92 lines
4.7 KiB
Markdown
92 lines
4.7 KiB
Markdown
# Neptune Kernel
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Inspired by xv6
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For a quick reference on RISC-V assembly:
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- https://risc-v.guru/instructions/
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Some low-level string format routines borrowed from [mini-printf](https://github.com/mludvig/mini-printf)
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### Directory outline
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The planned directory outline is as follows:
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```
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root
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├── kern # General kernel sources
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│ ├── libkern # Kernel specific library routines
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│ ├── memory # Memory related code (kalloc, buddy and friends)
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│ └── arch # Architecture specific functionality
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├── libc # Userspace C library
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└── user # Userspace programs
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```
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### Quick Start
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On a linux machine, make sure that you have `make` and `curl` in path. The rest
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of the tooling (gcc, qemu) will be handled by the build system. For extra convenience,
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you could also use `bear` for generating "compile_commands.json". This is useful for helping
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clangd pick up the correct paths, and will result in better in-editor diagnostics and warnings.
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```sh
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make get_toolchain # Dont worry, is will land locally inside project directory
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make -j$(nproc)
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make qemu
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```
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To generate "compile_commands.json":
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```sh
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make clean # To get a full build
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bear -- make -j$(nproc)
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```
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The project can be cleaned with:
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```sh
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make clean # Wipes .o, .d, .elf
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make distclean # Wipes the above, but also removes toolchain
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```
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### Toolchains:
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- https://github.com/xpack-dev-tools/riscv-none-elf-gcc-xpack
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- https://github.com/xpack-dev-tools/qemu-riscv-xpack/
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---
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> A word on terminology: Although the official x86 term is exception, xv6 uses the
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> term trap, largely because it was the term used by the PDP11/40 and therefore is the
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> conventional Unix term.
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### Notes/Misc
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| Register | Name | Privilege Level | Description |
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|-------------|----------------------------|------------------|-----------------------------------------------------------------------------|
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| `mstatus` | Machine Status Register | Machine | Holds global interrupt enable, previous privilege mode, etc. |
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| `mtvec` | Machine Trap-Vector Base | Machine | Holds the base address of the trap handler (exception/interrupt entry). |
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| `mepc` | Machine Exception PC | Machine | Stores the program counter at the time of the last trap. |
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| `mcause` | Machine Cause Register | Machine | Indicates the cause of the last trap (interrupt or exception). |
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| `satp` | Supervisor Address Translation and Protection | Supervisor | Controls page table base address and mode (e.g., Sv39, Sv48). |
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| `sstatus` | Supervisor Status Register | Supervisor | Like `mstatus`, but accessible from supervisor mode. |
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| `stvec` | Supervisor Trap-Vector Base| Supervisor | Like `mtvec`, but for supervisor mode traps. |
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| `sepc` | Supervisor Exception PC | Supervisor | Like `mepc`, but for supervisor mode. |
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| `scause` | Supervisor Cause Register | Supervisor | Like `mcause`, but for supervisor mode traps. |
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| `sscratch` | Supervisor Scratch | Supervisor | Can be used to store temporary state across traps in supervisor mode. |
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| `mscratch` | Machine Scratch | Machine | Like `sscratch`, but in machine mode. |
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| `mcycle` | Machine Cycle Counter | Machine | Counts the number of cycles executed. |
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| `mtime` | Machine Timer Register | Machine (via memory-mapped) | Used for timing and scheduling (not a CSR, but a memory-mapped register). |
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| `mip` | Machine Interrupt Pending | Machine | Indicates pending interrupts. |
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| `mie` | Machine Interrupt Enable | Machine | Controls which interrupts are enabled. |
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### TODO
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- Kassert, some macro in the form of `ASSERT(condition, "This went wrong")`
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- [FatFs](https://elm-chan.org/fsw/ff/)
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- Integrate [Kconfig](https://github.com/ulfalizer/Kconfiglib)
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- Klog, or some form of logging that is directable and more than just kprintf
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### Libc Implementations
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- [Libc reference](https://discapes.github.io/libc-reference/)
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- [uClibc](https://uclibc.org/)
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- [musl libc](https://musl.libc.org/)
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- [dietlibc](http://www.fefe.de/dietlibc/)
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[Comparison of C/POSIX standard library implementations for Linux](https://www.etalabs.net/compare_libcs.html) (by musl author)
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