80C186 INSTRUCTION SET ADDITIONS AND EXTENSIONS

The following listing gives the formal definition of the

ENTER instruction for all cases.

LEVEL denotes the value of the second operand.

Push BP

Set a temporary value FRAME_PTR: = SP

If LEVEL > 0 then

Repeat (LEVEL - 1) times:

BP:=BP - 2

Push the word pointed to by BP

End Repeat

Push FRAME_PTR

End if

BP:=FRAME_PTR

SP:=SP - first operand

Figure A-1. Formal Definition of ENTER

ENTER treats a reentrant procedure as a procedure calling another procedure at the same lexical level. A reentrant procedure can address only its own variables and variables of higher-level call- ing procedures. ENTER ensures this by copying only stack frame pointers from higher-level pro- cedures.

Block-structured high-level languages use lexical nesting levels to control access to variables of previously nested procedures. For example, assume for Figure A-2 that Procedure A calls Proce- dure B, which calls Procedure C, which calls Procedure D. Procedure C will have access to the variables of Main and Procedure A, but not to those of Procedure B because Procedures C and B operate at the same lexical nesting level.

The following is a summary of the variable access for Figure A-2.

1.Main has variables at fixed locations.

2.Procedure A can access only the fixed variables of Main.

3.Procedure B can access only the variables of Procedure A and Main. Procedure B cannot access the variables of Procedure C or Procedure D.

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.

A-3

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Intel 80C188XL, 80C186XL user manual Figure A-1. Formal Definition of Enter