= Kernel vs process memory layout
The Linux Kernel reserves two zones of virtual memory:
* one for kernel memory
* one for programs
The exact split is configured by `CONFIG_VMSPLIT_...`. By default:
* on 32-bit:
* the bottom 3/4 is program space: `00000000` to `BFFFFFFF`
* the top 1/4 is kernel memory: `C0000000` to `FFFFFFFF`, like this:
``
------------------ FFFFFFFF
Kernel
------------------ C0000000
------------------ BFFFFFFF
Process
------------------ 00000000
``
* on 64-bit: currently only 48-bits are actually used, split into two equally sized disjoint spaces. The Linux kernel just assigns:
* the bottom part to processes `00000000 00000000` to `008FFFFF FFFFFFFF`
* the top part to the kernel: `FFFF8000 00000000` to `FFFFFFFF FFFFFFFF`, like this:
``
------------------ FFFFFFFF
Kernel
------------------ C0000000
(not addressable)
------------------ BFFFFFFF
Process
------------------ 00000000
``
Kernel memory https://stackoverflow.com/questions/18953598/is-it-true-that-whole-system-space-address-space-in-linux-does-not-use-demand-pa[is also paged].
In previous versions, https://stackoverflow.com/questions/1658757/linux-3-1-virtual-address-split[the paging was continuous, but with HIGHMEM this changed].
There is no clear physical memory split: https://stackoverflow.com/questions/30471742/physical-memory-userspace-kernel-split-on-linux-x86-64