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Unformatted text preview: nstruction. STORAGE is the number of bytes of dynamic storage to allocate for local variables, and LEVEL is the lexical nesting level. PUSH EBP; FRAME_PTR ESP; IF LEVEL > 0 THEN DO (LEVEL - 1) times EBP EBP - 4; PUSH Pointer(EBP); (* doubleword pointed to by EBP *) OD; PUSH FRAME_PTR; FI; EBP FRAME_PTR; ESP ESP - STORAGE; The main procedure (in which all other procedures are nested) operates at the highest lexical level, level 1. The first procedure it calls operates at the next deeper lexical level, level 2. A level 2 procedure can access the variables of the main program, which are at fixed locations specified by the compiler. In the case of level 1, the ENTER instruction allocates only the requested dynamic storage on the stack because there is no previous display to copy. A procedure that calls another procedure at a lower lexical level gives the called procedure access to the variables of the caller. The ENTER instruction provides this access by placing a pointer to the calling procedure's stack frame in the display. A procedure that calls another procedure at the same lexical level should not give access to its variables. In this case, the ENTER instruction copies only that part of the display from the calling procedure which refers to previously nested procedures operating at higher lexical levels. The new stack frame does not include the pointer for addressing the calling procedure's stack frame. The ENTER instruction treats a re-entrant procedure as a call to a procedure at the same lexical level. In this case, each succeeding iteration of the re-entrant procedure can address only its own variables and the variables of the procedures within which it is nested. A re-entrant procedure always can address its own variables; it does not require pointers to the stack frames of previous iterations. By copying only the stack frame pointers of procedures at higher lexical levels, the ENTER instruction makes certain that procedures access only those variables of higher lexical levels, not those at parallel lexical levels (see Figure 6-6). Vol. 1 6-21 PROCEDURE CALLS, INTERRUPTS, AND EXCEPTIONS Main (Lexical Level 1) Procedure A (Lexical Level 2) Procedure B (Lexical Level 3) Procedure C (Lexical Level 3) Procedure D (Lexical Level 4) Figure 6-6. Nested Procedures
Block-structured languages can use the lexical levels defined by ENTER to control access to the variables of nested procedures. In Figure 6-6, for example, if procedure A calls procedure B which, in turn, calls procedure C, then procedure C will have access to the variables of the MAIN procedure and procedure A, but not those of procedure B because they are at the same lexical level. The following definition describes the access to variables for the nested procedures in Figure 6-6. 1. MAIN has variables at fixed locations. 2. Procedure A can access only the variables of MAIN. 3. Procedure B can access only the variables of procedure A and MAIN. Procedure B cannot access the variables of...
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This note was uploaded on 10/01/2013 for the course CPE 103 taught by Professor Watlins during the Winter '11 term at Mississippi State.
- Winter '11