Technotes


Accessing Globals From MPW Assembly Language


Technote PT 01January 1987


Revised by: March 1988
Written by: Jim Friedlander January 1987

This technical note demonstrates how to access MPW Pascal and MPW C globals from the MPW Assembler.


To allow access of MPW Pascal globals from the MPW Assembler, you need to identify the variables that you wish to access as external. To do this, use the {$Z+} compiler option. Using the {$Z+} option can substantially increase the size of the object file due to the additional symbol information (no additional code is generated and the symbol information is stripped by the linker). If you are concerned about object file size, you can "bracket" the variables you wish to access as external variables with {$Z+} and {$Z-}. Here's a trivial example:

Pascal Source

	PROGRAM MyPascal;
	USES
	   MemTypes,QuickDraw,OSIntf,ToolIntf;
	
	VAR
	   myWRect: Rect;
	{$Z+} {make the following external}
	   myInt: Integer;
	{$Z-} {make the following local to this file (not lexically local)}
	   err: Integer;

	PROCEDURE MyAsm; EXTERNAL; {routine doubles the value of myInt}

	BEGIN {PROGRAM}
	   myInt:= 5;
	   MyAsm; {call the routine, myInt will be 10 now}
	   writeln('The value of myInt after calling myAsm is ', myInt:1);
	END. {PROGRAM}
Assembly Source for Pascal 
	        CASE   OFF          ;treat upper and lower case identically
	MyAsm   PROC   EXPORT       ;CASE OFF is the assembler's default
	        IMPORT myInt:DATA   ;we need :DATA, the assembler assumes CODE
	        ASL.W  #1,myInt     ;multiply by two
	        RTS                 ;all done with this extensive routine, whew!
	        END

The variable myInt is accessible from assembler. Neither myWRect nor err are accessible. If you try to access myWRect, for example, from assembler, you will get the following linker error:

	### Link: Error   Undefined entry name:   MYWRECT.

C Source

In an MPW C program, one need only make sure that MyAsm is declared as an external function, that myInt is a global variable (capitalizations must match) and that the CASE ON directive is used in the Assembler: 

	#include <types.h>
	#include <quickdraw.h>
	#include <fonts.h>
	#include <windows.h>
	#include <events.h>
	#include <textedit.h>
	#include <dialogs.h>
	#include <stdio.h>

	extern MyAsm();    /* assembly routine that doubles value of myInt */
	short myInt;      /* we'll change the value of this from MyAsm */

	main()
	{
	WindowPtr MyWindow;
	Rect myWRect;

	myInt = 5;
	MyAsm();
	printf(" The value of myInt after calling myAsm is %d\n",myInt);
	} /*main*/
Assembly source for C 
	        CASE   ON           ;treat upper and lower case distinct
	MyAsm   PROC   EXPORT       ;this is how C treats upper and lower case
	        IMPORT myInt:DATA   ;we need :DATA, the assembler assumes CODE
	        ASL.W  #1,myInt     ;multiply by two
	        RTS                 ;all done with this extensive routine, whew!
	        END


Further Reference:

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