src/rp2_common/pico_stdio_semihosting/stdio_semihosting.c

/*
 * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

#include "pico/stdio/driver.h"
#include "pico/stdio_semihosting.h"
#include "pico/binary_info.h"

static void stdio_semihosting_out_chars(const char *buf, int length) {
    // must be volatile or the buffer gets put in registers & optimized away
    volatile struct {
        // https://developer.arm.com/documentation/dui0375/g/What-is-Semihosting-/SYS-WRITE--0x05-
        // arguments, in order:
        // word 0 = file handle (1 = stdout)
        // word 1 = pointer to buffer
        // word 2 = length of buffer
        size_t fd;
        const char *buf;
        size_t len;
    } args;

    args.fd = 1;  // 1 = stdout
    args.buf = buf;
    args.len = length;

    pico_default_asm (
#ifdef __riscv
        // a0 encodes the semihosting call number, 0x05 = SYS_WRITE
        "li a0, 0x05\n"
        // a1 points to the arguments
        "mv a1, %[args]\n"
        // Magic three-instruction sequence, containing a breakpoint. Note the
        // RISC-V unpriv spec implies the final instruction might encode the
        // call number (passed in a1) but openocd source shows this is just a
        // constant value of 0x07. These instructions must be uncompressed:
        ".option push\n"
        ".option norvc\n"
        "slli x0, x0, 0x1f\n"
        "ebreak\n"
        "srai x0, x0, 0x07\n"
        ".option pop\n"
        :
        : [args] "r" (&args)
        : "a0", "a1"
#else
        // r1 must contain a pointer to the arguments
        "movs r1, %[args]\n"
        // semihosting call number 0x05 = SYS_WRITE
        "movs r0, #5\n"
        // make the semihosting call: https://developer.arm.com/documentation/dui0375/g/What-is-Semihosting-/The-semihosting-interface
        "bkpt 0xab\n"
        :
        : [args] "r" (&args)
        : "r0", "r1", "cc", "memory"
#endif
    );
}

stdio_driver_t stdio_semihosting = {
        .out_chars = stdio_semihosting_out_chars,
#if PICO_STDIO_ENABLE_CRLF_SUPPORT
        .crlf_enabled = PICO_STDIO_SEMIHOSTING_DEFAULT_CRLF
#endif
};

void stdio_semihosting_init(void) {
#if !PICO_NO_BI_STDIO_SEMIHOSTING
    bi_decl_if_func_used(bi_program_feature("semihosting stdout"));
#endif
    stdio_set_driver_enabled(&stdio_semihosting, true);
}

void stdio_semihosting_deinit(void) {
    stdio_set_driver_enabled(&stdio_semihosting, false);
}