Kernel high memory

Question

In Operating System Design the kernel is most always mapped to a high virtual memory address, thus gaining control of the upper memory part. The space left below is for applications running in user space, as described in an excellent way in "Linux 3/1 virtual address split".

What I'd like to know is, why is this design decision made or why the kernel does not use the lower part of memory? This is not really clear to me, or maybe I've overseen something.

Edit: This question regards virtual addresses and not physical.

Answer 1

Some advantages of/reasons for such a design:

• Applications don't need to care about the size and location of the kernel and may pretend they're the only ones in the memory, starting at around 0 and spanning upwards, with minimal or no code and data relocations. Applications are hence easier to design and implement and they may be less likely to have bugs related to mememory management.
• Applications may use smaller/shorter addresses/pointers and hence save some memory.
• In the x86 CPU, 16-bit and 32-bit address spaces start at virtual address 0 and end at around 1MB (for real and virtual 8086 modes), 16 MB (16-bit protected mode on i80286+) and 4 GB (32-bit mode, unreal mode). Placing the kernel at lower addresses will reduce the range of addresses available to applications (ex: 16-bit app in 32-bit mode or 32-bit app in 64-bit mode) and/or complicate their memory management. Moving the kernel to the top of the virtual address space generally makes sense on the x86.

There may be other reasons, usually platform-specific. On some platforms there may be little to no difference between the two options. Yet on others the preferred kernel location may be at the lower virtual addresses. Details matter.