the stack
Program main1; var p: real; procedure A (X: integer); var Y: boolean; procedure C (Q: boolean); begin
end; {of proc C} begin
C(Y); end; {of proc A} procedure B (R: real); var S,T: integer; begin
A(S); end; {of proc B} begin {of MAIN_1 } B(P); end. {of MAIN_1}
Local variable
Parameter
Return (to main)
Dynamic Link
Static Link
Local variable
T
S
RARIfor B
Local variable P{ ARIfor MAIN
Parameter
Return (to B)
Dynamic Link
Static Link
Local variable
X
Y
ARIfor A
TOP
Parameter
Return (to A)
Dynamic Link
Static Link
Q
ARIfor C
Local variable
Parameter
Return (to main)
Dynamic Link
Static Link
Local variable
T
S
RARIfor B
Local variable P{ ARIfor MAIN
Parameter
Return (to B)
Dynamic Link
Static Link
Local variable
X
Y
ARIfor A
TOP
Parameter
Return (to A)
Dynamic Link
Static Link
Q
ARIfor C
Main calls BB calls AA calls C
main
B
CA
Implementing Nonlocal References
Display - the static links are collected in a single array called a display
Main calls BB calls AA calls C
Main
B
CA
ARIfor MAIN
stack
Each subprogram has a static depth
Main - 0
B - 1
C - 2A - 1
display
0
ARIfor B
ARIfor MAINMain calls B
B calls AA calls C
Main - 0
B - 1
C - 2A - 1
stack
1
display
0
ARIfor B
ARIfor MAINMain calls B
B calls AA calls C
Main - 0
B - 1
C - 2A - 1
stack
1
ARIfor A
display
0
ARIfor B
ARIfor MAIN
ARIfor A
ARIfor C
Main calls BB calls AA calls C
Main - 0
B - 1
C - 2A - 1
stack
2
1
display
0