crt0.S

/* Move section to .init that is allocatable and executable. */
.section .init, "ax"
.align 2
.global _start

_start:
    .cfi_startproc
    .cfi_undefined ra

    .option push
    .option norelax

    la gp, __global_pointer$
    .option pop

    la sp, __stack_top
    add s0, sp, zero
    j setup_mtrap

   jal zero, main
   .cfi_endproc
   .end

entry.S

.macro SAVE_CONTEXT
        sw ra, 24(sp)    # save return address to the stack
        sw t0, 20(sp)    # save temporary
        sw t1, 16(sp)    # save temporary
        sw t2, 12(sp)    # save temporary
        sw t3, 8(sp)     # save temporary
        sw a0, 4(sp)     # save first argument to function
.endm

.section .text
.align 4

.global setup_mtrap
setup_mtrap:
        # Setup machine trap handler address
        la t0, _trap_entry # load address of mtrap_handler

        # Set DIRECT_MODE access to
        slli t0, t0, 1 # shift address left by one ; LSB will be 0 (DIRECT_MODE)

        csrw mtvec, t0 # write handler and address mode

        # enable machine-mode interrupt (mstatus.MIE = 1)
        li t0, 8
        csrrs zero, mstatus, t0


_trap_entry:
        addi sp, sp, -28 # Allocate stack frame
        SAVE_CONTEXT
        csrr a0, mcause # write mcause to first function argument
        csrr a1, mtval # write mtval (exception specific information) to second function argument
        jal trap_handler

.end