asm.S

/*
    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:
    .cfi_startproc
    .cfi_undefined ra

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

    la sp, _stack_start

    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

_start_trap:
    addi sp, sp, -16*4

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

    jal ra, _start_trap_rust

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

    addi sp, sp, 16*4
    mret


/* Make sure there is an abort when linking */
.section .init
.globl abort
abort:
    jal zero, _start