Q$6($a$).$ A virtual memory system has an address space of $8K$ words, a memory space of $4K$

words, and main memory block sizes of $1K$ words. The following main memory block numbers

change occur during a given time interval. $($Only main memory block number changes are listed.

If the same block number is referenced again, it is not listed twice.$)$

$420126140102357$

Determine the four block that are resident in main memory after each block reference change if

the replacement algorithm used is

$($a$)$ FIFO

$($b$)$ LRU