Higher Half Kernel: Difference between revisions

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Line 2: {{Template:Kernel designs}} 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. ~~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~~some non-x86 ISA (the [[MIPS Overview\|MIPS]] [[User:Schol-r-lea/Understanding RISC vs CISC\|RISC architecture]] and [[AArch64\|ARM]]) which partly forces the issue. onOn MIPS and ARM 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: Line 11 ⟶ 10: * 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~~Mnemonic' invalid pointers such as ''0xCAFEBABE'', ''0xDEADBEEF'', ''0xDEADC0DE'', etc. can be used. == Bootloader support == To make things easier, some bootloaders natively support higher half kernels, by directly loading and mapping a kernel to the higher half in virtual memory. * [[BOOTBOOT]] only supports higher half kernels by design. It has example Hello World kernels written in [[C]], [[Pascal]], [[Rust]] and [[Go]] * The [[Limine]] protocol also only support higher half kernels by design. See [[Limine Bare Bones]] for a tutorial on how to write a simple 64-bit higher half kernel using Limine. == Initialization == To setup a higher half kernel, you have to map your kernel to the appropriate virtual address. ~~How~~When tousing doa ~~this~~boot ~~basically~~protocol ~~depends~~which onsupports ~~'''when'''~~higher ~~you'd~~half ~~like~~kernels ~~your~~directly, ~~kernel~~such toas ~~believe~~[[BOOTBOOT]], ~~it's~~or ~~in the higher end~~[[Limine]], ~~and~~your kernel ~~'''when'''~~will ~~you~~already ~~set~~be upproperly ~~paging~~mapped. 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. Without a boot loader help, you'll need a small trampoline code which runs in lower half, sets up higher half paging and jumps. == See Also == Line 21 ⟶ 28: * [[Higher Half x86 Bare Bones]] === ~~Forum~~ Threads === * [[Topic:11160\|I wrote a simple HigherHalf kernel]]