You are right that you can use qemu-system-aarch64 for achieving your goal.
Depending on what you exactly want to do, you have multiple options:
use the semihosting mode of qemu, along with the gcc --specs=rdimon.specs with newlib, or using another semihosting library, such as the one available in the Arm Trusted Firmware source code - the example hereafter uses this approach.
provide your own syscalls.c , and use the --specs=nosys.specs ld option, so that you can use newlib in baremetal programs: I would suggest to read the excellent article from Francesco Balducci on the Balau blog - the example hereafter uses this approach.
use a more baremetal-like approach, such as the one described hereafter: it does use sprintf() and the pl011 UART of the qemu-virt machine for displaying the resulting string.
gcc_arm64_ram.ld:
/******************************************************************************
* @file gcc_arm32.ld
* @brief GNU Linker Script for Cortex-M based device
* @version V2.0.0
* @date 21. May 2019
******************************************************************************/
/*
* Copyright (c) 2009-2019 Arm Limited. All rights reserved.
*
* SPDX-License-Identifier: Apache-2.0
*
* Licensed under the Apache License, Version 2.0 (the License); you may
* not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an AS IS BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
MEMORY
{
RAM (rwx) : ORIGIN = __RAM_BASE, LENGTH = __RAM_SIZE
}
/* Linker script to place sections and symbol values. Should be used together
* with other linker script that defines memory regions FLASH and RAM.
* It references following symbols, which must be defined in code:
* Reset_Handler : Entry of reset handler
*
* It defines following symbols, which code can use without definition:
* __exidx_start
* __exidx_end
* __copy_table_start__
* __copy_table_end__
* __zero_table_start__
* __zero_table_end__
* __etext
* __data_start__
* __preinit_array_start
* __preinit_array_end
* __init_array_start
* __init_array_end
* __fini_array_start
* __fini_array_end
* __data_end__
* __bss_start__
* __bss_end__
* __end__
* end
* __HeapLimit
* __StackLimit
* __StackTop
* __stack
*/
ENTRY(Reset_Handler)
SECTIONS
{
.text :
{
KEEP(*(.vectors))
*(.text*)
KEEP(*(.init))
KEEP(*(.fini))
/* .ctors */
*crtbegin.o(.ctors)
*crtbegin?.o(.ctors)
*(EXCLUDE_FILE(*crtend?.o *crtend.o) .ctors)
*(SORT(.ctors.*))
*(.ctors)
/* .dtors */
*crtbegin.o(.dtors)
*crtbegin?.o(.dtors)
*(EXCLUDE_FILE(*crtend?.o *crtend.o) .dtors)
*(SORT(.dtors.*))
*(.dtors)
*(.rodata*)
KEEP(*(.eh_frame*))
} > RAM
/*
* SG veneers:
* All SG veneers are placed in the special output section .gnu.sgstubs. Its start address
* must be set, either with the command line option ?--section-start? or in a linker script,
* to indicate where to place these veneers in memory.
*/
/*
.gnu.sgstubs :
{
. = ALIGN(32);
} > RAM
*/
.ARM.extab :
{
*(.ARM.extab* .gnu.linkonce.armextab.*)
} > RAM
__exidx_start = .;
.ARM.exidx :
{
*(.ARM.exidx* .gnu.linkonce.armexidx.*)
} > RAM
__exidx_end = .;
.copy.table :
{
. = ALIGN(16);
__copy_table_start__ = .;
LONG (__etext)
LONG (__data_start__)
LONG (__data_end__ - __data_start__)
/* Add each additional data section here */
/*
LONG (__etext2)
LONG (__data2_start__)
LONG (__data2_end__ - __data2_start__)
*/
__copy_table_end__ = .;
} > RAM
.zero.table :
{
. = ALIGN(16);
__zero_table_start__ = .;
/* Add each additional bss section here */
/*
LONG (__bss2_start__)
LONG (__bss2_end__ - __bss2_start__)
*/
__zero_table_end__ = .;
} > RAM
/**
* Location counter can end up 2byte aligned with narrow Thumb code but
* __etext is assumed by startup code to be the LMA of a section in RAM
* which must be 4byte aligned
*/
__etext = ALIGN(16);
.data : AT (__etext)
{
__data_start__ = .;
*(vtable)
*(.data)
*(.data.*)
. = ALIGN(16);
/* preinit data */
PROVIDE_HIDDEN (__preinit_array_start = .);
KEEP(*(.preinit_array))
PROVIDE_HIDDEN (__preinit_array_end = .);
. = ALIGN(16);
/* init data */
PROVIDE_HIDDEN (__init_array_start = .);
KEEP(*(SORT(.init_array.*)))
KEEP(*(.init_array))
PROVIDE_HIDDEN (__init_array_end = .);
. = ALIGN(16);
/* finit data */
PROVIDE_HIDDEN (__fini_array_start = .);
KEEP(*(SORT(.fini_array.*)))
KEEP(*(.fini_array))
PROVIDE_HIDDEN (__fini_array_end = .);
KEEP(*(.jcr*))
. = ALIGN(16);
/* All data end */
__data_end__ = .;
} > RAM
/*
* Secondary data section, optional
*
* Remember to add each additional data section
* to the .copy.table above to asure proper
* initialization during startup.
