HW8 Solutions

1. First read the first int of the first page, then the first int of
the second page (to create a new TLB entry), and so on through all six
pages (this is the inner loop below, with i=0). Then read the second int of
the first page, the second int of the second page, and so on through all
six pages. This access pattern is reflected in the code below:

    uint64_t tlb_unfriendly() {

        int *a = page_alloc(6 * PAGE_SIZE); 
        populate_array(a); // sets the integers in the array
        uint64_t sum = 0;

        /* YOUR CODE HERE: compute sum in the most TLB-unfriendly way possible */

        for (int i = 0; i < PAGESIZE/sizeof(int); i++) 
            for (int j = 0;  j < 6; j++) 
                sum += a[j*PAGESIZE/sizeof(int) + i];

        return sum;
    }


2. Lay out the allocated memory so that the last legitimately allocated
byte is on the last byte of the allocated page (this "wastes" the first
part of a page).  Mark the next virtual page (past the array) as "not
in use" (this does not cost physical memory). At that point, memory
references past the end of the array will generate page faults.

3.1

200 words/min x 1 min/60 seconds x 7 letters/word x 1 interrupt/character =
23 interrupts/second.

3.2

It should use interrupts. Your interrupts cost the computer 23
microseconds out of every second; this is a trivial fraction (23 parts
in one million). If the computer used polling, you (the human) would
notice the lag and get annoyed.