*****Listing 1*****


/* These are determined from the debug information */
#define AND_INSTRUCTION_OFFSET 0x10
#define OR_INSTRUCTION_OFFSET 0x15
set_plot_type(type)
     {
     unsigned char *pc;
     void draw_function();

/* The function you want to modify */


pc = (void *) draw_function;

switch(type)
     {
case XOR:
     /* Put NOPS for the masking operation */
     *(pc + AND_INSTRUCTION OFFSET) = 0x90; /* NOP */
     *(pc + AND_INSTRUCITON OFFSET + 1) = 0X90;
     *(pc + OR_INSTRUCTION_OFFSET) = 0x03;  /* XOR */
     break;
case OVERWRITE:
     *(pc + AND_INSTRUCTION_OFFSET) = 0x02;  /* AND */
     *(pc + AND_INSTRUCITON_OFFSET + 1) = 0x00
     /* Register */

     *(pc + OR_INSTRUCTION_OFFSET) = 0x08;  /* OR */
     break;
     }
return;
{


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