User's Manual

EXTENDED INSTRUCTION SET
MAIN PROGRAM (LEXICAL LEVEL 1)
PROCEDURE A (LEXICAL LEVEL 2)
PROCEDURE B (LEXICAL LEVEL 3)
PROCEDURE C (LEXICAL LEVEL 3)
PROCEDURE D (LEXICAL LEVEL 4)
Figure 4-2. Variable Access in Nested Procedures
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0
BPFOR
MAIN -
OLDBP
BPM'
SP_
'BPM - BP VALUE FOR MAIN
} DISPLAY
• DYNAMIC
STORAGE
Figure 4-2a. Stack Frame jor MAiN at Levei 1
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4. PROCEDURE C can access only the variables of PROCEDURE A and MAIN. PROCEDURE
C cannot access the variables of PROCEDURE B or PROCEDURE D.
5. PROCEDURE D can access the variables of PROCEDURE C, PROCEDURE A, and MAIN.
PROCEDURE D cannot access the variables of PROCEDURE B.
ENTER at the beginning of the MAIN PROGRAM creates dynami\; sturage space fuf MAIN but
copies no pointers. The first and only word in the display points to itself because there is no previous
value for LEAVE
to return to BP. See figure 4~2a.
After MAIN calls PROCEDURE A, ENTER creates a new display for PROCEDURE A with the
first word pointing
to the previous value of BP (BPM for LEAVE to return to the MAIN stack frame)
and the second word pointing to the current value of
BP. Procedure A can access variables in MAIN
since MAIN is at level 1. Therefore the base for the dynamic storage for MAIN is at [BP-2j. All
dynamic variables for
MAIN will be at a fixed offset from this value. See figure 4-2b.
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