maixbit (uart): serial is working with echo example

src/device/riscv/start64.S

@@ -0,0 +1,60 @@
+.section .init
+.global _start
+.type _start,@function
+
+_start:
+    // Load the stack pointer.
+    la sp,      _stack_top
+
+    // Load the globals pointer. The program will load pointers relative to this
+    // register, so it must be set to the right value on startup.
+    // See: https://gnu-mcu-eclipse.github.io/arch/riscv/programmer/#the-gp-global-pointer-register
+    la gp,      __global_pointer$
+
+    // Jump to runtime.main
+    call main
+
+.section .text.handleInterruptASM
+.global handleInterruptASM
+.type handleInterruptASM,@function
+handleInterruptASM:
+    // Save and restore all registers, because the hardware only saves/restores
+    // the pc.
+    // Note: we have to do this in assembly because the "interrupt"="machine"
+    // attribute is broken in LLVM: https://bugs.llvm.org/show_bug.cgi?id=42984
+    addi    sp, sp, -128
+    sd      ra, 120(sp)
+    sd      t0, 112(sp)
+    sd      t1, 104(sp)
+    sd      t2, 96(sp)
+    sd      a0, 88(sp)
+    sd      a1, 80(sp)
+    sd      a2, 72(sp)
+    sd      a3, 64(sp)
+    sd      a4, 56(sp)
+    sd      a5, 48(sp)
+    sd      a6, 40(sp)
+    sd      a7, 32(sp)
+    sd      t3, 24(sp)
+    sd      t4, 16(sp)
+    sd      t5, 8(sp)
+    sd      t6, 0(sp)
+    call    handleInterrupt
+    ld      t6, 0(sp)
+    ld      t5, 8(sp)
+    ld      t4, 16(sp)
+    ld      t3, 24(sp)
+    ld      a7, 32(sp)
+    ld      a6, 40(sp)
+    ld      a5, 48(sp)
+    ld      a4, 56(sp)
+    ld      a3, 64(sp)
+    ld      a2, 72(sp)
+    ld      a1, 80(sp)
+    ld      a0, 88(sp)
+    ld      t2, 96(sp)
+    ld      t1, 104(sp)
+    ld      t0, 112(sp)
+    ld      ra, 120(sp)
+    addi    sp, sp, 128
+    mret