How should modified pages be written out to secondary memory using Page Buffering?

Question

I am studying for my final OS exam and am currently stuck in a question:

Assume a system uses demand paging as its fetch policy.

The resident size is 2 pages.

Replacement policy is Least Recently Used (LRU).

Initial free frame list: 10, 20. 30, 40, 50

Assume a program runs with the following sequence of page references:
3(read), 2(read), 3(write), 1(write), 1(write), 0(write), 3(read)

I am asked to show the final contents of the free frame list, modified list, and the page table.

Here is the model answer.

This is what I managed to do.

The final Resident Set is correct, but the free frame list and the modified list are not. I just cannot see how the modified list does not contain page number 0 (as in it got written to memory), while page number 1 was not written even though it was referenced before it.

Any help would be appreciated.

Answer 1

Why do you recycle 3(10) to the free list in step 4? It was the least recently used (and is dirty) so you would want to keep it, and get rid of 2(20). That appears to be what the model answer is based on.