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Annotation of os2sdk/demos/examples/csalias/csalias.c, revision 1.1

1.1     ! root        1: /* This example illustrates the use of the following 286DOS function:
        !             2:  *     DOSCREATECSALIAS
        !             3:  *
        !             4:  * A data segment is allocated by calling DOSALLOCSEG. A small assembly 
        !             5:  * language routine is created in this data segment. An alias code selector 
        !             6:  * for this data segment is obtained by calling DOSCREATECSALIAS. Using the 
        !             7:  * alias code selector, the routine in the data segment is called. After 
        !             8:  * returning from the routine, the alias code selector and the data segment 
        !             9:  * are freed by calling DOSFREESEG.
        !            10:  *
        !            11:  * This program illustrates the function - it does not do anything useful.
        !            12:  * A more likely use of aliasing code to data is where the code is actually
        !            13:  * generated at run time, rather than statically declared in this example.
        !            14:  *
        !            15:  * Copyright (C) Microsoft Corp. 1986
        !            16:  */
        !            17: 
        !            18: #include       <doscalls.h>            /* CP/DOS function declarations      */
        !            19: #include       <stdio.h>               /* printf C lib function declaration */
        !            20: #include       <dos.h>                 /* FP_OFF and FP_SEG macros          */
        !            21: 
        !            22: #define        PRIVATE 0               /* segment will not be shared */
        !            23: #define        SOMENUM 3               /*to be squared by proc in CSAlias'ed segment*/
        !            24: 
        !            25: unsigned char square[] = { /*a routine that computes the square of an integer*/
        !            26:        0x55,                           /* push bp             */
        !            27:        0x8b, 0xec,                     /* mov  bp,sp          */
        !            28:        0x8b, 0x46, 0x06,               /* mov  ax,[bp+6]      */
        !            29:        0xf7, 0xe0,                     /* mul  ax             */       
        !            30:        0x8b, 0xe5,                     /* mov  sp,bp          */
        !            31:        0x5d,                           /* pop  bp             */
        !            32:        0xcb,                           /* retf                */
        !            33:        '\0'                            /* terminate with null */
        !            34: };
        !            35: 
        !            36: main ()
        !            37: {
        !            38:        unsigned        DataSelector,   /* selector to data segment    */
        !            39:                        CodeSelector;   /* alias code segment selector */
        !            40:        char            *DataSeg;       /* pointer to data segemnet    */
        !            41:        long            (*proc)(int);   /* pointer to a procedure      */
        !            42:        long            SquareOfInt;    /* to hold square of an integer*/
        !            43: 
        !            44:        /* allocate a segment */
        !            45:        DOSALLOCSEG (sizeof(square), &DataSelector, PRIVATE);
        !            46: 
        !            47:        /* construct a far pointer to the data segment */
        !            48:        FP_SEG(DataSeg) = DataSelector; 
        !            49:        FP_OFF(DataSeg) = 0;
        !            50: 
        !            51:        /* copy an assembly language procedure into the data segment */
        !            52:        strcpy(DataSeg, square); 
        !            53:        
        !            54:        /* get a csalias selector for the data segment */
        !            55:        DOSCREATECSALIAS (DataSelector, &CodeSelector);
        !            56:        
        !            57:        /* construct the address to the procedure in the csalias'ed segment */
        !            58:        FP_SEG(proc) = CodeSelector;
        !            59:        FP_OFF(proc) = 0; 
        !            60: 
        !            61:        /* execute the code in the csalias'ed data segment */
        !            62:        if ((SquareOfInt = (*proc)(SOMENUM)) != (SOMENUM * SOMENUM))
        !            63:          printf ("*** error: procedure in CSAlias'ed segment failed ***\n");
        !            64: 
        !            65:        /* free the alias code selector */
        !            66:        DOSFREESEG(CodeSelector);
        !            67: 
        !            68:        /* free the data segment */
        !            69:        DOSFREESEG(DataSelector);
        !            70: }

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