Demo: Reserve, Allocate, Map

Two demos for part 04: physical memory arrives at the first access, one page at a time.

Demo	What it shows
reserve-allocate/	Reserve 4 MB, touch half the pages: RSS goes up by 2 MB, and 512 page faults happened.
page-faults/	The same, counted from inside: 0 faults for the mmap(), 1024 for the first write to each page, 0 for the second.

The first waits at prompts, for a second terminal; the second just prints.