p = bitmap_data_for_current_scanline;
for (i = 0; i < bytes_in_shift_register; i++) {
    eight_bits = *p++;
    port_pin = eight_bits_0;
    clock_pin = high;
    clock_pin = low;
    port_pin = eight_bits_1;
    clock_pin = high;
    clock_low = low;
    ...
}