Chapter3:MemoryManagement
(calculated bysize >> 1). The front half of the initial area is inserted in thefree_arealist for
order=4.
5
4
3
2
1
page
mem_map
Area
5
4
3
2
1
page
mem_map
Area
5
4
3
2
1
page
mem_map
Area
Figure 3-34: Steps performed byexpandwhen splitting a memory area.
Only the firstpageinstance of a memory area is needed by the buddy system for
management purposes; the size of the areais derived automatically from the list in
which the page is located.
- The index of the rear memory area withsize = 16can be calculated by addingsizetoindex,
thus skipping the next 16 bits in the allocation bitmap. Because allpageinstances are in lin-
ear succession in memory, the pointer topagemust also be increased by 16 to arrive at the
correspondingpageinstance. The position of thepagepointer is indicated by an arrow in
Figure 3-34.
- The next loop pass places the first half of the remaining 16 units on thefree_arealist with
size=8.Bothindexandpageare then increased by 8 units. The function has now arrived at
the desired size unit, and thepagepointer can be returned as the result. From the figure, it
is evident that the last 8 pages of the original area of 32 pages are used; all other pages are in
the appropriatefree_arealists of the buddy system.
