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/* ################################################################################################# */
/* # << NEORV32 - crt0.S - Start-Up Code >>                                                        # */
/* # ********************************************************************************************* # */
/* # BSD 3-Clause License                                                                          # */
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/* # Copyright (c) 2020, Stephan Nolting. All rights reserved.                                     # */
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/* # The NEORV32 Processor - https://github.com/stnolting/neorv32              (c) Stephan Nolting # */
/* ################################################################################################# */

.file   "crt0.S"
.section .text.boot
.balign 4
.global _start


// IO region
.equ IO_BEGIN, 0xFFFFFF80 // start of processor-internal IO region


_start:
.cfi_startproc
.cfi_undefined ra

// *********************************************************
// Clear register file
// Assume 'worst case': rv32e
// *********************************************************
__crt0_reg_file_clear:
//addi  x0,  x0, 0 // hardwired to zero
  addi  x1,  x0, 0
  addi  x2,  x0, 0
  addi  x3,  x0, 0
  addi  x4,  x0, 0
  addi  x5,  x0, 0
  addi  x6,  x0, 0
  addi  x7,  x0, 0
  addi  x8,  x0, 0
  addi  x9,  x0, 0
//addi x10,  x0, 0
//addi x11,  x0, 0
//addi x12,  x0, 0
//addi x13,  x0, 0
  addi x14,  x0, 0
  addi x15,  x0, 0


// *********************************************************
// No interrupts, thanks
// *********************************************************
__crt0_status_init:
  li x10, 0x00001800    // clear mstatus and set mpp(1:0)
  csrrw zero, mstatus, x10
  csrrw zero, mie, zero // clear mie


// *********************************************************
// Setup pointers using linker script symbol
// *********************************************************
__crt0_pointer_init:
.option push
.option norelax
  la    sp, __crt0_stack_begin
  andi  sp, sp, 0xfffffffc // make sure this is aligned
  addi  fp, sp, 0          // frame pointer = stack pointer
  la gp, __global_pointer$ // global pointer
.option pop


// *********************************************************
// Init trap handler base address
// *********************************************************
__crt0_neorv32_trap_init:
  la    x11, __crt0_dummy_trap_handler
  csrw  mtvec, x11 // set address of first-level exception handler


// *********************************************************
// Reset/deactivate IO/peripheral devices
// Devices, that are not implemented, will cause a store access fault
// which is captured but actually ignored due to the dummy handler.
// *********************************************************
__crt0_reset_io:
  li x11, IO_BEGIN // start of processor-internal IO region

__crt0_reset_io_loop:
  sw   zero, 0(x11)
  addi x11, x11, 4
  bne  zero, x11, __crt0_reset_io_loop


// *********************************************************
// Clear .bss section (byte-wise) using linker script symbols
// *********************************************************
__crt0_clear_bss:
  la x11, __crt0_bss_start
  la x12, __crt0_bss_end

__crt0_clear_bss_loop:
  bge  x11, x12, __crt0_clear_bss_loop_end
  sb   zero, 0(x11)
  addi x11, x11, 1
  j    __crt0_clear_bss_loop

__crt0_clear_bss_loop_end:


// *********************************************************
// Copy initialized .data section from ROM to RAM (byte-wise) using linker script symbols
// *********************************************************
__crt0_copy_data:
  la x11, __crt0_copy_data_src_begin  // start of data area (copy source)
  la x12, __crt0_copy_data_dst_begin  // start of data area (copy destination)
  la x13, __crt0_copy_data_dst_end    // last address of destination data area

__crt0_copy_data_loop:
  bge  x12, x13,  __crt0_copy_data_loop_end
  lb   x14, 0(x11)
  sb   x14, 0(x12)
  addi x11, x11, 1
  addi x12, x12, 1
  j    __crt0_copy_data_loop

__crt0_copy_data_loop_end:


// *********************************************************
// Call main function (with argc = argv = 0)
// *********************************************************
__crt0_main_entry:

  addi x10, zero, 0 // argc = 0
  addi x11, zero, 0 // argv = 0

  jal ra, main


// *********************************************************
// Go to endless sleep mode if main returns
// *********************************************************
__crt0_this_is_the_end:
  csrrci zero, mstatus, 8 // mstatus: disable global IRQs (MIE)
  wfi
  j . // in case WFI is not available


// *********************************************************
// dummy trap handler (for exceptions & IRQs)
// tries to move on to next instruction
// *********************************************************
.global __crt0_dummy_trap_handler
.balign 4
__crt0_dummy_trap_handler:

  addi  sp, sp, -8
  sw      x8, 0(sp)
  sw      x9, 4(sp)

  csrr  x8, mcause
  blt   x8, zero, __crt0_dummy_trap_handler_irq  // skip mepc modification if interrupt

  csrr  x8, mepc

// is compressed instruction?
__crt0_dummy_trap_handler_exc_c_check:
  lh    x9, 0(x8)   // get compressed instruction or lower 16 bits of uncompressed instruction that caused exception
  andi  x9, x9, 3   // mask: isolate lowest 2 opcode bits (= 11 for uncompressed instructions)

  addi  x8, x8, +2  // only this for compressed instructions
  csrw  mepc, x8    // set return address when compressed instruction

  addi  x8, zero, 3
  bne   x8, x9, __crt0_dummy_trap_handler_irq // jump if compressed instruction
  
// is uncompressed instruction
__crt0_dummy_trap_handler_exc_uncrompressed:
  csrr  x8, mepc
  addi  x8, x8, +2  // add another 2 (making +4) for uncompressed instructions
  csrw  mepc, x8

__crt0_dummy_trap_handler_irq:

  lw    x9, 0(sp)
  lw    x8, 4(sp)
  addi  sp, sp, +8

  mret

.cfi_endproc
.end