title: Raspberry 5 baremetal Hello World example
keywords:raspi5,asm,c,kernel,arm64

# Raspberry 5 baremetal Hello World example

## Intro

Here is base example that will allow to print out message to serial console,
and debug application. This example is picked up from variouse resources to
make easy start into baremetal programming for raspi5, similar route is for
all other models. All commands are tested from raspi4, adjust sources 
for your system. 

## Source code


### boot.S

```
.section ".text.boot"

.global _start

_start:
    // read cpu id, stop slave cores
    mrs     x1, mpidr_el1
    and     x1, x1, #3
    cbz     x1, 2f
    // cpu id > 0, stop
1:  wfe
    b       1b
2:  // cpu id == 0

    // set top of stack just before our code (stack grows to a lower address per AAPCS64)
    ldr     x1, =_start
    mov     sp, x1

    // clear bss
    ldr     x1, =__bss_start
    ldr     w2, =__bss_size
3:  cbz     w2, 4f
    str     xzr, [x1], #8
    sub     w2, w2, #1
    cbnz    w2, 3b

    // jump to C code, should not return
4:  bl      main
    // for failsafe, halt this core too
    b       1b

```

### kernel.c

```c
void main()
{
	while (1) {

	}
}
```

### Makefile
```Makefile
TOOLCHAIN=aarch64-linux-gnu-
AS = ${TOOLCHAIN}as
LD = ${TOOLCHAIN}ld
SRCS = $(wildcard *.c)
OBJS = $(SRCS:.c=.o)
CFLAGS = -Wall -O2 -ffreestanding
CC=$(TOOLCHAIN)gcc

all: clean kernel8.img

boot.o: boot.S
	${CC} ${CFLAGS} -c boot.S -o boot.o 

%.o: %.c
	${CC} ${CFLAGS} -c $< -o $@

kernel8.img: boot.o ${OBJS}
	${LD} boot.o ${OBJS} -T linker.ld -o kernel8.elf
	${TOOLCHAIN}objcopy -O binary kernel8.elf kernel8.img

clean:
	rm -rf *.o *.img *.elf
```

### config.txt
```
enable_jtag_gpio=1
```

### Compile source code
```sh
make
```

### Prepare for a boot



```sh
cp
```



## Debugging with OpenOCD+GDB

The default openocd doesn't configration for raspi5.

### OpenOCD config for bcm2712.cfg

If OpenOCD doesn't have the raspi5 config by default here is one 
that suppose to work. Copy it for example in default openocd script location in:

```
/usr/share/openocd/scripts/target/
```


```tcl
# bcm2712.cfg
# SPDX-License-Identifier: GPL-2.0-or-later
# OpenOCD target config file
# This file is based on target/bcm2711.cfg
# I have checked that it works with Open On-Chip Debugger 0.12.0 
# using the Raspberry Pi Debug-Probe interface

transport select swd
adapter speed 1000

if { [info exists CHIPNAME] } {
    set  _CHIPNAME $CHIPNAME
} else {
    set  _CHIPNAME bcm2712
}

if { [info exists CHIPCORES] } {
    set _cores $CHIPCORES
} else {
    set _cores 4
}

if { [info exists USE_SMP] } {
    set _USE_SMP $USE_SMP
} else {
    set _USE_SMP 0
}

if { [info exists DAP_TAPID] } {
    set _DAP_TAPID $DAP_TAPID
} else {
    set _DAP_TAPID 0x2ba00477
}

# swj_newdap $_CHIPNAME cpu -irlen 4 -ircapture 0x1 -irmask 0xf -expected-id $_DAP_TAPID
swd newdap $_CHIPNAME cpu -expected-id $_DAP_TAPID
dap create $_CHIPNAME.dap -chain-position $_CHIPNAME.cpu

# MEM-AP for direct access
target create $_CHIPNAME.ap mem_ap -dap $_CHIPNAME.dap -ap-num 0

# these addresses are obtained from the ROM table via 'dap info 0' command
set _DBGBASE {0x80010000 0x80110000 0x80210000 0x80310000}
set _CTIBASE {0x80020000 0x80120000 0x80220000 0x80320000}

set _smp_command "target smp"

for { set _core 0 } { $_core < $_cores } { incr _core } {
    set _CTINAME $_CHIPNAME.cti$_core
    set _TARGETNAME $_CHIPNAME.cpu$_core

    cti create $_CTINAME -dap $_CHIPNAME.dap -ap-num 0 -baseaddr [lindex $_CTIBASE $_core]
    target create $_TARGETNAME aarch64 -dap $_CHIPNAME.dap -ap-num 0 -dbgbase [lindex $_DBGBASE $_core] -cti $_CTINAME

    set _smp_command "$_smp_command $_TARGETNAME"
}

if {$_USE_SMP} {
    eval $_smp_command
}

# default target is cpu0
targets $_CHIPNAME.cpu0
```

Connecting pico probe and doint step by step analysis of basic loop

### Running OpenOCD 

Start openocd server

```
openocd -f interface/cmsis-dap.cfg -f target/bcm2712.cfg
```

Connect with gdb

```
gdb
```

and run inside gdb shell

```
tar ext :3333
```


## Links


https://www.raspberrypi.com/documentation/computers/config_txt.html
https://wiki.osdev.org/Raspberry_Pi_Bare_Bones

https://www.rpi4os.com/part1-bootstrapping/

https://www.valvers.com/open-software/raspberry-pi/bare-metal-programming-in-c-part-1/

https://www.raspberrypi.com/documentation/microcontrollers/debug-probe.html

https://macoy.me/blog/programming/RaspberryPi5Debugging
https://gist.github.com/tnishinaga/219122a5f1e3973668ee78c0fb1c7bf9
https://forums.raspberrypi.com/viewtopic.php?p=2172522#p2172522

https://github.com/DSERIOUSGUY/HobOS