Linux Kernel Architecture

Chapter3:MemoryManagement

which callssetup_bootmem_allocatorto initialize the bootmem allocator, whereas AMD64 uses

contig_initmem_init.

The code flow diagram in Figure 3-20 illustrates the individual steps involved in the initialization of the

bootmem allocator on IA-32 systems, and the corresponding diagram for AMD64 is shown in Figure 3-21.

Determine usable low memory page frames

Call reserve_bootmem to reserve special regions

setup_memory

reserve_bootmem(bootmap,bootmap_size)

register_bootmem_low_pages

init_bootmem

setup_bootmem_allocator

Figure 3-20: Initialization of the bootmem allocator on IA-32 machines.

Find suitable memory area

contig_initmem

bootmem_bootmap_pages

init_bootmem

register_bootmem_with_active_regions

reserve_bootmem (bootmap,bootmap_size)

Figure 3-21: Initialization of the bootmem allocator on

AMD64 machines.

Initialization for IA-32

setup_memoryanalyzes the detected memory regions to find the maximal page frame number in the low

memory region; high memory is of no use for the bootmem allocator because it is too complicated to

handle. The global variablemax_low_pfnholds the number of the highest mappable page. The kernel

reports the amount of memory found in its boot logs.

wolfgang@meitner>dmesg

...

0MB HIGHMEM available.

511MB LOWMEM available.

...