/* ################################################################################################# */ /* # << NEORV32 - crt0.S Start-Up Code >> # */ /* # ********************************************************************************************* # */ /* # BSD 3-Clause License # */ /* # # */ /* # Copyright (c) 2023, Stephan Nolting. All rights reserved. # */ /* # # */ /* # Redistribution and use in source and binary forms, with or without modification, are # */ /* # permitted provided that the following conditions are met: # */ /* # # */ /* # 1. Redistributions of source code must retain the above copyright notice, this list of # */ /* # conditions and the following disclaimer. # */ /* # # */ /* # 2. Redistributions in binary form must reproduce the above copyright notice, this list of # */ /* # conditions and the following disclaimer in the documentation and/or other materials # */ /* # provided with the distribution. # */ /* # # */ /* # 3. 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This is required if this code // is part of a program uploaded by the on-chip debugger (potentionally taking control from the // bootloader). We setup a new stack pointer here and WE DO NOT WANT TO trap to an outdated trap // handler with a modified stack pointer. // ************************************************************************************************ csrrwi zero, mstatus, 0 // clear mstatus; disable machine-level interrupts // ************************************************************************************************ // Setup CPU core CSRs // ************************************************************************************************ __crt0_cpu_csr_init: csrw mie, zero // disable all interrupt sources la x1, __crt0_trap_handler // configure early-boot trap handler csrw mtvec, x1 // ************************************************************************************************ // Initialize integer register file // ************************************************************************************************ __crt0_pointer_init: .option push .option norelax // Setup pointers using linker script symbols la sp, __crt0_stack_begin // stack pointer la gp, __global_pointer$ // global pointer .option pop __crt0_reg_file_init: //addi x0, x0, 0 // hardwired to zero //addi x1, x0, 0 // implicitly initialized within crt0 //addi x2, x0, 0 // stack pointer sp //addi x3, x0, 0 // global pointer gp 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 // implicitly initialized within crt0 //addi x11, x0, 0 // implicitly initialized within crt0 //addi x12, x0, 0 // implicitly initialized within crt0 //addi x13, x0, 0 // implicitly initialized within crt0 //addi x14, x0, 0 // implicitly initialized within crt0 //addi x15, x0, 0 // implicitly initialized within crt0 #ifndef __riscv_32e // initialize upper half (if E extension is not implemented) addi x16, x0, 0 addi x17, x0, 0 addi x18, x0, 0 addi x19, x0, 0 addi x20, x0, 0 addi x21, x0, 0 addi x22, x0, 0 addi x23, x0, 0 addi x24, x0, 0 addi x25, x0, 0 addi x26, x0, 0 addi x27, x0, 0 addi x28, x0, 0 addi x29, x0, 0 addi x30, x0, 0 addi x31, x0, 0 #endif // ************************************************************************************************ // Copy initialized .data section from ROM to RAM (word-wise, section begins and ends on word boundary) // ************************************************************************************************ __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 beq x11, x12, __crt0_copy_data_loop_end // nothing to do if source and destination address are the same __crt0_copy_data_loop: bge x12, x13, __crt0_copy_data_loop_end lw x14, 0(x11) sw x14, 0(x12) addi x11, x11, 4 addi x12, x12, 4 j __crt0_copy_data_loop __crt0_copy_data_loop_end: // ************************************************************************************************ // Clear .bss section (word-wise, section begins and ends on word boundary) // ************************************************************************************************ __crt0_clear_bss: la x14, __crt0_bss_start la x15, __crt0_bss_end __crt0_clear_bss_loop: bge x14, x15, __crt0_clear_bss_loop_end sw zero, 0(x14) addi x14, x14, 4 j __crt0_clear_bss_loop __crt0_clear_bss_loop_end: // ************************************************************************************************ // Call constructors // ************************************************************************************************ #ifndef make_bootloader // constructors are not supported for bootloader __crt0_call_constructors: la x8, __init_array_start la x9, __init_array_end __crt0_call_constructors_loop: bge x8, x9, __crt0_call_constructors_loop_end lw x1, 0(x8) jalr x1, 0(x1) addi x8, x8, 4 j __crt0_call_constructors_loop __crt0_call_constructors_loop_end: #endif // ************************************************************************************************ // Setup arguments and call main function // ************************************************************************************************ __crt0_main_entry: addi x10, zero, 0 // x10 = a0 = argc = 0 addi x11, zero, 0 // x11 = a1 = argv = 0 jal x1, main // call actual app's main function __crt0_main_exit: // main's "return" and "exit" will arrive here csrw mie, zero // disable all interrupt sources csrw mscratch, a0 // backup main's return code to mscratch (for debugger) // ************************************************************************************************ // Call destructors // ************************************************************************************************ #ifndef make_bootloader // destructors are not supported for bootloader __crt0_call_destructors: la x8, __fini_array_start la x9, __fini_array_end __crt0_call_destructors_loop: bge x8, x9, __crt0_call_destructors_loop_end lw x1, 0(x8) jalr x1, 0(x1) addi x8, x8, 4 j __crt0_call_destructors_loop __crt0_call_destructors_loop_end: #endif // ************************************************************************************************ // Call "after main" handler (if there is any) if main really returns // ************************************************************************************************ #ifndef make_bootloader // after_main handler not supported for bootloader __crt0_main_aftermath: .weak __neorv32_crt0_after_main la ra, __neorv32_crt0_after_main beqz ra, __crt0_main_aftermath_end // check if an aftermath handler has been specified jalr ra // execute handler with main's return code still in a0 __crt0_main_aftermath_end: #endif // ************************************************************************************************ // Go to endless sleep mode // ************************************************************************************************ __crt0_shutdown: wfi // go to sleep mode j . // endless loop // ************************************************************************************************ // Dummy trap handler (for all traps during very early boot stage) // - does nothing but trying to move on to the next instruction // ************************************************************************************************ .balign 4 // the trap handler has to be 32-bit aligned __crt0_trap_handler: addi sp, sp, -8 sw x8, 0(sp) sw x9, 4(sp) csrr x8, mcause blt x8, zero, __crt0_trap_handler_end // skip mepc modification if interrupt // update mepc csrr x8, mepc 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 // mepc +2 only for compressed instructions csrw mepc, x8 // set new return address addi x8, zero, 3 bne x8, x9, __crt0_trap_handler_end // done if compressed instruction csrr x8, mepc addi x8, x8, +2 // add another +2 to mepc (making +4) for uncompressed instructions csrw mepc, x8 __crt0_trap_handler_end: lw x8, 0(sp) lw x9, 4(sp) addi sp, sp, +8 mret .cfi_endproc .end