Ch10-1

Chapter 10
Implementing Subprograms
ISBN 7-111-10161-8
Chapter 10 Topics
1. The General Semantics of Calls and Returns 2. Implementing FORTRAN 77 Subprograms 3. Implementing Subprograms in ALGOL-like Languages 4. Blocks 5. Implementing Dynamic Scoping
Copyright © 2006 Shujuan Jiang. All rights reserved. 1-5
10.2 Implementing FORTRAN 77 Subprograms
• The semantics of a FORTRAN 77 subprogram call requires the following actions: - Save the execution status of the current program
Figure 10.3 A type activation record for an ALGOL-like language Local variables parameters dynamic link static link return address
Copyright © 2006 Shujuan Jiang. All rights reserved. 1-14
Copyright © 2006 Shujuan Jiang. All rights reserved.
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10.2 Implementing FORTRAN 77 Subprograms
• Activation record instance（活动记录实例） is a concrete(具体的) example of an Activation Record
Copyright © 2006 Shujuan Jiang. All rights reserved.
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• A subprogram Return must include the mechanism
10.1The General Semantics of Calls and Returns
Copyright © 2006 Shujuan Jiang. All rights reserved.
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10.2 Implementing FORTRAN 77 Subprograms
• The call and return actions require storage for the following
- If local variables are not static, it must cause storage to be allocated for the locals declared in the called subprogram and bind those variables to that storage - It must save the execution status of the calling program unit
- Status information about the caller - Parameters - Return address - Functional value for function
Copyright © 2006 Shujuan Jiang. All rights reserved.
Copyright © 2006 Shujuan Jiang. All rights reserved.
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10.1The General Semantics of Calls and Returns
• Subprogram linkage: The subprogram call and return operations of a language • A subprogram call must include the mechanism
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10.2 Implementing FORTRAN 77 Subprograms
• A FORTRAN 77 subprogram consists of two separate parts:
- The actual code of the subprogram, which is constant - Local variables data listed above, which can change when the subprogram is executed. - Both of these parts have fixed sizes
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10.2 Implementing FORTRAN 77 Subprograms
• The semantics of a FORTRAN 77 subprogram return requires the following actions
- If pass-by-value-result parameters are used, the current values of those parameters are moved to the corresponding actual parameters - If the subprogram is a function, the functional value is moved to a place accessible to the caller - The execution status of the caller is restored(恢复) - Control is transferred back to the caller
stack top
10.3 Implementing Subprograms in ALGOL-like Languages
• The return address often consists of a pointer to the code segment of the caller and an offset address in that code segment of the instruction following the call • The static link (sometimes called a static scope pointer) points to the bottom of the activation record instance of an activation of the static parent • The dynamic link is a pointer to the top of the activation record instance of the caller
unit - Carry out the parameter-passing process - Pass the return address to the callee - Transfer control to the callee
Copyright © 2006 Shujuan Jiang. All rights reserved.
Copyright © 2006 Shujuan Jiang. All rights reserved.
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10.3 Implementing Subprograms in ALGOL-like Languages
In the case of the ALGOL-like languages, activation record instances must be created dynamically
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10.2 Implementing FORTRAN 77 Subprograms
• The executable program put together by the linker • When the linker is called for a main program
– First, find the files that contain the translated subprograms referenced in that program, along with their activation record instances, and load them into memory – Determine the size of all COMMON blocks and allocate storage for them – Then, set the target addresses of all calls to those subprograms in the main program to the entry addresses of those subprograms
Copyright © 2006 Shujuan Jiang. All rights reserved. 1-12
10.3 Implementing Subprograms in ALGOL-like Languages
• More complex Activation Records
- Parameters can usually be passed by two different methods（ by value or by reference） - Variables declared in subprograms are often dynamically allocated - Recursion adds the possibility of multiple simultaneous(同时发生的) activations of a subprogram - Use static scoping to provide access to nonlocal variables