-> got 100 pages
-> unpin backward 90..80 OK
-> unpin backward 70..60 OK
-> unpin backward 50..40 OK
-> unpin backward 30..20 OK
-> unpin backward 10..0 OK
-> purge return 50 pages OK
-> pin 0..10 OK ret 1
-> pin 10..20 OK ret 0
-> pin 20..30 OK ret 1
-> pin 30..40 OK ret 0
-> pin 40..50 OK ret 1
-> pin 50..60 OK ret 0
-> pin 60..70 OK ret 1
-> pin 70..80 OK ret 0
-> pin 80..90 OK ret 1
-> pin 90..100 OK ret 0