Higher Half Kernel
It is traditional and generally good to have your kernel mapped in every user process. Linux and many other Unices, for instance, reside at virtual addresses 0xC0000000 - 0xFFFFFFFF of every address space, leaving the range 0x00000000 - 0xBFFFFFFF for user code, data, stacks, libraries, etc. Kernels that have such design are said to be "in the higher half" by opposition to kernels that use lowest virtual addresses for themselves, and leave higher addresses for the applications.
In addition, there is at least one non-x86 ISA (the MIPS RISC architecture) which partly forces the issue. on MIPS systems, addresses using the high bit (either bit 31 or bit 63, depending on the system word width) are reserved for use in Supervisor mode, and are exception trapped when in User mode.
Advantages of a higher half kernel are:
- It's easier to set up VM86 processes since the region below 1 MB is userspace.
- More generally, user applications are not dependent on how much memory is kernel space (your application can be linked to 0x400000 regardless of whether kernel is at 0xC0000000, 0x80000000 or 0xE0000000 ...), which makes the ABI nicer.
- If your OS is 64-bit, then 32-bit applications will be able to use the full 32-bit address space.
- 'mnemonic' invalid pointers such as 0xCAFEBABE, 0xDEADBEEF, 0xDEADC0DE, etc. can be used.
To setup a higher half kernel, you have to map your kernel to the appropriate virtual address. How to do this basically depends on when you'd like your kernel to believe it's in the higher end, and when you set up paging.