J E L L Y E N T
Bare Metal Programming on Raspberry Pi 3

Whats up there! This tutorial assortment are made for folk that can perchance perchance presumably neatly cherish to assemble their very gather as factual with naked steel utility
for the Raspberry Pi.

The procedure procedure audience is hobby OS builders, who are recent to this hardware. I’ll most traditional you examples on how you’d attain the
total components, cherish writing to the serial console, discovering out keystrokes from it, environment veil chance and discipline to
the linear physique buffer. I am furthermore going to demonstrate you basically the most functional association one can gather the hardware’s serial quantity, a hardware-backed random
quantity, and the blueprint you’d be taught recordsdata from the boot partition.

Upright here is no longer an tutorial on how you’d write an OS. I obtained’t veil subject matters cherish memory administration and digital file programs,
or how you’d build into function multi-tasking. Absorb to you suggest to put in writing your gather as factual with OS for the Raspberry Pi, I counsel to withhold out some
be taught forward of you continue. This tutorial is strickly about interfacing with the hardware, and no longer about OS notion.
For that, I could perchance perchance maybe presumably neatly also imply raspberry-pi-os.

I rob you may perchance perchance furthermore just maybe presumably neatly presumably even have a ravishing GNU/Linux records on how you’d assemble functions and fabricate disk and file blueprint pictures. I
obtained’t veil those intimately, even supposing I’ll most traditional you gracious a pair of hints about how you’d jam up a disagreeable-compiler for this structure.

Why Raspberry Pi 3?

I’ve choosen this board for several causes: to begin with, or not it is low-impress and uncomplicated to gather. 2nd, or not it is a 64 bit
machine. I gave up programming for 32 bit long long time previously. The 64 bit is to this point more chunky of life, because or not it is
purchase care of apartment is increadibly abundant, better than the storage doable which permits us to develop educate of some chunky of life recent
options. Third, uses highest MMIO which makes it easy to program.

For 32 bit tutorials, I could perchance perchance maybe presumably neatly also imply:

Cambridge tutorials (ASM and 32 bit highest),

David Welch’s tutorials (largely C, with some 64 bit examples),

Peter Lemon’s tutorials (ASM highest, furthermore for 64 bit) and

Leon de Boer’s tutorials (C and ASM, furthermore for 64 bit, more evolved examples cherish USB and OpenGL).

Why no longer C++?

The C language in « freestanding » mode permits us to function straight to the hardware. With C++ ultimate here’s no longer
that you just gracious may perchance perchance furthermore just maybe presumably neatly presumably also keep in mind, on legend of that requires a runtime library. Absorb to you may perchance perchance perchance be on this, then I counsel to employ a be taught about
on the unheard of Circle C++ library, which no longer highest contains the quite a lot of
C++ runtime, nonetheless furthermore implements every and every Raspberry Pi functionalities we’re about to level of curiosity on in these tutorials (and some distance more).

Why no longer Rust?

Merely on legend of my inner most notion is that Rust is a splendid better diploma language than preferable for naked steel, one thing cherish
with C++. But must you present the desired runtime libraries, you may perchance perchance furthermore just maybe presumably neatly presumably also attain it. My multiplatform blueprint boot loader has an
example Rust kernel too and @andre-richter ported these tutorials to Rust.
He has added considerably more code to his Rust repository which
is a extraordinarily very top function must you may perchance perchance perchance be on this language.

Requirements

Sooner than you may perchance perchance furthermore just maybe presumably neatly presumably also function, you may perchance perchance perchance be going to be searching a disagreeable-compiler (retract 00_crosscompiler itemizing for fundamental capabilities)
and a Micro SD card with firmware recordsdata on a FAT filesystem.

Every itemizing has a Makefile.gcc and a Makefile.clang. Web obvious the Makefile symlink capabilities to the version per
the disagreeable-compiler of your deciding on. I could perchance perchance maybe presumably neatly also cherish to brand attributable to @laroche for testing these
tutorials for the notion time with Clang too.

I imply to gather a Micro SD card USB adapter
(many producers ship SD playing playing cards with such an adapter), so that you just gracious may perchance perchance furthermore just maybe presumably neatly presumably also join the card to any desktop laptop very top
cherish an USB stick, no explicit card reader interface required (even supposing many laptops have those on on basically the most traditional time and age). Absorb to you abominate
dd, then for writing pictures I imply USBImager which is easy GUI app with a
transportable executable accessible for Home windows, MacOSX and Linux.

You are going to be prepared to function an MBR partitioning blueprint on the SD card with an LBA FAT32 (vogue 0x0C) partition, layout it
and reproduction bootcode.bin, function.elf and fixup.dat onto it. Or alternatively you may perchance perchance furthermore just maybe presumably neatly presumably also produce a raspbian image,
dd it to the SD card, mount it and delete the pointless .img recordsdata. Whichever you capture. What’s fundamental, you may perchance perchance perchance be going to
fabricate kernel8.img with these tutorials which need to be copied to the foundation itemizing on the SD card, and no assorted .img
recordsdata may perchance perchance furthermore just maybe presumably neatly also silent exists there.

