void PrintStack(int sp, int mem_base)
{
        int argc;
        int addr;
        int i;
        int str_at;
        /*
         * print three words of zeros at stack top
         */
        printf("sp:\t%8d: | %8d |\n",sp,*(int* )(&main_memory[sp+mem_base]));
        printf("\t%8d: | %8d |\n",sp+4,*(int *)(&main_memory[sp+4+mem_base]));
        printf("\t%8d: | %8d |\n",sp+8,*(int *)(&main_memory[sp+8+mem_base]));
        argc=*(int *)(&main_memory[sp+12+mem_base]);
        printf("\t%8d: | %8d | argc\n",
                sp+12,*(int *)(&main_memory[sp+12+mem_base]));
        printf("\t%8d: | %8d | &argv[0]\n",
                sp+16,*(int *)(&main_memory[sp+16+mem_base]));
        printf("\t%8d: | %8d | &envp[0]\n",
                sp+20,*(int *)(&main_memory[sp+20+mem_base]));
        for(i=0; i < argc; i++) {
                addr = sp+24+(i*4)+mem_base;
                str_at=*(int *)(&main_memory[addr]);
                printf("\t%8d: | %8d | argv[%d] -> %s\n",
                        sp+24+(i*4),*(int *)(&main_memory[addr]),
                        i,(char *)&main_memory[str_at+mem_base]);
        }
        return;

}