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A.7 External Calls

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The assembly code on the page after next was produced by the MPE XL Pascal compiler from the Pascal source code shown on the next page. A few additional notes concerning the code sample:

In the source code, an external call situation has been simulated by assuming that the callee (one) resides in a different load module than the caller (two).

The assembly listing has been abbreviated to include only the code associated directly with the source code. In the complete listing, there would be calling and called stubs for all calls to process initialization and termination procedures (which occur in the outer block/main program as noted below) preceding the section of code shown here.

Because the use of the CALLX millicode is transparent to the user, it has been just referenced as being in system space to avoid all of the excess detail that would be necessary to use actual addressing. A similar liberty has been taken in a few other cases; where actual offsets appear in the code, they have been eliminated to achieve simplicity.

The code sample is accompanied (in the left margin) by arrows that follow the flow of control. These arrows function exactly as those used in the flow diagrams in the text; in this case, the starting point is in the main program block, near the bottom of the code sample. Furthermore, all critical points have been labeled with numbers and documented on the page following the assembly code (just as was done in the local call example).

The assembly code is lightly documented; the code used in the stubs is documented in detail in Chapter 5, Import and Export Stubs, and the small amount of code present other than in stubs is basically the same as that used in any local procedure call.

A.7.1 Pascal Source Code


  program ext call;
  :
  :
   (* procedure one is in module 2; therefore an external call
      is necessary in order for the call from two to be successful *)

  procedure one (a,b : integer);
  begin
  end;
  :
  :
   (* procedure two is in module 1, i.e. the same module
      as the main program block. Bar calls one *)

  procedure two;
  begin
      one (1,2);
  end;

  begin
    two;
  end.

A.7.2 Assembly Code

The numbers in parenthesis follow the flow of control and are used as labels for the documentation that follows.

[A.7.2]



A.6 Other Compiler-Generated Information

A.8 Assembly Documentation