I could perchance perchance maybe presumably neatly also furthermore imply to gather an USB serial debug cable. You join it to the
GPIO pins 14/15, and trudge minicom on your desktop laptop cherish

minicom -b 115200 -D /dev/ttyUSB0

Emulation

Sadly official qemu binary would no longer enhance Raspberry Pi 3 nonetheless. But very top files, I’ve implemented that, so
or not it is coming quick (UPDATE: accessible in qemu 2.12). Till then, or not it is fundamental to
assemble qemu from basically the most traditional present. As rapidly as compiled, you may perchance perchance furthermore just maybe presumably neatly presumably also educate it with:

qemu-blueprint-aarch64 -M raspi3 -kernel kernel8.img -serial stdio

Or (with the file blueprint tutorials)

qemu-blueprint-aarch64 -M raspi3 -kernel kernel8.img -pressure file=$(yourimagefile),if=sd,layout=raw -serial stdio

-M raspi3
The predominant argument tells qemu to emulate Raspberry Pi 3 hardware.

-kernel kernel8.img
The 2d tells the kernel filename to be outdated.

-pressure file=$(yourimagefile),if=sd,layout=raw
In 2d case this argument tells the SD card image too, which most frequently is a susceptible rasbian image as efficiently.

-serial stdio

-serial null -serial stdio
Within the fracture the leisure argument redirects the emulated UART0 to the licensed enter/output of the terminal working qemu, so that the total pieces
despatched to the serial line may be displayed, and every and every key typed in the terminal may be purchased by the vm. Handiest works with the
tutorials 05 and above, as UART1 is no longer redirected by default. For that, you may perchance perchance perchance be going to must add one thing cherish
-chardev socket,host=localhost,port=1111,identity=aux -serial chardev:aux (thanks @godmar for the records),
or just educate two -serial arguments (thanks @cirosantilli).

!!!WARNING!!! Qemu emulation is rudimentary, highest genuinely basically the most total peripherals are emulated! !!!WARNING!!!

Touching on to the hardware

There are hundreds of pages on the produce describing the Raspberry Pi 3 hardware intimately, so I will be short and
veil highest the fundamentals.

The board is shipped with a BCM2837 SoC chip.
That capabilities a

  • VideoCore GPU
  • ARM-Cortex-A53 CPU (ARMv8)
  • Some MMIO mapped pheripherals.

Curiously the CPU is no longer genuinely the largest processor on the board. When or not it is powered up, first GPU runs. When or not it is
done with the initialization by executing the code in bootcode.bin, it goes to load and steal out the function.elf executable.
That is no longer an ARM executable, nonetheless compiled for the GPU. What pursuits us is that function.elf appears to be like to be for entirely assorted
ARM executables, all starting with kernel and ending in .img. As we’re going to program the CPU in AArch64 mode,
we’re going to desire kernel8.img highest, which is the leisure to scrutinize. As rapidly because or not it is loaded, the GPU triggers the reset line on
the ARM processor, which begins executing code at purchase care of 0x80000 (or more precisely at 0, alternatively the GPU locations an ARM
initialisation and jump code
there first).

The RAM (1G for the Raspberry Pi 3) is shared among the many many CPU and the GPU, this capacity that one can be taught what the alternative has
written into memory. To present a enhance to some distance from confusion, a efficiently outlined, so often known as mailbox interface
is established. The CPU writes a message into the mailbox, and tells the GPU to be taught it. The GPU (vivid that the
message is fully in memory) interprets it, and areas a response message on the same purchase care of. The CPU has
to poll the memory to know when the GPU is done, and then it goes to be taught the response.

Similarily, all peripherals communicates in memory with the CPU. Every has or not it is devoted memory purchase care of starting from
0x3F000000, nonetheless it definitely’s no longer in loyal RAM (often known as Memory Mapped IO). Now there is not any longer any mailbox for peripherals, as a change every and every
instrument has or not it is gather as factual with protocol. What’s total for these gadgets that their memory need to be be taught and written in 32 bit
objects at 4 bytes aligned addresses (so often known as phrases), and every and every has withhold an scrutinize on/direct and records phrases. Sadly
Broadcom (the producer of the SoC chip) is necessary adversarial at documenting their products. The succesful we now have purchased is the
BCM2835 documentation, which is shut abundant.

There may be furthermore a Memory Administration Unit in the CPU which permits growing digital purchase care of areas. It’ll reputedly presumably neatly even be programmed
by explicit CPU registers, and care need to be taken if you happen to discipline these MMIO addresses correct staunch into a digital purchase care of condominium.

About a of the more chunky of life MMIO addresses are:

0x3F003000 - Machine Timer
0x3F00B000 - Interrupt controller
0x3F00B880 - VideoCore mailbox
0x3F100000 - Vitality administration
0x3F104000 - Random Quantity Generator
0x3F200000 - Customary Reason IO controller
0x3F201000 - UART0 (serial port, PL011)
0x3F215000 - UART1 (serial port, AUX mini UART)
0x3F300000 - External Mass Media Controller (SD card reader)
0x3F980000 - Universal Serial Bus controller

For more records, retract Raspberry Pi firmware wiki and documentation on github.

https://github.com/raspberrypi

Proper success and skills hacking along with your Raspberry! 🙂

bzt

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