*/
/*
__etext2 = ALIGN(16);
.data2 : AT (__etext2)
{
. = ALIGN(16);
__data2_start__ = .;
*(.data2)
*(.data2.*)
. = ALIGN(16);
__data2_end__ = .;
} > RAM2
*/
.bss :
{
. = ALIGN(16);
__bss_start__ = .;
*(.bss)
*(.bss.*)
*(COMMON)
. = ALIGN(16);
__bss_end__ = .;
} > RAM AT > RAM
/*
* Secondary bss section, optional
*
* Remember to add each additional bss section
* to the .zero.table above to asure proper
* initialization during startup.
*/
/*
.bss2 :
{
. = ALIGN(16);
__bss2_start__ = .;
*(.bss2)
*(.bss2.*)
. = ALIGN(16);
__bss2_end__ = .;
} > RAM2 AT > RAM2
*/
.heap (COPY) :
{
. = ALIGN(16);
__end__ = .;
PROVIDE(end = .);
. = . + __HEAP_SIZE;
. = ALIGN(16);
__HeapLimit = .;
} > RAM
.stack (ORIGIN(RAM) + LENGTH(RAM) - __STACK_SIZE) (COPY) :
{
. = ALIGN(16);
__StackLimit = .;
. = . + __STACK_SIZE;
. = ALIGN(16);
__StackTop = .;
} > RAM
PROVIDE(__stack = __StackTop);
/* Check if data + heap + stack exceeds RAM limit */
ASSERT(__StackLimit >= __HeapLimit, "region RAM overflowed with stack")
}
qemu-virt-aarch64.ld:
__RAM_BASE = 0x40000000;
__RAM_SIZE = 0x08000000;
__STACK_SIZE = 0x00100000;
__HEAP_SIZE = 0x00100000;
INCLUDE gcc_arm64_ram.ld
startup.s:
.title startup64.s
.arch armv8-a
.text
.section .text.startup,"ax"
.globl Reset_Handler
Reset_Handler:
ldr x0, =__StackTop
mov sp, x0
bl main
wait: wfe
b wait
.end
pl011.c:
#include <stdint.h>
static volatile unsigned int * const UART0DR = ( unsigned int * ) ( uintptr_t * ) 0x9000000;
int putchar(int c)
{
*UART0DR = c; /* Transmit char */
return c;
}
void putchar_uart0( int c )
{
*UART0DR = c; /* Transmit char */
}
void putc_uart0( int c )
{
*UART0DR = c; /* Transmit char */
}
void print_uart0( const char * s )
{
while( *s != '' ) /* Loop until end of string */
{
*UART0DR = ( unsigned int ) ( *s ); /* Transmit char */
s++; /* Next char */
}
}
void puts_uart0( const char * s )
{
while( *s != '' ) /* Loop until end of string */
{
*UART0DR = ( unsigned int ) ( *s ); /* Transmit char */
if (*s == '
') {
*UART0DR = ( unsigned int ) ( '
' );
}
s++; /* Next char */
}
}
pl011.h:
#pragma once
#ifdef __cplusplus
extern "C" {
#endif
void putchar_uart0( int c );
void print_uart0( const char * s );
void putc_uart0( int c );
void puts_uart0( const char * s );
#ifdef __cplusplus
}
#endif
qemu-virt-aarch64.c:
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>
#include "pl011.h"
// angel/semihosting interface
#define SYS_WRITE0 0x04
static uint64_t semihosting_call(uint32_t operation, uint64_t parameter)
{
__asm("HLT #0xF000");
}
// syscall stubs
int _close (int fd)
{
errno = EBADF;
return -1;
}
int _isatty (int fd)
{
return 1;
}
int _fstat (int fd, struct stat * st)
{
errno = EBADF;
return -1;
}
off_t _lseek (int fd, off_t ptr, int dir)
{
errno = EBADF;
return (off_t) -1;
}
int _read (int fd, void *ptr, size_t len)
{
errno = EBADF;
return -1;
}
int _write (int fd, const char *ptr, size_t len)
{
for (size_t i = 0; i < len; i++) {
putchar_uart0(ptr[i]);
}
return len;
}
void main()
{
char buffer[BUFSIZ];
uint64_t regCurrentEL;
__asm volatile ("mrs %0, CurrentEL" : "=r" (regCurrentEL));
// UART0
sprintf(buffer, "Hello EL%d World!
", (regCurrentEL >> 2) & 0b11);
puts_uart0(buffer);
// angel/semihosting interface
sprintf(buffer, "Hello semi-hosted EL%d World!
", (regCurrentEL >> 2) & 0b11);
semihosting_call(SYS_WRITE0, (uint64_t) (uintptr_t) buffer);
// newlib - custom syscalls.c, with _write() using UART0
printf("Hello EL%d World! (syscalls version)
", (regCurrentEL >> 2) & 0b11);
}
Please note that the code responsible for initializing the .bss section was omitted.
Compiling:
/opt/arm/9/gcc-arm-9.2-2019.12-x86_64-aarch64-none-elf/bin/aarch64-none-elf-gcc -I. -O0 -ggdb -mtune=cortex-a53 -nostartfiles -ffreestanding --specs=nosys.specs -L. -Wl,-T,qemu-virt-aarch64.ld -o virt.elf startup.s pl011.c qemu-virt-aarch64.c
Running:
/opt/qemu-5.2.0/bin/qemu-system-aarch64 -semihosting -m 128M -nographic -monitor none -serial stdio -machine virt,gic-version=2,secure=on,virtualization=on -cpu cortex-a53 -kernel virt.elf
Hello EL3 World!
Hello semi-hosted EL3 World!
Hello EL3 World! (syscalls version)
