Technical Note PT27

MPW: {$LOAD}; _DataInit;%_MethTables

CONTENTS {$LOAD} The _DataInit Segment %_MethTables and %_SelProcs References Downloadables

This technical note discusses the Pascal {$LOAD} directive as well as how to unload the _DataInit and %_MethTables segments. [Nov 01 1986]

{$LOAD}

MPW Pascal has a {$LOAD} directive that can dramatically speed up compiles.

will combine symbol tables of all units following this directive (until another {$LOAD} directive is encountered), and dump them out to HD:MPW:PLibraries:PasSymDump. In order to avoid using fully specified pathnames, you can use {$LOAD} in conjunction with the -k option for Pascal:

combined with the following lines in myfile:

    USES
        {$LOAD PasSymDump}
            MemTypes,QuickDraw, OSIntf, ToolIntf, PackIntf,
        {$LOAD} {This "turns off" $LOAD for the next unit}
            NonOptimized,
        {$LOAD MyLibDump}

will do the following: the first time a program containing these lines is compiled, two symbol table dump files (in this case PasSymDump and MyLibDump) will be created in the directory specified by the -k option (in this case {PLibraries}). No dump file will be generated for the unit NonOptimized. The compiler will compile MemTypes, QuickDraw, OSIntf, ToolIntf, PackIntf (quite time consuming) and dump those units' symbols to PasSymDump and it will compile the interface to MyLib and dump its symbols to MyLib. For subsequent compiles of this program (or any program that uses the same dump file(s)), the interface files won't be recompiled, the compiler will simply read in the symbol table.

Compiling a sample five line program on a Macintosh Plus/HD20SC takes 62 seconds without using the {$LOAD} directive. The same program takes 10 seconds to compile using the {$LOAD} directive (once the dump file exists). For further details about this topic, please see the MPW Pascal Reference Manual .

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The _DataInit Segment

The Linker will generate a segment whose resource name is %A5Init for any program compiled by the C or Pascal compilers. This segment is called by a program's main segment. This segment is loaded into the application heap and locked in place. It is up to your program to unload this segment (otherwise, it will remain locked in memory, possibly causing heap fragmentation). To do this from Pascal, use the following lines:

    PROCEDURE _DataInit;EXTERNAL;
    ...

    BEGIN     {main PROGRAM}
    UnloadSeg(@_DataInit);
    {remove data initialization code before any allocations}

From C, use the following lines:

    extern _DataInit();
    ...
    { /* main */
        UnloadSeg(_DataInit);
        /*remove data initialization code before any allocations*/

For further details about Data Initialization, see the MPW Reference Manual.

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%_MethTables and %_SelProcs

Object use in Pascal produces two segments which can cause heap problems. These are %_MethTables and %_SelProcs which are used when method calls are made. MacApp deals with them correctly, so this only applies to Object Pascal programs that don't use MacApp. You can make the segments locked and preloaded (probably the easiest route), so they will be loaded low in the heap, or you can unload them temporarily while you are doing heap initialization. In the latter case, make sure there are no method calls while they are unloaded. To reload %_MethTables and %_SelProcs, call the dummy procedure %_InitObj. %_InitObj loads %MethTables --calling any method will then load %_SelProcs.

Make sure that any external routines that you reference are capitalized the same in both the external routine and the external declaration (especially in C). If the capitalization differs, you will get the following link error (library routine = findme, program declaration = extern FindMe();):

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References

MPW Reference Manuals

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Downloadables