70 lines
3.2 KiB
Markdown
70 lines
3.2 KiB
Markdown
# Writing software for PepperOS
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## Why would you want to do that?
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Honestly I have no idea. Maybe you have too much free time.
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Keep in mind that the Pepper kernel is a personal project and it's full of bugs, inconsistencies, weird ways of doing things (and I don't care because it's my toy).
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Now if you still want to write something for this OS, thank you. Follow along.
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## Headers available in userspace
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Of course, all of the freestanding headers are available:
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- `<float.h>`: macros for floating-point types
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- `<limits.h>`: macros for integer types
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- `<iso646.h>`: macros for bitwise and logical operators
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- `<stdarg.h>`: variadic function support
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- `<stddef.h>`: definitions for `size_t`, `ptrdiff_t`, and others
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- `<stdbool.h>`: definitions for boolean types
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- `<stdint.h>`: definitions for `int_t` and `uint_t` types
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Also available is the `<syscall.h>` header that gives access to low-level system call interface, notably the `syscallX` function family, X being the amount of arguments to use.
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## 1. Write the source code
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PepperOS is able to run programs written in x86 assembly, and C programs.
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### x86 Assembly
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Start your assembly file with the `bits 64` instruction, to emit 64-bit code.
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You can add sections `.text`, `.data`, `.bss` as you need.
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The three things to take in consideration here are:
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- PepperOS does not use the `syscall` instruction, instead it uses the old-fashioned `int 0x80` to trigger a system call.
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- The entry point should be labelled as `_start`.
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- At the end of the file, there should be an exit system call followed by a loop, like so:
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```nasm
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.end:
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mov rax, 0x3C
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mov rdi, 0x0
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int 0x80
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.loop:
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jmp .loop
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```
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For an example, look at the file [pedicel.S](../user/pedicel.S).
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### C program
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You will find relevant headers in the `libc` directory. They contain system call wrappers, utility functions, and more. See what's implemented there and what's not.
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To invoke a system call you can use the functions defined in `libc/syscall.h`.
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## 2. Add the Makefile rule and variable
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Now that your code is complete, add a Makefile rule to `user/Makefile` with your program name. You can just copy-paste the rule that applies to you (either from an Assembly source or C source) and change the name of the files (.raw, .elf, etc...) in the rule.
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For clarity, raw binaries have the `.raw` extension, and ELF ones have `.elf`.
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You also now have to add the name of the executable to the `USER_PROGRAMS` variable at the top of the global Makefile.
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Finally, do `make user` to compile your program.
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## 3. Run your program
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You can now boot up PepperOS, in a VM or on real hardware, and use the kernel's shell to `list` files in the filesystem (to see if your executable was properly added), and then, run it with the `load` command. Congratulations, you made a program for a random hobby OS!
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## 4. (Optional) debugging
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Use GDB with the `make debug` rule!
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For your information, user programs are loaded at `0x400000`. Can be good to know to set breakpoints.
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## 5. (Optional) contribute!
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If you like what you've done and you think it could be nice to add it to PepperOS, send it to me by e-mail: `xamidev (at) riseup (dot) net`. It may or may not be added in a future release... who knows? |