asm.S

#if __riscv_xlen == 64
# define STORE    sd
# define LOAD     ld
# define LOG_REGBYTES 3
#else
# define STORE    sw
# define LOAD     lw
# define LOG_REGBYTES 2
#endif
#define REGBYTES (1 << LOG_REGBYTES)

/*
    Entry point of all programs (_start).

    It initializes DWARF call frame information, the stack pointer, the
    frame pointer (needed for closures to work in start_rust) and the global
    pointer. Then it calls _start_rust.
*/

.section .init, "ax"
.global _start

_start:
    /* Jump to the absolute address defined by the linker script. */
    // for 32bit
    .if __riscv_xlen == 32
    lui ra, %hi(_abs_start)
    jr %lo(_abs_start)(ra)
    .endif

    // for 64bit
    .if __riscv_xlen == 64
.option push
.option norelax // to prevent an unsupported R_RISCV_ALIGN relocation from being generated
1:
    auipc ra, %pcrel_hi(1f)
    ld ra, %pcrel_lo(1b)(ra)
    jr ra
    .align  3
1:
    .dword _abs_start
.option pop
    .endif

_abs_start:
    .cfi_startproc
    .cfi_undefined ra

    csrw mie, 0
    csrw mip, 0

    li  x1, 0
    li  x2, 0
    li  x3, 0
    li  x4, 0
    li  x5, 0
    li  x6, 0
    li  x7, 0
    li  x8, 0
    li  x9, 0
    li  x10,0
    li  x11,0
    li  x12,0
    li  x13,0
    li  x14,0
    li  x15,0
    li  x16,0
    li  x17,0
    li  x18,0
    li  x19,0
    li  x20,0
    li  x21,0
    li  x22,0
    li  x23,0
    li  x24,0
    li  x25,0
    li  x26,0
    li  x27,0
    li  x28,0
    li  x29,0
    li  x30,0
    li  x31,0

    .option push
    .option norelax
    la gp, __global_pointer$
    .option pop

    // Check hart id
    csrr a2, mhartid
    lui t0, %hi(_max_hart_id)
    add t0, t0, %lo(_max_hart_id)
    bgtu a2, t0, abort

    // Allocate stacks
    la sp, _stack_start
    lui t0, %hi(_hart_stack_size)
    add t0, t0, %lo(_hart_stack_size)
#ifdef __riscv_mul
    mul t0, a2, t0
#else
    beqz a2, 2f  // Jump if single-hart
    mv t1, a2
    mv t2, t0
1:
    add t0, t0, t2
    addi t1, t1, -1
    bnez t1, 1b
2:
#endif
    sub sp, sp, t0

    // Set frame pointer
    add s0, sp, zero

    jal zero, _start_rust

    .cfi_endproc

/*
    Trap entry point (_start_trap)

    Saves caller saved registers ra, t0..6, a0..7, calls _start_trap_rust,
    restores caller saved registers and then returns.
*/
.section .trap, "ax"
.global _start_trap
/* Make it .weak so PAC/HAL can provide their own if needed. */
.weak _start_trap

_start_trap:
    addi sp, sp, -16*REGBYTES

    STORE ra, 0*REGBYTES(sp)
    STORE t0, 1*REGBYTES(sp)
    STORE t1, 2*REGBYTES(sp)
    STORE t2, 3*REGBYTES(sp)
    STORE t3, 4*REGBYTES(sp)
    STORE t4, 5*REGBYTES(sp)
    STORE t5, 6*REGBYTES(sp)
    STORE t6, 7*REGBYTES(sp)
    STORE a0, 8*REGBYTES(sp)
    STORE a1, 9*REGBYTES(sp)
    STORE a2, 10*REGBYTES(sp)
    STORE a3, 11*REGBYTES(sp)
    STORE a4, 12*REGBYTES(sp)
    STORE a5, 13*REGBYTES(sp)
    STORE a6, 14*REGBYTES(sp)
    STORE a7, 15*REGBYTES(sp)

    add a0, sp, zero
    jal ra, _start_trap_rust

    LOAD ra, 0*REGBYTES(sp)
    LOAD t0, 1*REGBYTES(sp)
    LOAD t1, 2*REGBYTES(sp)
    LOAD t2, 3*REGBYTES(sp)
    LOAD t3, 4*REGBYTES(sp)
    LOAD t4, 5*REGBYTES(sp)
    LOAD t5, 6*REGBYTES(sp)
    LOAD t6, 7*REGBYTES(sp)
    LOAD a0, 8*REGBYTES(sp)
    LOAD a1, 9*REGBYTES(sp)
    LOAD a2, 10*REGBYTES(sp)
    LOAD a3, 11*REGBYTES(sp)
    LOAD a4, 12*REGBYTES(sp)
    LOAD a5, 13*REGBYTES(sp)
    LOAD a6, 14*REGBYTES(sp)
    LOAD a7, 15*REGBYTES(sp)

    addi sp, sp, 16*REGBYTES
    mret

.section .text
.global default_setup_interrupts

default_setup_interrupts:
    // Set trap handler
    la t0, _start_trap
    csrw mtvec, t0
    ret

/* Make sure there is an abort when linking */
.globl abort
abort:
    j abort