6.42

same like

for (i = 0; i < 640; i++)
  for (j = 0; j < 480; j++)
    buffer[i][j].r = 0;
    buffer[i][j].g = 0;
    buffer[i][j].b = 0;
    buffer[i][j].a = 0;

C = 64KB, B = 4B, sizeof(piexl) = 4

buffer[i][j].r = 0;

miss, cache one piexl, so

buffer[i][j].g = 0;
buffer[i][j].b = 0;
buffer[i][j].a = 0;

all hit

miss rate is 1/4