boot2

Playing with the boostrap
git clone https://git.ryansepassi.com/git/boot2.git
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crt_arch.h (1203B)

      1 /* tcc-build riscv64 crt_arch.h replacement.
      2  *
      3  * Stock musl uses several things tcc 0.9.26's riscv64 assembler can't:
      4  *
      5  *   1. `.option push/norelax/pop` directives — tcc has no .option
      6  *      handling at all. We don't run linker relaxation, so dropping
      7  *      these is harmless.
      8  *
      9  *   2. `lla gp, __global_pointer$` — sets the global pointer for
     10  *      gp-relative addressing under linker relaxation. Without
     11  *      relaxation, no code uses gp; leaving it at process-entry value
     12  *      is fine for static-only boot4 binaries.
     13  *
     14  *   3. `tail _start_c` — the `tail` pseudo (auipc+jalr indirect) is
     15  *      not in tcc's mnemonic table. Replace with `jal zero, _start_c`
     16  *      which tcc accepts and which reaches any in-TU target (crt1.c
     17  *      defines _start_c in the same translation unit).
     18  *
     19  *   4. _DYNAMIC handling — boot4 only links static binaries, so
     20  *      _start_c's second argument is unused. Don't pass it.
     21  *
     22  * What's left is the minimum kernel-to-userland glue: pass sp to
     23  * _start_c so it can read argc/argv/envp/auxv, then jump.
     24  */
     25 __asm__(
     26 ".text\n"
     27 ".global " START "\n"
     28 ".type " START ",%function\n"
     29 START ":\n"
     30 "	mv a0, sp\n"
     31 "	jal zero, " START "_c\n"
     32 );