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7.5. IMPLICATIONS FOR THE COMPILER / INTERPRETER use none none generating torender none on noneasp.net upc-a generator 3. Local bindings remain none for none static until control leaves the block at block exit time. A given name refers to the same storage object from block entry until block exit.

At block exit, all local storage objects are deallocated and local identi ers revert to their prior bindings (possibly unde ned ). The freed storage is made available for reuse by popping the run-time stack. The number of storage objects on the stack changes only when blocks are entered or exited, and then only changes by the number of names (locals + parameters) declared in that block.

Compiling Block Structure. The implementation of block structure in a compiled language is not quite the same as the implementation in an interpreted language. The di erence is that the compiler deals with storage that will be allocated rather than storage that is allocated.

The address at which a stack frame will start is completely unpredictable at compile time. However, the number, size, and order of the items in each future stack frame is known to the compiler. A common frame-management scheme is to have a central core in each stack frame that contains everything except parameters and local variables.

A calling program puts parameters on the stack and increments the stack pointer (which is kept in a machine register which we will call the SP ). Then the calling program lls in the static link, dynamic link, and return address in the core area of the stack frame. It puts a pointer to the dynamic link eld of the new frame into a machine register.

(Let us call this register the FP, for frame pointer.) The contents of the new dynamic link is the current value of FP. Finally, the program branches to the subroutine.

The subroutine can now nd its stack frame because the address is in the FP. It can use the SP to nd the top of the stack. The rst action of the subroutine is to increment the SP to allocate space for local variables.

The subprogram code is compiled to refer to its parameters and local variables using addresses that are relative to the FP. The last parameter might be FP 2, and the rst might be FP 12. The local variables will be in locations such as FP + 4 and FP + 20.

Stack locations past the SP are available for use as a scratch pad and will be used for temporary storage during calculations. At block exit, several things happen: The return value, if any, of the subprogram is loaded into a machine register. (It is often called the return register.

) The dynamic link eld is copied back into the FP. The SP is decremented by the size of the stack frame. A branch is taken to the return address.

This erases all trace of the subprogram.. Bar Code Scanner Environments Exercises 1. What is a name W none none hat is the meaning of a name Do languages follow a one object-one name rule Explain..

CHAPTER 7. NAMES AND BINDING 2. Do compilers need none none names to execute a program Explain. What is the semantic aspect of a name Why is this important for human interpretation of a program 3.

What is the role of the symbol table How are new names added to it 4. In APL, why are names typeless 5. What are primitive symbols Why are they necessary 6.

How do names relate to objects in a compiled language In an interpreted one 7. What is binding How is it invoked 8. What do we call storage that is allocated once at load time and remains until program exit, but whose usage is restricted to one function 9.

De ne and contrast static binding, dynamic binding, and block structured binding. 10. In modern functional languages, assignment is not supported, and in APL, it is rarely used.

Dynamic binding takes the place of assignment. Explain how this is implemented. 11.

In block structured languages, when is a name visible Invisible 12. This question and the next seven involve the following skeletal program written in C. How many name scopes are de ned in this program skeleton Draw a box around each block and label these blocks with the letters A, B, C, etc.

int x, y, z; fun1() { int j, k, l; { int m, n, x; ...

} } fun3() { int y, z, k; ...

} main() { ...

} 13. Name a block that is nested within another block. 14.

In which scopes are the global variables x, y, and z all accessible 15. What variables are accessible to the main function . 7.5. IMPLICATIONS FO R THE COMPILER / INTERPRETER 16.

Are k in fun1 and k in fun3 the same variable Why or why not 17. Are the global variables y and z accessible in fun3 Why or why not 18. In which blocks does j refer to a global variable In which blocks is j a local variable 19.

In which block or blocks can we use both m and k . 20. Name a language none none in which storage can be shared by two functions, neither of which is enclosed within the other, but kept private from all other functions and from the main program. Explain how to accomplish this goal in your language.

21. How are FORTRAN s named common storage and Ada s packages examples of nonhierarchical data facilities 22. What is the function of a constant declaration Why is it useful to the programmer 23.

What is the di erence between de ning a true constant whose meaning is a constant expression and using a macro to de ne a symbolic name for that expression 24. How are constants implemented in these languages: Pascal, Ada, FORTRAN, FORTH 25. What is a naming con ict 26.

How did the concept of scope and modularity resolve the problem of naming con icts What is the di erence between an identi er and a complete name 27. What is a block structure What are nested blocks What is the scope of the complete name of identi ers declared within a block 28. Consider a language in which all variables must be declared with fully speci c types.

Is this language more likely to be interpreted or compiled Why 29. Why is the scope of a name not equal to its lifetime Explain. 30.

Explain how the compiler and interpreter each determine the semantics of an ambiguous name..
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